How it works
A home battery has two limits, and you need to size for both. Energy (kWh) decides how long it lasts; power (kW) decides how much it can run at once. A battery with plenty of kWh but too few kW will shut off when the well pump and microwave start together.
For energy, the calculator adds up each appliance's watts × the hours it actually runs in a day, multiplies by the days you want to cover and adds a safety margin for cold weather, battery ageing and the inverter's own use. If you have solar, it subtracts the solar harvest from the second day on, since an outage can begin at night or in a storm.
For power, it adds up the running watts of everything ticked (assuming they could all run together) and the largest start-up surge, then works out how many units of common home batteries cover both limits.
The formula
daily kWh = Σ (watts × hours a day) ÷ 1,000 capacity = (daily kWh + (days − 1) × max(0, daily kWh − solar kWh)) × (1 + margin) power (kW) = Σ running watts ÷ 1,000 surge (kW) = power + biggest (starting − running) ÷ 1,000 units = larger of capacity ÷ battery kWh and power ÷ battery kW, rounded up
Worked example
A winter outage: fridge, internet, lights, phones and laptop, sump pump and a gas furnace fan (the calculator's defaults), with a 20% margin.
| Step | Working | Result |
|---|---|---|
| Refrigerator | 150 W × 8 h | 1.20 kWh |
| Wi-Fi router and modem | 15 W × 24 h | 0.36 kWh |
| LED lights (6 bulbs) | 60 W × 6 h | 0.36 kWh |
| Phones and laptop | 60 W × 4 h | 0.24 kWh |
| Sump pump (1/2 HP) | 1,050 W × 1 h | 1.05 kWh |
| Furnace fan (gas furnace) | 800 W × 6 h | 4.80 kWh |
| Use per day | 8.01 kWh | |
| One day of backup | 8.01 × 1.2 | 9.6 kWh |
| Three days, no solar | 8.01 × 3 × 1.2 | 28.8 kWh |
| Power needed | all running together | 2.1 kW (3.7 kW surge) |
One Tesla Powerwall 3 (13.5 kWh, 11.5 kW) covers one day comfortably. Three days without solar needs about 29 kWh: 3 Powerwalls, 2 FranklinWH aPower 2 units or 6 Enphase 5P units. With 5 kW of solar on a reasonably sunny day, one battery could cover the same three days.
Frequently asked questions
How much battery backup do I need for my house?
For essentials (fridge, internet, lights, a sump pump and furnace fan) most homes need about 10 to 15 kWh a day. Running central air or electric heating can take 30 kWh a day or more, which usually means several batteries or a generator.
How big a battery do I need to run my whole house?
The average US home uses about 28 kWh a day, so a full day without cutting back needs roughly 30 to 35 kWh of usable storage and enough continuous power (often 10 kW or more) for the AC and kitchen. Most people back up essential circuits instead.
Is one Powerwall enough?
One Powerwall 3 (13.5 kWh, 11.5 kW) runs essentials for about a day, or indefinitely with solar in good weather. It will not run central air and an electric range together for long.
Do I need solar with a home battery?
Not for short outages, but solar is what makes multi-day outages manageable: it refills the battery each sunny day. Without it, size the battery for the full length of outage you expect.
What is the difference between kWh and kW?
kWh is the amount of energy stored (the size of the tank); kW is how fast the battery can deliver it (the size of the tap). You need enough of both.
Sources
- Battery capacity and output: manufacturer specifications for Tesla Powerwall 3, FranklinWH aPower 2 and Enphase IQ Battery 5P (checked October 2026).
- Appliance running and starting watts: generator makers' wattage guides and US Department of Energy, estimating appliance energy use.
Related guides
Estimates are for planning only and are not financial advice. Get written quotes before you buy.