# How to size a home battery without over-buying capacity you'll never use

> Installers often quote by panel count, not by your actual overnight load. Here's the back-of-envelope method.

- URL: https://solarexaminer.com/posts/battery-sizing-guide/
- Site: Solar Examiner (https://solarexaminer.com/)
- Category: Equipment
- Published: 2026-09-06
- Author: Marcus Feld


The fastest way to overspend on a battery is to size it to your total daily usage instead of your overnight critical-load usage, which is usually a third of the total.

## The method

List what you actually want powered during an outage: refrigerator, a few lights, internet, and phone charging is typical. Add up the wattage, multiply by the hours of outage you're planning for, and that's your target capacity in watt-hours — not your whole home's daily draw.

## A worked example

A refrigerator (150W average), router (15W), and five LED lights (50W total) run about 215W continuous. For an 8-hour overnight outage, that's roughly 1.7 kWh, well under a single standard battery module's capacity in most residential systems.

## When to size bigger

If you have a well pump, medical equipment, or live somewhere with multi-day outages, size to that specific load rather than the general critical-load estimate above.

## Related

- [kW vs kWh: what each means on your bill and solar quote](/posts/kw-vs-kwh/), for the watt versus watt-hour math used above.
- [How many solar panels do I need?](/posts/how-many-solar-panels-do-i-need/), for sizing the panels that charge the battery.
- [Solar payback period without the federal tax credit](/posts/solar-payback-period-without-federal-tax-credit/), before you add a battery to a quote.

Image credit: Photo by Vanya Smythe on [Unsplash](https://unsplash.com/photos/rows-of-batteries-with-red-and-blue-terminals-CH7kRmyBQ4I?utm_source=solarexaminer&utm_medium=referral)