Backup Power for Home: Sizing a Battery or Generator

The Short Answer

Backup power for home use splits into three problems, and they have different answers. Keeping food cold and a phone alive. Keeping a furnace fan, a sump pump or a well pump running. And keeping a medical device running. Very few households need all three solved at the same standard.

Start here:

Measure before you shop. A plug-in energy meter ($25–$40) left on your refrigerator for 24 hours tells you more than any buying guide. Everything below is arithmetic once you have real numbers.
Essential load under about 1.5 kWh per day, outages under 24 hours: a 1–2 kWh LiFePO4 battery station with a pure sine wave inverter rated 1,500 W or higher. Silent, safe indoors, no fuel, no exhaust, no maintenance beyond a recharge every few months.
Essential load above about 3 kWh per day, or outages that run for days, or anything involving heat: an inverter generator in the 2,200–3,500 W class. Add a licensed-electrician-installed interlock or transfer switch if you need hardwired loads like a furnace or a well pump.
The best answer for most people who can afford it: both. The generator runs in two or three short blocks a day at efficient load, recharging the battery and pulling the refrigerator and freezer back down. The battery carries the quiet overnight loads. Fuel use drops by roughly two-thirds compared with running the generator around the clock, and nobody listens to an engine at 2 a.m.
A 240 V well pump, central air conditioning, an electric water heater, an electric range or resistive space heating are a different category entirely. They need 5,000 W and up. Resistive heat is not a battery problem at all: a 1,500 W space heater run for 24 hours is 36 kWh, roughly ten times what a large portable station holds.

The rest of this guide gets you from your appliances to a number, and from a number to a purchase you will not regret on the second night. If you are building household preparedness from scratch, Start Here sequences this alongside water, food and first aid.

Measure First: Building Your Real Load Table

The nameplate on an appliance is the maximum the manufacturer will admit to, not what the thing draws. A refrigerator labelled 6.5 A at 115 V (750 W) typically runs at 100–200 W when the compressor is on, and the compressor is off most of the time. Size to the nameplate and you buy three times the generator you need. Size to the instantaneous reading and you miss the duty cycle. You need both numbers, plus a third.

The three numbers for every load:

Running watts — steady draw while operating. Read it off the meter.
Starting watts (surge) — what a motor pulls for a fraction of a second as it spins up. Induction motors draw roughly 3–8 times running current. Modern variable-speed inverter compressors soft-start and may surge barely at all. Most plug-in meters will not catch a 200-millisecond spike; treat published motor figures as your guide and leave margin.
Watt-hours per day — running watts multiplied by hours actually run. This is the only number that sizes a battery, and it is the one nobody measures. Leave the meter connected for a full 24 hours and read the accumulated kWh.

Typical ranges to sanity-check your own readings (verify yours, these are class figures, not a substitute for metering):

Refrigerator, 18–22 cu ft, ten years old: 100–200 W running, 700–1,200 W surge, 1.2–2.0 kWh/day. A recent efficient model: 0.8–1.3 kWh/day.
Chest freezer, 7 cu ft: 0.6–1.2 kWh/day.
Gas furnace with a PSC blower: 400–600 W running, 1,000–2,000 W starting. With an ECM blower: 80–300 W. The hot surface igniter adds 300–500 W for a few seconds per cycle.
Boiler circulator pump: 60–120 W.
Sump pump, 1/3 hp: 700–900 W running, 1,300–2,500 W starting.
Submersible well pump, 1/2 hp, 240 V: 900–1,200 W running, 2,500–4,000 W starting.
Router plus modem or ONT: 10–25 W continuous.
LED bulb: 6–12 W. Laptop: 30–70 W. One phone charge: 15–25 Wh.
CPAP without heated humidifier or heated hose: 25–45 W. With both: 60–110 W.
Oxygen concentrator: 300–600 W continuously — 7 to 14 kWh per day.
Microwave: a "1,000 W" microwave is rated on cooking output. It draws 1,400–1,700 W from the wall. Same trap on kettles and coffee makers at 900–1,500 W.
Window air conditioner, 8,000 BTU: 600–900 W running, 1,500–2,500 W starting unless it is an inverter model.

Two things the plug-in meter cannot do. It will not measure a hardwired load — furnace, well pump, boiler — and it will not measure 240 V. For those, use a clamp meter on the circuit, a whole-panel energy monitor, or the manufacturer's data plate with a generous allowance. Do not open a panel unless you are qualified to; see the disclaimer.

