How to Size a Home Battery Backup System: A Load Math Guide

TL;DR: Most homes need 20-40 kWh of battery capacity for whole-home backup during a multi-day outage, while a 10-13.5 kWh unit (like one Tesla Powerwall) covers only critical loads such as a fridge, some lights, and internet for roughly 12-24 hours. Size your system by adding up actual appliance wattage, not by picking whatever capacity the installer quotes first.

_Last reviewed: July 2026 · 7 min read_

You've priced out a battery system and the numbers all over the internet don't agree, one site says 10 kWh is plenty, another says you need 40. The honest answer is that it depends entirely on what you plan to run and for how long, and that math is doable in about fifteen minutes with your last electric bill and a notepad.

Okoniq Property Hub logs your home's appliance inventory and panel details in one place, so when you're comparing battery quotes you already have the load numbers instead of guessing.

How much power does your home actually use during an outage?

Start by listing what you actually need running, not everything the house owns. A refrigerator draws roughly 150-400 watts running but spikes to 800-1,200 watts on startup. A window AC unit pulls 500-1,500 watts. LED lighting for a whole house is often under 200 watts total. A well pump can spike to 2,000-3,000 watts. Add these up and you get your "critical load" wattage, usually 1,500-3,000 watts for a modest household running fridge, lights, phone chargers, and a router.

Whole-home backup is a different exercise. Central AC alone can draw 3,000-5,000 watts, an electric range 3,000-8,000 watts, and an electric water heater 3,000-4,500 watts. Try to run all of that at once off a battery and you'll trip the system's output limit before you drain the capacity. This is why installers ask about your panel and largest appliances before quoting a size, similar to how 100 vs 200 amp service determines what your panel can even deliver in the first place.

How many kilowatt-hours of battery capacity do you need?

Multiply your critical load wattage by the hours you want to run it, then convert to kilowatt-hours. A 1,500-watt critical load running 16 hours a day needs 24 kWh of stored energy for one day. Most manufacturers rate usable capacity slightly below the sticker number, so a "13.5 kWh" battery might deliver closer to 12.5 kWh usable.

A single battery unit (10-13.5 kWh) typically covers 12-24 hours of critical loads only: fridge, some lighting, phone charging, a router, maybe a sump pump. Two units (20-27 kWh) stretch that to a full day of critical loads or a half-day of near-whole-home use. Three or more units (30-40+ kWh), often paired with solar panels to recharge daily, are what's needed for multi-day whole-home backup during a week-long outage. Installed cost runs roughly $9,000-$16,000 per battery unit as of 2024-2025, before any solar panels or tax credits.

Should you back up your whole house or just critical circuits?

Most owner-operators are better served backing up critical circuits only, not the whole panel. A critical-loads subpanel isolates the fridge, a few lights, the router, and maybe a sump pump or well pump onto a separate panel that the battery powers, while everything else (central AC, electric dryer, oven) stays off the battery during an outage. This cuts your required capacity by half or more and lowers installed cost significantly, since you're sizing for 1,500-3,000 watts instead of 8,000+.

| Setup | Typical Capacity Needed | Runs During Outage | |---|---|---| | Critical loads only | 10-20 kWh | Fridge, lights, router, sump pump | | Whole-home backup | 30-40+ kWh | All of the above plus AC, oven, dryer |

Whole-home backup makes more sense in regions with frequent, long outages, or for households with medical equipment that can't tolerate any interruption. Otherwise, a critical-loads panel gets you 80% of the peace of mind for a fraction of the battery capacity and cost.

Does your home's electrical panel support a battery backup system?

Your panel's amperage and condition matter as much as the battery you buy. Battery inverters connect through your main panel or a subpanel, and older 100-amp services can be tight once you add EV charging or a battery inverter's own load requirements on top of existing circuits. If your panel is original to a home built before the 1990s, get it inspected before signing a battery contract, and check for outdated wiring types while you're at it, since knob-and-tube wiring or ungrounded outlets can complicate or block a battery install entirely. An electrician can confirm in about an hour whether your panel needs an upgrade first, and issues like 2-prong outlets that still need grounding upgrades are worth flagging in the same visit since they often get discovered together.

Battery backup vs. generator: which one actually lasts longer?

A battery runs silently and instantly when the grid drops, but it eventually depletes unless paired with solar or a generator to recharge it. A gas or propane generator can run indefinitely as long as fuel supply holds, typically 8-20 hours per tank depending on load and generator size, but it needs manual starting (unless it's a standby unit), makes noise, and requires fuel storage. Many owner-operators in areas with outages longer than 2-3 days pair a smaller battery (for the quiet overnight hours) with a generator (for sustained daytime draws), rather than choosing one system to do everything.

FAQ

How many kWh does a house need per day?

An average US home uses about 29-30 kWh per day total, but during an outage most households only need 3-8 kWh per day if they limit usage to critical circuits like the fridge, lights, and router.

Is a 10 kWh battery enough for a house?

A 10 kWh battery covers critical loads only, such as a fridge, some lighting, and a router, for roughly 12-24 hours; it will not run central AC, an electric oven, or a dryer for any meaningful stretch.

How much does a whole-home battery backup system cost installed?

Expect $9,000-$16,000 per battery unit installed as of 2024-2025, so a whole-home setup needing 3-4 units can run $30,000-$60,000 before incentives, while a critical-loads-only setup with one or two units often lands between $10,000-$25,000.

Can I add battery backup to an existing 100-amp panel?

Sometimes, but many installers recommend upgrading to 200-amp service first if you're also adding solar, an EV charger, or plan whole-home backup, since the combined load can exceed what a 100-amp panel safely handles.

Do I need solar panels to make a battery backup system worthwhile?

No, a battery alone can store grid power charged overnight at off-peak rates and discharge it during an outage or peak-price hours, though pairing it with solar lets the battery recharge daily during a multi-day outage instead of running down once.

This is educational information, not electrical or financial advice. Consult a licensed electrician for a panel and load assessment, and get quotes from at least two installers before committing to a battery size.