The verdict, in one paragraph
Most households sizing a battery for a critical-load keep-list — refrigerator, freezer, furnace blower, lighting, network, charging — land between 3kWh and 15kWh, depending on how many days they want. The sum is (daily watt-hours of your keep-list) x (days of cover) / 0.85. Sizing a battery for a whole house behaving normally is a different question with a much larger answer: the EIA puts average US residential use at 10,791 kWh a year, about 899 kWh a month, which is roughly 30kWh a day. The inverter number is separate, and it is set by your largest simultaneous load plus its starting surge.
Two numbers, and people only check one
A battery has a kilowatt rating and a kilowatt-hour rating, and they answer different questions:
- Kilowatts (the inverter) decides what you can run. Too small and the furnace will not start, no matter how large the battery is.
- Kilowatt-hours (the cells) decides for how long. Too small and everything works beautifully until it stops.
Oversizing one does nothing for the other. This is the single most common sizing error, and it is the same error in reverse as sizing a portable power station, where people buy capacity and get caught by the inverter.
Step 1 — write the keep-list, not the house
Sizing starts with a decision, not a calculation: what has to keep running. Three lists, and almost everybody belongs on the first:
- Keep the house from being damaged. Sump pump, well pump, furnace blower, freezer. Skipping these costs money in ruined food, frozen pipes or a flooded basement.
- Keep the house livable. Refrigerator, lighting, network, phone charging, a fan or one small heater.
- Keep the house normal. Central air, electric range, dryer, water heater. This is the list that turns a 5kWh question into a 50kWh one.
Everything below sizes list one plus list two. If list three is genuinely non-negotiable, the honest comparison is against a standby generator — generator versus home battery runs the economics rather than the arithmetic.
Step 2 — get real watt-hours a day
A nameplate wattage is what a device draws when it is on. A battery cares about watt-hours a day, which means wattage multiplied by how long it is actually running — the duty cycle. The gap is enormous. A refrigerator rated 600W on the compressor runs maybe a fifth of the time.
This site's working averages, from the pages linked beside each figure, are a starting point. Your own measurements beat them every time:
| Load | Running average | Watt-hours a day | Starting surge |
|---|
| Refrigerator | About 150 W | 1,200 - 3,600 Wh | 500 - 1,000 W |
|---|
| Chest freezer, well filled | About 100 W | 1,000 - 2,000 Wh | 500 - 1,000 W |
|---|
| Gas furnace blower | About 800 W when running | 2,500 - 6,000 Wh in cold weather | 800 - 1,600 W |
|---|
| Sump pump | 800 - 1,050 W when running | Entirely dependent on water | 2,000 W and up |
|---|
| Well pump | 1,000 W and up, 240V | 500 - 1,500 Wh typical household | Up to the 5,000 W class |
|---|
| Router, modem, ONT | 20 - 40 W | 500 - 1,000 Wh | None |
|---|
| LED lighting, a few rooms | 40 - 100 W | 300 - 600 Wh | None |
|---|
| Phones, laptops, tablets | Intermittent | 200 - 500 Wh | None |
|---|
These are this site's working averages, each derived on the per-appliance page linked in the next section. They are a starting point for arithmetic, not a substitute for measuring your own appliances.
Every row has a page behind it: refrigerator, freezer, furnace, sump pump and well pump. The appliance wattage chart covers everything else, and reading a nameplate explains why the sticker usually overstates the number you need.
Step 3 — the inverter number
This one is not an average. Add up everything that could plausibly be running at the same moment, then check the largest starting surge against that total:
- 120V keep-list, nothing heated: fridge 150W + freezer 100W + blower 800W + lights 100W + network 40W is about 1,200W running. A 1,800W inverter covers it; 2,400W covers it with the blower starting mid-cycle.
- Add one 1,500W space heater: 2,700W running. You now need a 3,000W-class inverter, and this is the single cheapest decision to reverse — turn the heater off instead.
- Add a well pump: the running total barely moves, but the surge requirement jumps to the 5,000W class and the whole machine has to produce 240V. Starting watts versus running watts is why.
If the surge requirement is what pushes you up a tier, price a soft starter first — soft starter versus a bigger machine is frequently the cheaper answer.
Step 4 — the capacity sum
Now the arithmetic. Published battery capacity is at the cells; the inverter loses some of it on the way to the outlet. This site uses 85% inverter efficiency as a standing convention, which means:
Battery kWh needed = (daily keep-list kWh x days) / 0.85
Four worked households:
- Suburban, gas heat, one night. Fridge 2.4kWh + freezer 1.5kWh + blower 3kWh + lights 0.5kWh + network 0.7kWh + devices 0.3kWh = 8.4kWh a day. One night at 0.85 is about 10kWh. Two nights is 20kWh.
- Apartment, no heat to run, one night. Fridge 2.4kWh + lights 0.4kWh + network 0.7kWh + devices 0.4kWh = 3.9kWh. At 0.85, about 4.6kWh — the DELTA Pro 3 band, or an Elite 300 with the fridge door kept shut.
- Rural with a well, two days. 8.4kWh of the suburban list + 1kWh of pumping = 9.4kWh a day, two days, at 0.85 = about 22kWh. One F3800 plus five expansion batteries, or a DELTA Pro Ultra with two.
- Whole house, central air, one day. Using the EIA average of roughly 30kWh a day and no load shedding at all, one day at 0.85 is about 35kWh — and summer air conditioning pushes the daily figure up, not down. This is the case where a standby generator usually wins on cost per hour.
Step 5 — the recharge check, which decides whether it lasts
This is the step that gets skipped, and it is the one that turns a correctly sized battery into a disappointing one. Capacity is how much you have once. Recharge is whether you get it twice.
- From the wall. Most units in this class accept around 1,800W of AC input. A 10kWh bank is therefore five and a half hours of grid time; a 22kWh bank is over twelve. If your outages come in clusters with short restorations between them, this is your real constraint.
- From solar. Published maximums range from 500W on a DELTA 3 to 2,600W on a DELTA Pro 3, and a panel never produces its rating. How much solar you need for an outage has the honest derating.
- From a generator. The fastest path, and the reason hybrid setups exist — Anker publishes 6,000W of AC charging on the F3800 Plus via generator or Home Power Panel. Running a battery and a generator together covers which systems accept it and which do not.
The rule of thumb that falls out of this: beyond about 15kWh, a battery without either serious solar or generator input is a very expensive single-use device.
Buy one size up, once
The same conclusion as every sizing page on this site, for the same reason. Load lists grow — a new freezer, a home office, a medical device — and expansion is almost always more expensive per kilowatt-hour than buying the next tier at the outset. Going up one rung is one decision. Going up one rung twice is two purchases and a leftover battery.
Where you cannot buy up, buy expandable: expandable power stations covers which ecosystems let you add later and what the lock-in costs, and outage duration and capacity turns your own utility's outage history into the days figure this page multiplies by.