Measure in the season you are planning for. A furnace blower runs three times as long in January as in October, and a refrigerator in a 90°F (32°C) kitchen works considerably harder than in a 68°F (20°C) one. If you have to measure out of season, add 25%.

The Arithmetic, Including the Losses Nobody Counts

Watts are a rate. Watt-hours are a quantity. A battery stores watt-hours; a generator delivers watts. Confusing the two is the single most common sizing error.

Amp-hours are meaningless without a voltage. A 100 Ah battery is 100 Ah × 12.8 V = 1,280 Wh. Always convert to watt-hours before comparing anything.

Then apply the derates.

Depth of discharge. A flooded or AGM lead-acid battery should not go below about 50% state of charge if you want it to survive. That 100 Ah lead-acid battery delivers roughly 600 usable Wh, weighs about 60 lb (27 kg), and gives you a few hundred cycles. A 100 Ah LiFePO4 gives roughly 1,150 usable Wh, weighs about 30 lb (14 kg), and is commonly rated for 3,000+ cycles to 80% capacity. Per usable watt-hour over its life, lithium is the cheaper battery.
Discharge rate. Lead-acid capacity is quoted at a 20-hour rate. Pull it down in five hours and you may get only 75–85% of the rated amp-hours. LiFePO4 barely notices.
Inverter efficiency. 85–92% for a decent pure sine wave inverter. Plan on 85%.
Inverter standby draw. The number buried in the manual: 5–25 W any time the AC output is on. This matters enormously for small loads. A station holding 1,024 Wh, powering a 15 W router, with a 10 W standby draw, is really carrying 25 W — about 34 hours of runtime, not the 57 hours the naive division suggests. Switch the AC output off when nothing needs it, and run 12 V devices from the DC ports where you can.
Temperature. Lithium capacity drops in the cold. Below freezing, expect noticeably less usable energy, and most quality stations will refuse to charge at all below 32°F (0°C) because charging a frozen lithium cell plates metallic lithium and permanently damages it.

The two formulas you need:

Daily requirement (Wh) = Σ (each load's watts × hours it runs per day), then divide by 0.85 for inverter losses.
Runtime (hours) = (rated Wh × 0.85) ÷ average load in watts.

Write your load table on paper. Nine loads and three columns. That sheet is your specification, and it is worth more than any product comparison.

A Worked Example: 48 Hours in February

Three people, a gas furnace with a standard PSC blower, municipal water, one CPAP, an eight-year-old refrigerator. Two days without power in a cold snap. Here is the whole calculation.

Pass one — everything the household considers essential, per 24 hours:

Refrigerator/freezer: 1.5 kWh
Furnace blower, 500 W, cycling about 20 minutes per hour in the cold = 8 hours of run time: 4.0 kWh
Router and ONT, 18 W × 24 h: 0.43 kWh
LED lighting, 40 W × 5 h: 0.20 kWh
Phones and one laptop: 0.30 kWh
CPAP, humidifier off, 35 W × 7 h: 0.25 kWh

Subtotal at the appliance: 6.68 kWh/day. Divide by 0.85 for inverter losses: 7.9 kWh/day from the battery, or about 15.7 kWh over 48 hours.

That is four to five large portable stations, or roughly $8,000–$12,000 of battery. It is not a battery problem. It is a generator problem — or a furnace problem.

Pass two — remove the furnace. 2.68 kWh/day at the appliance, 3.15 kWh/day from the battery, 6.3 kWh over 48 hours. A 2 kWh station now covers about 15 hours. A 3.6 kWh station with one expansion battery covers about 27 hours. Still short of two days, and you now have a cold house.

Pass three — apply the cold-chain rules instead of brute force. Per FoodSafety.gov, an unopened refrigerator holds safe temperature for about 4 hours, a full freezer for about 48 hours, and a half-full freezer for about 24 hours. For a 48-hour outage with a full freezer, you may not need to power the freezer at all. Keeping the doors shut costs nothing.

The realistic essential load becomes: refrigerator 1.5, lighting 0.2, communications 0.25, CPAP 0.25 = 2.2 kWh/day at the appliance, about 2.6 kWh/day from the battery. A 2 kWh station carries that comfortably through a night and most of a day.

