How big should a home battery be? A sizing guide

A home battery is the right size when it can store the solar surplus you produce on a typical day and release it during the hours when your panels produce nothing. In practice that means looking at two numbers and taking the smaller: how much surplus your solar system exports on an average sunny day, and how much electricity you use between sunset and sunrise. A battery larger than both stays partly empty for most of the year.

This guide is about fixed home batteries connected by an installer to a rooftop solar system. For small batteries next to a plug-in kit, see plug-in solar with a battery.

Step 1: gather your data

What you need	Where to find it
Annual electricity use (kWh)	your bills or supplier account
Use in the evening and at night	smart meter data in half-hourly or hourly intervals, if available
Solar system size (kWp)	installer quote or existing system data
Expected solar yield	installer quote, or the European Commission’s PVGIS tool
Daily surplus on sunny days	inverter app or export readings from your meter
Future changes	heat pump, electric car, home working

If you have a smart meter, ask your supplier for interval data. It shows exactly how much you use in the evening and overnight, which is the energy a battery can replace.

Step 2: apply the two limits

Limit 1, the night-time demand. A battery cannot save more per day than you consume while the panels are idle. If your home uses 4 kWh between sunset and sunrise, a 10 kWh battery will rarely be emptied.

Limit 2, the daily surplus. A battery can only store what the panels produce beyond your daytime consumption. If you export 3 kWh on a typical spring day, a 10 kWh battery will rarely be full.

The useful capacity lies around the smaller of the two figures. Add a margin for usable capacity, which is often a little below the nominal capacity, and for battery ageing over the years.

A rule of thumb, used with care

A widely quoted rule of thumb from German solar research says that, for a typical household, battery capacity in kWh should be roughly equal to the solar system size in kWp and to annual consumption in MWh, for example 5 kWh of storage for a 5 kWp system and 5,000 kWh of annual use. It is a starting point for the conversation, not a sizing method. Households with high evening use, a heat pump or an electric car can deviate a lot.

Step 3: check more than capacity

Feature	Why it matters
Continuous output (kW)	decides whether the battery can cover a cooker or heat pump, or only base load
Usable capacity	the share of nominal capacity you can actually use
Round-trip efficiency	losses when storing and releasing energy
Backup or off-grid function	needed if the battery should supply power in a blackout
Grid charging and tariff control	needed for dynamic or time-of-use tariffs
Expandability	lets you start smaller and add modules later
Warranty	years, cycles and guaranteed remaining capacity

If you are interested in tariffs that reward flexible charging, see dynamic electricity tariffs. Some households also connect their battery to an aggregator that uses many small batteries as a virtual power plant.

Backup power changes the sizing

If you want the battery to keep essentials running during a power cut, sizing works differently: you add up the devices you need and the hours they must run. A battery sized for daily savings may be too small for long outages, and backup requires a system with a switch that separates your home from the grid. See backup power for a blackout.

Installation is not a DIY job

A fixed home battery connects to the home’s electrical installation and, depending on the country, must be notified to or approved by the network operator. Placement, fire safety, ventilation and cable sizing are part of the installer’s responsibility. Use this guide to prepare the questions you ask, and ask each installer to explain how they arrived at the size they recommend.

Questions to ask an installer

1. How did you calculate the capacity, and with which consumption data?
2. What share of the year will the battery be full, and what share will it be empty?
3. What is the usable capacity and the continuous output?
4. Does the system support backup, and which circuits would it supply?
5. Can it charge from the grid at low prices, and is that worthwhile on my tariff?
6. What does the warranty cover after ten years?