The size you need equals your backed-up daily load in kWh divided by usable depth of discharge times inverter efficiency, then multiplied by days of autonomy.
Two numbers decide everything: how much energy you use (kWh) and how much power you draw at once (kW). The first sets runtime; the second decides which appliances can run together without tripping the system. Skip either one and you buy the wrong box.
Most guides sizing whole-home backup land around 20–40+ kWh for a typical setup, with 60–100 kWh for multi-day autonomy. Energy Scout’s sizing guide walks the whole method if you want to work it line by line: whole-home battery backup sizing guide. Below is the short version.
How Do You Calculate Battery Capacity?
Required battery kWh = daily load kWh ÷ (usable depth of discharge × inverter efficiency) × days of backup. That formula is the whole game.
Work it in steps:
- List only the circuits you want backed up.
- Estimate their daily energy: watts × hours ÷ 1,000 = kWh.
- Pick your outage duration in hours or days.
- Divide by usable fraction and inverter efficiency for nameplate capacity.
- Check inverter continuous and surge kW against the largest simultaneous loads.
LFP batteries are often sized to about 80% depth of discharge for daily cycling, even though their technical limit is higher. So a 10 kWh battery might only give you 8 kWh of usable energy.
What Should You Back Up First?
Start with essentials: refrigerator, lights, Wi-Fi, key outlets, medical devices, sump pump. Add HVAC, water heating, or EV charging only after those are covered.
An average US home uses roughly 20–30 kWh per day. True whole-home backup often lands at 20–40+ kWh. Whole-home with HVAC commonly runs 27–40 kWh or more, and larger all-electric homes with water heating and EV charging can need 40–60 kWh. Power matters too: whole-house continuous draw is often 8–12 kW, and a central AC surge can spike to roughly 6,000–9,000 W at startup.
| Backup Scope | Typical Capacity Range | Runtime Reality |
|---|---|---|
| Critical loads only | 10–20 kWh | About a day on essentials |
| Whole-home, no HVAC | 20–40+ kWh | Roughly a day of normal use |
| Whole-home with HVAC | 27–40+ kWh | Depends on AC duty cycle |
| Large all-electric home with EV | 40–60 kWh | Higher draw, shorter runtime |
| Multi-day autonomy | 60–100 kWh | Pair with solar to recharge |
Which Sizing Mistakes Cost The Most?
Four errors account for most undersized systems, and each one is avoidable with ten minutes of math.
- Using annual average household use instead of the actual circuits you plan to back up.
- Ignoring motor and compressor startup surge, especially from AC and pumps.
- Sizing for kWh only and never checking inverter kW output.
- Forgetting solar recharge for multi-day outages — without solar, runtime is capped at whatever charge you had when the grid dropped.
One more limit sits outside the math: residential battery placement can be capped by code. If your plan bumps that ceiling, your installer will need to adjust the layout.
Once you’ve run your numbers, comparing real hardware is the next move. Our roundup of the best backup power supply for a home covers tested options by capacity and output.
FAQ
Can I size a battery without knowing my exact appliance watts?
Yes, using nameplate ratings on each appliance label. A fridge runs about 150 W while cycling, a sump pump higher during operation. Total watts × hours of use ÷ 1,000 gives you daily kWh. If a label is missing, manufacturer spec sheets list it. Add 10–20% margin for estimating error.
Does a bigger battery always mean longer runtime?
No. Runtime is limited by whichever runs out first: stored energy or inverter output. A large battery paired with a small inverter still can’t start a central AC. Check both ratings against your largest simultaneous loads before buying, and confirm surge capability covers compressor startup.
How much does whole-home backup usually cost to install?
Cost varies widely by capacity, inverter class, and local labor. A 20 kWh system installed typically runs higher than a 10 kWh critical-load setup, and multi-day configurations climb further. Get two or three local quotes, since installation labor and permitting often move the total more than the battery itself.
References & Sources
- Energy Scout. “Whole-Home Battery Backup Sizing Guide.” Supports kWh sizing formula, load benchmarks, DoD assumptions, and NFPA 855 placement caps.