Pass four — add a generator for heat and resupply. A small inverter generator delivers roughly 3–5 kWh of electricity per gallon (3.8 L) of gasoline at moderate load; that figure comes straight out of manufacturer runtime charts, where a 2,200 W-class unit burns about 0.20–0.25 gal/hr at half load. Running it in two three-hour blocks a day covers the furnace, recharges the battery and pulls the refrigerator down for 1.2–1.5 gallons (4.5–5.7 L) per day. Running the same generator continuously at light load burns 3–4.5 gallons (11–17 L) per day and produces the same useful result.

At $3.50 a gallon and 4 kWh per gallon, backup electricity costs you about $0.88 per kWh — roughly five times grid price. That is the real cost of resilience, and it is worth knowing before you plan to run the whole house.

Triage: What Actually Needs Power, in Order

Work down this list and stop when you run out of capacity. Most people build their plan in the reverse order, which is how households end up with a powered television and a flooded basement.

1. Medical equipment. Anything keeping someone alive is tier zero and sits outside the arithmetic above.

CPAP: 25–45 W with the humidifier and heated hose off, which is how you should run it on backup. Many units accept 12 V DC directly, skipping the inverter and cutting consumption further. Whether a night or two without it is acceptable is a question for the prescribing clinician, not for us.
Oxygen concentrator: 300–600 W continuous, 7–14 kWh/day. This is not a portable-power problem. Talk to your equipment supplier about cylinder backup and plan to relocate.
Refrigerated medication: follow the manufacturer's labelled storage guidance for that specific product. Do not generalise from a website, including this one.
Register with your utility's medical or priority-restoration program. Registration is worth doing and it is not a plan — it does not guarantee earlier restoration.

2. Water. On a well, no power means no water at all, and a 240 V pump is beyond every portable battery and most small generators. Storing water is enormously cheaper than powering the pump: containers for a week's supply cost under $50. Plan on one gallon (3.8 L) per person per day as a floor — see how much water to store per person. If a boil-water notice accompanies the outage, the EPA's emergency disinfection guidance covers boiling and bleach ratios, and boiling takes a stove, not a generator.

3. The cold chain. Put a thermometer in the refrigerator and one in the freezer now, not during the outage — you cannot judge 40°F (4°C) by hand. Keep the refrigerator at or below 40°F (4°C) and the freezer at or below 0°F (−18°C). Perishables held above 40°F for more than two hours should be discarded. Before a forecast storm, turn both colder and fill freezer voids with jugs of water: thermal mass is free, and the jugs become drinking water. Food that has thawed but still holds ice crystals or is at or below 40°F can generally be refrozen with some quality loss; if you plan to salvage a thawed freezer by canning, use tested procedures from the National Center for Home Food Preservation rather than improvising. Our 30-day pantry plan is built so that a failed freezer is an inconvenience rather than a crisis.

4. Heat, or cooling. In winter the target is the furnace blower or the boiler circulator, not the whole HVAC system. In summer, fans plus one shaded room beat any attempt to run central air off a portable source; the CDC and the Red Cross both publish heat-illness guidance worth reading before you need it. In a hard freeze, protecting plumbing may matter more than comfort.

5. Sump pump. If a wet basement is your actual failure mode, a dedicated battery-backup sump pump is cheaper, more reliable and more automatic than any general-purpose backup plan. Buy the specific solution to the specific problem.

6. Communications and light. A router at 15–25 W is one of the cheapest loads you own. Assume cell service may degrade; a battery or hand-crank radio tuned to National Weather Service broadcasts needs no infrastructure at all. Lighting belongs in your 72-hour kit, not on your generator.

7. Everything else. Cooking, coffee, entertainment. Cheap on a generator during a run block, expensive on a battery. Cooking is better solved outdoors on a camp stove — never indoors or in a garage — and if the weather is against you, fire in wet conditions covers doing that badly-conditioned.

Battery Stations: Reading the Spec Sheet Honestly

A portable power station is a battery, an inverter, a charger and a controller in one box. Six specifications decide whether it will do your job.

