What Size Home Battery Do I Need? A California Calculator
Straight answer: for critical loads only — fridge, Wi‑Fi, lights, medical devices — most California homes need only about one ~13.5 kWh battery, since that covers roughly a day’s worth of essentials. Want whole-home backup with AC and electric cooking? That usually means two or more. Use the calculator below to size yours, then see how usable capacity and solar recharging change the math.
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- Battery size is about usable kWh, not the nameplate number — a “13.5 kWh” unit like a Tesla Powerwall 3 gives you about 13.5 kWh you can actually draw.
- Backing up critical loads only (fridge, Wi‑Fi, a few lights, medical devices) usually needs roughly 20–30% of a day’s use; whole‑home backup with AC and cooking often needs two or more batteries.
- Solar can recharge the battery during a daytime outage, so one battery often stretches further than the raw math suggests.
- This is a planning estimate, not an engineering load calculation — a site visit sizes the real system.
Quick battery-capacity estimator
Pick what you want to keep running, enter your average daily electricity use, and choose how many days of backup you want. The tool returns a recommended usable capacity and how many typical ~13.5 kWh batteries that works out to. It’s a ballpark for planning — a rooftop and panel-box visit gives the real number.
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 homeHow this is different from “do I need a battery for outages?”
If you’re still deciding whether a battery is worth it for PSPS shutoffs and grid outages, start with our companion guide. This page assumes you’ve decided to add storage and just want to know how big.
Our battery, outages & PSPS guide covers the “is it worth it” question — how Public Safety Power Shutoffs work, what a battery does during a blackout, and how it pairs with solar under NEM 3.0. This article is the next step: a straightforward way to translate “I want to keep the fridge and a few lights on” or “I want the whole house to ride through a shutoff” into a number of kilowatt-hours and batteries.
Usable vs. nominal kWh: the number that actually matters
Every battery has two capacity figures. Size around the usable one.
The nominal (or nameplate) figure is the total energy in the cells. The usable figure is what you can actually draw before the system stops to protect the battery. Manufacturers hold back roughly 10–20% as a buffer, so a pack with a higher nameplate can deliver less than it seems. A Tesla Powerwall 3, for example, is rated at 13.5 kWh usable — Tesla manages the charge window internally, so that 13.5 kWh is the number you plan around. When you compare batteries, always compare usable kWh, and when you size a system, size it in usable kWh. The calculator above already works in usable terms.
Critical-loads vs. whole-home backup
The single biggest driver of how many batteries you need is how much of the house you want to keep alive.
| Backup goal | What it typically runs | ~Share of daily use | ~Usable kWh (1 day) | ~13.5 kWh batteries |
|---|---|---|---|---|
| Critical loads only | Fridge, Wi‑Fi/router, phone & laptop charging, a few LED lights, medical devices | ~20–30% | ~5–7.5 | 1 |
| Partial home | The above plus more outlets and one room of AC or a well/sump pump | ~50–60% | ~12.5–15 | 1–2 |
| Whole home | Central AC, electric cooking, EV-adjacent loads — essentially the whole panel | ~100% | ~25+ | 2–3 |
Shares and capacities are illustrative planning figures based on EnergySage and general industry sizing guidance; your home’s loads and habits change the result. Critical-load circuits are chosen with your installer and depend on your panel and applicable code.
One battery or two?
For most Southern California homes that just want to ride out a PSPS shutoff or a short outage without losing the fridge and the internet, a single ~13.5 kWh battery is plenty — especially paired with solar, which tops the battery back up each sunny day. Homeowners who want central air conditioning, electric cooking, or an electric water heater to keep running through a multi-hour or overnight outage usually land on two batteries, and larger all-electric homes sometimes three. A useful rule of thumb: air conditioning and electric heat are the loads that push you from one battery to two. If those can be off (or run briefly) during an outage, one battery covers a lot of ground.
A battery plus solar isn’t the same as a battery alone
A standalone battery has to carry the whole outage on its stored charge. With rooftop solar, the panels can recharge the battery during daylight, so a single 13.5 kWh unit may — with a well-designed solar-plus-storage system, enough sunshine, and controlled loads — be recharged by daytime solar and stretch an outage beyond one night, but that is not guaranteed multi-day backup. That’s why we size critical-load backup on about a day’s worth of usable capacity rather than piling on batteries for “days” of storage. Cloudy stretches and heavy AC use shorten it.
