Model your electric bill, solar system, battery and payback using current California utility data — with the assumptions, rates and official sources shown behind every result. No sign-up.
What size home battery do you need?
Updated August 7, 2026 · using a typical 13.5 kWh battery and 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 usable kWh — roughly three 13.5 kWh batteries. 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 on a typical 13.5 kWh battery — load shares labeled. See every figure it uses ↓
Assumptions: critical loads ≈ 20–30% of daily use (we use 25%); partial home ≈ 55%; whole home = 100%. We size in usable kWh and count typical ~13.5 kWh usable units, then round up. Keep a reserve — don’t plan to run every battery to empty. If you have solar, it can recharge the battery during a daytime outage, so real-world 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 usable kWh, then into a battery count. Here is every figure it uses — the load-share fractions are labeled Cali Energy planning assumptions.
| Typical unit | 13.5 kWh (e.g., Tesla Powerwall 3) |
| 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 |
| Recommended capacity | ÷ 0.85 for reserve and losses |
| Batteries | ÷ 13.5, rounded up |
The 13.5 kWh unit reflects common batteries like the Tesla Powerwall 3; dividing by 0.85 sizes capacity so your energy fills ~85%, leaving a 15% margin for depth-of-discharge, conversion losses, temperature and aging. 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 | Recommended capacity | Batteries (13.5 kWh) |
|---|---|---|---|
| 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 energy fills about 85% of the battery, a 15% margin for real-world losses. Dividing by 13.5 kWh gives the number of typical batteries. 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 usable kWh including the reserve; the battery count rounds up to whole units. 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 kWh (monthly bill ÷ 30 works), the backup scope you want, and how many days of autonomy you’re planning for.
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
- EnergySage — Best batteries for whole-home backup (2026)
- EnergySage — Tesla Powerwall 3 (13.5 kWh usable)
- CPUC — Self-Generation Incentive Program (SGIP)
- U.S. EIA — California electricity profile
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.
Read the full guide →