Capacity in watt-hours, and usable capacity. Assume roughly 85% of the printed number reaches your AC outlets. A "2,048 Wh" station is a 1,700 Wh station in practice.
Chemistry. LiFePO4 (LFP) is the right default: commonly rated 3,000–4,000 cycles to 80% capacity, thermally stable, tolerant of sitting at partial charge. NMC packs more energy into less weight and bulk but is typically rated 500–800 cycles. For a unit that lives in a closet and gets cycled twenty times a year, cycle life matters less than calendar aging and storage behaviour, and LFP wins there too.
Inverter rating: continuous watts, surge watts, and waveform. You need continuous ≥ the sum of your simultaneous running loads, and surge ≥ your largest starting load. Check that the surge figure comes with a duration; "3,000 W peak" for 20 milliseconds will not start a compressor. Insist on pure sine wave. Modified sine wave can overheat motors, confuse variable-speed appliance electronics and is a poor choice for anything medical.
Charging inputs. AC input rate is what makes a generator-plus-battery setup work: a station that accepts 1,200 W refills 2 kWh in under two hours of generator run time. A station limited to 300 W input needs seven hours, which defeats the whole architecture. Also check maximum solar input voltage and wattage, and whether the MPPT controller is any good.
Standby draw and passthrough. Find the idle consumption in the manual. Check whether the unit offers UPS-style passthrough with a switchover fast enough (typically under 20 ms) to keep a desktop or a device alive, and whether the manufacturer actually endorses leaving an inductive load like a refrigerator on passthrough indefinitely. Many do not.
Temperature limits. Typical: charge between 32°F and 113°F (0°C to 45°C), discharge from about −4°F to 113°F (−20°C to 45°C) with reduced capacity at the cold end. A station stored in an unheated garage in January may refuse to charge when you need it.

Storage discipline matters more than the spec sheet. Keep the station between 50% and 80% charge for long-term storage, top it up every three to six months, and bring it up to full when a storm is forecast. A station sitting at 15% since last summer is the most common way a good purchase becomes useless.

What a portable station cannot do: 240 V loads, whole-house backup, resistive heating, central air, or a well pump. It also cannot recharge itself through a week of overcast winter. If your plan depends on any of those, you need a generator, a home standby system, or a different plan. Our gear testing method explains what we do and do not verify ourselves before a product appears in Gear, Tested.

Generators: Inverter vs Conventional, and the Fuel Math

Inverter generators rectify and re-synthesise the output, producing clean power (total harmonic distortion typically under 3%) and throttling engine speed to match load. They are quieter — often around 50–60 dBA at rated distance versus 70–80 for an open-frame unit — lighter, more fuel-efficient at part load, and safe for sensitive electronics. They are also more expensive per watt and usually top out around 4,500 W.

Conventional open-frame generators run at a fixed 3,600 rpm regardless of load. They are cheaper per watt, available in larger sizes with 240 V output, noticeably louder, thirstier at light load, and produce dirtier power — distortion figures in the 5–25% range are common. Fine for a sump pump or a well pump, questionable for a variable-speed furnace board or a medical device.

Size to running watts plus the largest single surge, never to the peak number on the box. Worked example: refrigerator 150 W running / 800 W surge, furnace blower 500 W running / 1,500 W surge, lighting and router 100 W, device chargers 100 W.

Total running load: 850 W.
When the furnace starts: (850 − 500) + 1,500 = 1,850 W momentary.
If the refrigerator compressor happens to start at the same instant: add another 650 W = 2,500 W.

A unit rated 1,800 W continuous / 2,250 W peak handles the furnace start alone and trips on the coincidence. A 2,800–3,000 W continuous unit has real margin. Coincidence is not rare over 48 hours; buy the margin.

Fuel. Work from the manufacturer's runtime chart, which gives hours at a stated load, and convert: gallons per hour = tank size ÷ runtime. The useful derived figure for a small inverter generator at moderate load is roughly 3–5 kWh of electricity per gallon (3.8 L) of gasoline. Three days at 6 kWh/day is therefore 4–6 gallons (15–23 L), which is two approved cans — not the twenty gallons people imagine.

Gasoline degrades. Untreated fuel becomes questionable in 3–6 months; stabiliser extends that, and ethanol-free fuel is more stable. Rotate stored fuel into your car every six months. Store only in approved containers, and check your local fire code — residential storage quantity limits vary by jurisdiction and are sometimes lower than people assume.
Propane does not degrade, which makes it ideal for equipment that sits unused for years. A 20 lb (9 kg) cylinder holds about 4.6 gallons (17 L) and roughly 430,000 BTU, typically 8–12 hours of moderate-load running on a small inverter generator. Expect about 10% less power output on propane than on gasoline. In deep cold — below roughly 20°F (−7°C) — a 20 lb cylinder can struggle to vaporise fast enough at high draw; use a larger cylinder or switch to gasoline. A dual-fuel generator is the sensible default for most households.