What about the cost — and SGIP?
A single home battery typically adds about $10,000–$18,000 installed before incentives, depending on brand and size; a second unit adds less per battery because the install labor and electrical work are largely shared. California’s Self-Generation Incentive Program (SGIP) can offset part of the cost, and higher-value paths exist for medically vulnerable and fire-threat customers — but the general market budgets availability changes by program category and administrator, so confirm the live SGIP budget status before treating any rebate as available — don't bank on it in your project economics. See our SGIP battery incentives guide for the live picture and who still qualifies. Numbers here are illustrative planning figures, not a quote.
A quick worked example
Say a Northridge home averages 25 kWh a day. For critical-loads backup, we plan on ~25% of that — about 6.3 kWh of usable capacity for a day, which one 13.5 kWh battery covers comfortably with headroom to spare. Want the whole home including central AC to ride through an overnight outage? That’s ~25 kWh usable, which rounds up to two 13.5 kWh batteries. Add solar and the critical-load setup can span multiple sunny days, because the panels refill the battery each afternoon. Your real usage — especially AC and any EV charging — moves these figures, which is exactly what a site visit nails down.
Get a free battery estimateFrequently asked
What size home battery do I need in California?
It depends on what you want to keep running. For critical loads only (fridge, Wi‑Fi, lights, a few outlets, medical devices), as a rough planning shortcut critical loads are often around 20–30% of a day’s electricity use — roughly 5–8 kWh usable, which one ~13.5 kWh battery covers. For whole-home backup including central AC and electric cooking, you usually need 25 kWh or more of usable capacity — often two or three batteries once you add a planning reserve. For a real design, size by the specific appliances you want to run, their run-hours and startup power, not a flat percentage. The estimator above gives a ballpark.
What is the difference between usable and nominal battery capacity?
Nominal (nameplate) capacity is the total energy in the cells; usable capacity is what you can actually draw before the system stops to protect the battery. Manufacturers describe this differently — some publish nominal and usable energy separately, while others publish an AC battery-energy figure under specified conditions (Tesla lists 13.5 kWh AC for the Powerwall 3). Don’t assume every maker withholds the same percentage; use the exact capacity definition in the current datasheet and warranty when you compare batteries.
Do I need one battery or two?
For riding out a PSPS shutoff or short outage without losing the fridge and internet, one ~13.5 kWh battery is usually enough — especially with solar, which recharges it each sunny day. If you want central AC, electric cooking, or an electric water heater to keep running through a long or overnight outage, most homes step up to two batteries, and larger all-electric homes sometimes three. Air conditioning and electric heat are the loads that push you from one battery to two.
Does solar change how big a battery I need?
Yes. A standalone battery has to carry the whole outage on stored charge, but with rooftop solar the panels recharge the battery during daylight. With a properly designed solar-plus-storage system, enough sunshine and controlled loads, daytime solar may extend backup beyond one night — but it’s not guaranteed multi-day backup. Actual duration depends on your PV size, weather, season, shading, whether the system can charge in island mode, and the loads running during the outage.
Does SGIP still help pay for a battery in 2026?
Sometimes. California’s SGIP rebate can offset part of a battery’s cost, with higher-value paths for medically vulnerable and high fire-threat customers. But SGIP availability changes by program category and administrator, so confirm the live budget status and your reservation eligibility before you include any rebate in the project economics.
How much does a home battery cost?
A single home battery typically adds about $10,000–$18,000 installed before incentives, depending on brand and size. A second unit usually costs less per battery because the install labor and electrical work are largely shared. These are illustrative planning figures — a site visit gives a real, itemized price.
Related reading
Get a home battery sized right for your house
Tell us what you want to keep running during an outage and we'll size the battery and any panel work - a real, itemized number, no pressure.
Get a free estimatePrepared by Cali Energy, August 2026. This article is for general educational purposes only and is not legal, tax, financial, engineering, or utility advice. Rates, incentives, codes, permit requirements, equipment specifications, prices, and program terms may change; figures and timelines are estimates, not guarantees. Confirm current requirements with the applicable utility, AHJ, program administrator, manufacturer, or a licensed professional. See our Content Disclaimer. Cali Energy · 19201 Parthenia St Unit E, Northridge, CA 91324 · CSLB #1032379 (B, C-10, C-39)