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What size home battery do you need?
Updated September 17, 2026 · counting real batteries at the capacity each manufacturer publishes, with Cali Energy load-share assumptions · by Cali Energy
What size home battery do you need? To back up a whole home using 30 kWh a day for one day, plan on about 35 kWh of capacity — roughly three Tesla Powerwall 3 units at 13.5 kWh nominal apiece. Critical loads only (fridge, lights, internet) need far less: about one to two. The right size depends on which loads you keep and for how long.
✓ An energy-only estimate in real, currently sold batteries, each with the capacity label its maker publishes — load shares labeled. See every figure it uses ↓
Assumptions: the backup shares — critical loads 25%, partial home 55%, whole home 100% of daily use — are planning heuristics from our own installs, not a measured statistic, and they are the weakest input here — in a knowable direction. They scale a roughly fixed load with a variable that has little to do with it: a fridge, a router and a few lights draw much the same 3–5 kWh a day whether the house uses 10 or 60. So below roughly 20 kWh a day the critical-loads share undersizes, and well above it, it oversizes. Enter the usage of the day you expect the outage on, too, not your annual average — PSPS shutoffs land on heat-wave days when the air conditioning runs hardest. For a real design the share should be replaced by the appliances you actually intend to run: our backup load builder does that, and also answers the power question this tool deliberately does not. We then divide by 0.85, so your load fills about 85% of the planned capacity and 15% is left as headroom rather than designing around running the pack flat, and round up to whole units. Capacity labels differ by manufacturer — Tesla publishes 13.5 kWh as nominal battery energy, Enphase and FranklinWH publish usable figures — so unit counts across makes are not strictly like for like. If you have solar it can recharge the battery during a daytime outage, so real runtime is often longer. A planning ballpark, not an engineering load calc or a quote.
Get a battery sized for your homeThe rates and figures this calculator usestap to verify
The tool turns your daily usage, backup scope and days into the kWh to plan for, then into a count of whichever battery you pick. Here is every figure it uses — the load-share fractions are labeled Cali Energy planning assumptions.
| Selected unit | Each maker’s own published figure — Powerwall 3 13.5 kWh nominal; aPower 2 15 kWh usable; IQ Battery 10C 10 kWh usable; IQ Battery 5P 5 kWh usable |
| Sizing margin | need ÷ 0.85 — your energy fills ~85% of capacity |
| Critical loads only | ~25% of daily use |
| Partial home | ~55% |
| Whole home | 100% |
| Energy needed | daily use × scope share × days |
| Capacity to plan for | ÷ 0.85 — your load fills about 85%, leaving headroom |
| Batteries | ÷ the selected unit’s published capacity, rounded up |
Each battery is counted at the capacity its own manufacturer publishes, and the labels are not equivalent — Tesla publishes a nominal figure for the Powerwall 3 where Enphase and FranklinWH publish usable ones, so unit counts across makes are not strictly like for like. Dividing by 0.85 sizes capacity so your load fills about 85%, leaving 15% as headroom rather than designing around emptying the pack. The load-share fractions (25% / 55% / 100%) are Cali Energy planning assumptions, not a measured audit of your home — your true critical-loads share depends on which circuits you back up. This is an energy-only estimate: it does not size appliance startup/surge power, your panel capacity, or whether solar can recharge the battery during a daytime outage. For the power side, use the backup-load builder.
Example results you can reproduce
Each row assumes 30 kWh of daily use — enter the same usage, scope and days above to reproduce it.
| Backup scope | Days | Capacity to plan for | Powerwall 3 units (13.5 kWh nominal) |
|---|---|---|---|
| Critical (~25%) | 2 | ~17.6 kWh | 2 |
| Partial (~55%) | 1 | ~19.4 kWh | 2 |
| Whole home (100%) | 1 | ~35.3 kWh | 3 |
How this calculator works
It multiplies your daily usage by the share of it you want to back up (25% critical, 55% partial, 100% whole home) and by the number of days, then divides by 0.85 so your load fills about 85% of the planned capacity, leaving 15% as headroom. Dividing by the published capacity of whichever battery you pick gives the unit count. It’s deliberately an energy calculation — kilowatt-hours, how much you store — not a power calculation of whether the battery can start a big motor. These are Cali Energy planning figures.
How to read your result
The recommended capacity is the energy to plan for, headroom included; the battery count rounds up to whole units of the battery you selected. Treat “whole home for multiple days” numbers with care — they climb fast and often point to solar-plus-storage (so the battery recharges by day) rather than a bigger and bigger battery bank. Pair this with the power check before you commit.
What moves the result the most
Backup scope is the biggest lever — whole-home is four times the critical-loads share. Then days of autonomy (each extra day adds a full day’s energy) and your daily usage. Solar changes the picture entirely: with panels recharging the battery each day, a smaller bank can carry a longer outage.
Common questions
What do I need to use this calculator?
Your average daily electricity use in kilowatt-hours, the backup scope you want, and how many days of autonomy you’re planning for. Take the kWh for the billing period off the bill itself — not the dollar amount, and not the total due, which on an LADWP bill can also include water and sanitation — and divide by the number of days that period covers. LADWP bills most homes every two months, so that is usually about 60 days, not 30.
What counts as “critical loads”?
Typically the refrigerator, some lights, internet/Wi-Fi, phone charging and maybe a medical device — the essentials on a few dedicated circuits, roughly a quarter of daily use.
How many days should I plan for?
Most grid outages are hours, not days. One day of critical-loads backup covers the common case; plan more only if outages in your area are long and you have no solar recharge.
Does solar recharge the battery in an outage?
Yes — a properly configured solar-plus-battery system recharges by day during an outage, so a smaller battery can span a longer event. This tool sizes energy without assuming that, to stay conservative.
Is this the power rating too?
No — this is energy (kWh). Whether the battery can actually start your AC or well pump is a power question; check it with our backup-load builder.
Does a battery without solar pay off?
Usually slowly on arbitrage alone — see our battery payback calculator. Its value is mostly backup and any SGIP incentive.
Sources
- Tesla — Powerwall 3 datasheet (nominal battery energy 13.5 kWh AC)
- Enphase — IQ Battery 5P datasheet (total and usable capacity, reserve footnote)
- Enphase — IQ Battery 10C datasheet
- FranklinWH — aPower 2 datasheet (usable system energy 15 kWh AC)
- U.S. EIA — electricity use in homes (end-use shares of residential consumption)
- U.S. EIA — California electricity profile
- California SGIP — program metrics and step status
- IRS — Residential Clean Energy Credit
Estimates only, not a quote. Assumptions, rates, incentives and program rules can change — confirm current details with your utility, the program administrator or a licensed professional.
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