Oil is how generators actually die. Small engines commonly need their first oil change at 5–20 hours and changes every 50–100 hours thereafter. A three-day outage is 20–70 run hours. Keep the correct oil, a funnel and a rag physically strapped to the generator, not in the garage you may not be able to reach.

Maintenance that prevents the most common failure: run the unit under load for 20–30 minutes every month or two, or store it with the fuel valve closed and the carburettor run dry. Gummed carburettors from stale ethanol fuel are the overwhelming cause of "it won't start" on day one of an outage.

Carbon Monoxide and Backfeed: The Two Things That Kill People

This section is safety-critical. Treat everything in it as a hard rule rather than a preference.

Carbon monoxide. After the storm itself, CO poisoning from generators is the leading cause of death in outages. CO is colourless and odourless, and the onset is insidious — headache, dizziness, weakness, nausea, confusion, sleepiness — which is exactly why people do not react in time.

Never run a generator indoors, in a garage (even with the door open), a shed, a basement, a crawlspace, a breezeway, or under an open porch or carport. Enclosure is not the only factor; proximity is.
Operate it outdoors, at least 20 feet (6 m) from the house, with the exhaust directed away from doors, windows, vents and air intakes — and away from your neighbours' too. This figure is published guidance from Ready.gov and the CDC; read both directly.
Install battery-backed CO alarms on every level and outside sleeping areas. Test them during your drill and check the printed replacement date — CO sensors expire, typically at 5–10 years.
If anyone shows symptoms, get everyone outside into fresh air immediately and call emergency services. Do not go back inside.
The same rule covers charcoal grills, camp stoves and unvented fuel heaters. No combustion indoors or in an attached garage, in any weather, for any reason.
Never refuel a running or hot generator. Spilled gasoline on a hot muffler ignites. Shut it down and let it cool.

Backfeed. Never plug a generator into a wall outlet using a cord with male plugs on both ends. It energises your home wiring, bypasses every breaker, and can push power back through the utility transformer at lethal voltage onto lines a lineworker believes are dead. It has killed utility workers and homeowners.

The four legitimate ways to connect:

Appliance cords run directly from the generator. Outdoor-rated, correctly gauged, kept out of standing water and up off wet ground. Cord gauge matters: for a 15 A load over 100 ft (30 m), use 12 gauge, not the 16 gauge lamp cord in the drawer. Undersized cords drop voltage, overheat motors and start fires.
A generator interlock kit on the main panel — a mechanical plate that makes it physically impossible to have the main breaker and the generator breaker closed at the same time. The cheapest compliant option.
A manual transfer switch feeding a selected set of circuits.
A permanently installed standby generator with an automatic transfer switch.

The last three must be installed by a licensed electrician, permitted and inspected. In the US they fall under NEC Article 702 plus local amendments, and requirements differ by state, county and utility. Grounding and neutral-bonding on portable generators can conflict with transfer switch wiring and defeat GFCI protection — this is genuinely a professional's job, not a weekend project. Rules vary by jurisdiction and we cannot tell you what applies at your address; see the disclaimer and ask your local building department.

Renters and apartment dwellers: in most cases a generator is not an option at all, on a balcony or anywhere else. Battery stations exist precisely for this case, and they are the reason the arithmetic in this guide matters so much to you.

Solar and Other Recharging: What a Panel Rating Really Delivers

A panel's wattage is measured at standard test conditions — 1,000 W/m² of irradiance, a 25°C cell temperature, a clear reference spectrum. Your roof, your driveway and your January are none of those.

In practice, expect a well-angled panel in clear conditions to deliver roughly 70–85% of its rating while the sun is high, and rather less if it is lying flat on the grass. Then multiply by usable sun hours, which vary enormously by latitude and season: broadly, a good summer day in much of the continental US gives the equivalent of four to five hours at full output, a clear winter day in the northern half gives perhaps one and a half to two and a half, and a storm-overcast day can give 10–20% of that. These are planning rules of thumb, not measurements — look up the solar resource for your own location before you rely on them.

So 400 W of portable panels might deliver 1.4–1.8 kWh on a clear summer day and 0.1–0.3 kWh on an overcast winter day. That asymmetry is the whole point, and it is where most solar-backup plans quietly fail: the weather system that takes your power out in January is usually the same weather system that takes your sun.

What that means in practice:

Solar is excellent for a long summer outage, for wildfire-season public safety power shutoffs, and for indefinitely extending a small load like lighting and communications.
Solar is not a plan for a three-day winter storm. Pair it with an AC charge source.
Car charging is worse than it looks. A 12 V accessory socket is typically fused at 10–15 A, giving 120–180 W. One kWh therefore takes six to nine hours of idling, and idling burns roughly 0.2–0.5 gal/hr — considerably worse fuel-to-kWh than a generator. An inverter clamped directly to the battery terminals with the engine running does better but loads the alternator hard and needs care. Never idle a vehicle in an attached garage.
Electric vehicles and some hybrids with power export are the exception — output ratings run from about 1.5 kW to 9.6 kW, which is genuine whole-house territory. Check whether your specific vehicle supports it, what inlet or adapter it needs, and whether powering a house requires a transfer switch installation.

Before a forecast event, top everything up. Watch the National Weather Service rather than a social feed, and treat "charge the battery, fill the cans, fill the bathtub" as one routine triggered by a watch or warning.

The Hybrid Setup: Generator Blocks Plus Battery Overnight

This is the architecture we would recommend to most households that can afford both pieces, and it is not how generators are usually sold.

The principle: a generator is most efficient at 40–60% load and least efficient idling along at 10%. A battery station is efficient at any load but has a hard energy ceiling. Run the engine hard and briefly; let the battery handle the long, quiet tail.

A worked daily schedule for a 48–72 hour winter outage:

07:00–10:00, generator running. Battery station charges at its maximum AC input. Refrigerator and freezer pull down to temperature. Furnace brings the house up. Cooking, hot water, laundry and every device charge happen inside this window.
10:00–18:00, generator off. Battery carries the router, lighting as needed, and the refrigerator on its own duty cycle. The house drifts down in temperature.
18:00–21:00, generator running. Second charge, second pull-down, evening meal, house back up to temperature.
21:00–07:00, generator off. Battery carries the CPAP, the router and a light. Silence.

The fuel difference: six hours a day at a healthy load is 1.2–1.5 gallons (4.5–5.7 L). Twenty-four hours a day at light load doing the same useful work is 3–4.5 gallons (11–17 L). Over three days that is roughly four gallons versus twelve — the difference between two cans in the shed and a queue at a gas station that has no power either.

What it requires: a battery station that accepts at least 1,000 W of AC input, an outdoor-rated cord of adequate gauge, and a decided place for the generator to live that satisfies the 20 ft (6 m) rule and lets you chain and lock it.

Where the architecture breaks. If any load must run continuously and exceeds your inverter — a well pump, a sump pump during active flooding, medical equipment beyond the station's rating — block scheduling does not work. You need a transfer switch and a generator running continuously, and you should size and fuel accordingly.

How cold the house gets between blocks is the one variable we cannot compute for you. It depends on your insulation, your air sealing, your outdoor temperature and your thermal mass. Measure it during the drill below and you will have your own number instead of a guess.

The Drill: Blackout Saturday, With Pass Conditions

A plan you have not run is a hypothesis. This drill takes six hours, costs nothing, and reliably finds the two or three things that would have ruined a real outage.

Setup, the day before: put thermometers in the refrigerator and the freezer. Fill the generator and check its oil. Charge the battery station. Print your load table. Tell the household — a surprise test teaches less than a rehearsed one, and the objective is to find failures, not to catch people out.

Procedure:

08:00. Switch off the main breaker, or unplug the relevant circuits if you would rather not. Note the time. From here, only what is on your load table, powered only from your actual backup source.
Do not open the refrigerator or freezer except as your plan requires.
Every hour, record: battery state of charge, indoor air temperature, and anything anybody reached for and could not use. The last column is the valuable one.
At four hours and six hours, record refrigerator and freezer temperatures.
14:00. Restore power. Write up the failures within the hour, while the detail is still fresh.

Pass conditions — all seven must be true:

Refrigerator interior is at or below 40°F (4°C) at the four-hour mark, and the freezer is at or below 0°F (−18°C), matching the FoodSafety.gov thresholds.
The battery station reads 30% or higher at hour six while carrying every load on your table, meaning you have genuine margin for an overnight.
Your generator, on fuel that has been sitting since the last drill, starts within 10 seconds of electric start or five pulls, accepts its rated load, and runs 30 minutes without surging or stalling.
Every person in the household located a working light source within 60 seconds of the lights going out, without using a phone as a torch.
Every CO alarm passes its test button, and none is past its printed replacement date.
You can state, from your own written notes, how many hours your setup covers your essential load — as a number, not a feeling.
Nothing required an improvised connection, an undersized extension cord, or a trip to a store.

Also record, even though it is not pass/fail: the rate the indoor temperature fell, in degrees per hour, against the outdoor temperature that day. That single figure lets you estimate how long your house stays habitable between generator blocks in genuinely cold weather, which is the number no guide can give you.

A failed drill is information. First attempts most often fail on one of three things: the generator will not start on old fuel, the battery has been sitting at 20% since last summer, or the load table turns out to be missing something obvious — the gas furnace, the electric garage door, the medication in the refrigerator door.

Run it twice a year. Tie it to the clock change so you do not have to remember. If a family member has medical needs, run the drill with their equipment in the loop and with your first aid kit somewhere you can reach it in the dark. Our general approach to documenting conditions and failures is set out in How We Test.

Common Mistakes and Failure Modes

Almost every backup power failure we can identify falls into one of these. Each is listed with the mechanism, because knowing why is what stops it recurring.

Sizing from nameplates. The label is the maximum draw, often three to five times the real running load. Result: an oversized generator running at 10% load, burning fuel inefficiently and costing three times what was needed.
Ignoring starting surge. A refrigerator that meters at 150 W will still trip a 300 W inverter the instant the compressor kicks in. Size to running load plus the largest single surge, and check what duration the surge rating covers.
Confusing amp-hours with watt-hours, or ignoring depth of discharge. A "100 Ah battery" tells you nothing until you multiply by voltage and then subtract the unusable half if it is lead-acid.
Forgetting inverter losses and standby draw. Roughly 15% goes to conversion, plus 5–25 W burnt continuously just having the AC output switched on. On small loads this can halve your runtime.
Backfeeding through a wall outlet. Lethal to utility workers, bypasses all overcurrent protection, and illegal essentially everywhere. Use an interlock or transfer switch installed by a licensed electrician.
Running a generator too close to the house. In a garage with the door open, under a deck, on a porch, next to an open window. CO does not require full enclosure to kill. 20 feet (6 m), exhaust pointed away.
No CO alarm, or an expired one. Sensors have a service life of roughly 5–10 years and the alarm keeps chirping like it works long after the sensor has drifted.
Stale fuel and a gummed carburettor. The most common single reason a generator does not start on the first night of an outage. Rotate gasoline every six months, use stabiliser, or run the carburettor dry before storage. Propane sidesteps the problem entirely.
No oil, no spare oil, no idea of the service interval. A three-day outage can exceed the first oil-change interval on a new small engine.
Planning to run resistive heat off a battery. 1,500 W for 24 hours is 36 kWh. No portable station is within an order of magnitude. Heat is a fuel problem or an insulation problem, not a battery problem.
Buying solar without checking the season. Panels sized off summer performance deliver a small fraction of that during the winter storm that caused your outage.
Undersized or indoor-rated extension cords. Voltage drop overheats motors and the cord itself. Match gauge to current and length, use outdoor-rated cord, and keep connections dry and off the ground.
Powering the entertainment and not the sump pump. Decide the triage order before the outage, in writing, when you are calm.
Forgetting the well pump is 240 V. No portable battery and few small generators can run it. Store water instead; it is a fraction of the cost.
Storing the battery station at 0% or 100%, in a hot or freezing garage. Both degrade capacity, and lithium will refuse to charge below freezing when you finally need it.
Never running the drill. Everything above is discoverable in six hours on a Saturday and expensive to discover at midnight in February.

One last note on scope. Equipment selection, installation and fuel storage are all regulated differently depending on where you live, and the electrical work in this guide is described so you can talk sensibly to an electrician — not so you can do it yourself. Read the disclaimer, check your local code, and see the rest of Self-Reliance at Home for the adjacent problems: heat, water and food when the grid is the thing that failed. The general household preparedness framework these fit into is covered by FEMA and Ready.gov, and both are worth reading in the original.