M&E Contrax Resources

Solar Battery Sizing Guide

Resource Highlights:

The best battery size depends on your evening and overnight electricity use, not just annual usage.

A well sized battery should regularly charge and discharge, rather than sitting underused.

Battery sizing should consider solar system size, seasonal generation, and future plans like EV charging.

How to choose the right solar battery capacity for your home or business

Battery storage can significantly increase the amount of solar electricity you use yourself, but only if the battery is sized correctly. Too small and it empties quickly, too large and it may rarely charge or discharge fully, which can reduce value.

This guide explains how to size a solar battery properly, what data you need, and the practical questions that determine the best capacity for your property.

Start with how you use electricity

Battery sizing is mostly about when you use power.

A battery is most valuable when it can cover a meaningful portion of your non daylight usage, typically:

late afternoon and evening

early morning

overnight base load

What to gather

your annual electricity usage (kWh)

a sense of your typical evening usage

any high load items you run after dark, cooking, heating, EV charging, hot tubs, workshop equipment

If you have smart meter data, that is ideal. For businesses, half hourly data is even better.

Understand what the battery is trying to do

A battery has two jobs:

store surplus solar generated in the day

discharge later to reduce imports from the grid

This means there are two limits that matter:

how much surplus you have to charge it

how much evening demand you have to use it

A battery needs both to deliver value. If you do not generate surplus, it cannot charge. If you do not use much power in the evening, it will not discharge.

Check your solar system size and expected surplus

Battery size should make sense alongside your solar PV system.

If your solar system is small and you use most of the electricity during the day, you may have limited surplus to store. In that case, a smaller battery is often the most sensible starting point.

If your solar system is larger and you regularly export energy during the day, you may have plenty of surplus available, which can justify a larger battery, provided you will use it later.

A simple way to think about sizing

A practical battery size is usually linked to how much electricity you want to cover after solar generation drops.

A useful way to approach it is:

estimate how much electricity you use from late afternoon to bedtime, plus overnight base load

choose a battery that can cover a good portion of that most days, without being so large that it rarely cycles

The goal is consistent use, because regular cycling is what turns storage into savings.

Consider battery power as well as battery capacity

People often focus only on battery capacity, but power matters too.

Capacity (kWh)

How much energy the battery can store.

Power (kW)

How much energy it can deliver at once.

If you have high demand items running together, such as cooking appliances, showers, EV charging, workshop tools, the battery power rating influences how much of that load it can support at any one time.

This is why battery sizing should consider:

how much energy you want to store

what loads you want to run from the battery

whether the battery is designed to support those loads

Seasonal behaviour

In summer, solar generation is higher and batteries charge more easily. In winter, generation is lower and days are shorter, so charging can be limited.

A good battery sizing decision considers:

how the battery performs in winter when generation is reduced

whether the goal is summer self consumption or year round reduction in imports

whether you still need grid imports in winter, which is normal for most systems

It is best to have realistic expectations. Even with a battery, most properties will still import some electricity in winter.

Batteries and EV charging

EV charging is a common reason people consider a larger battery, but it needs careful thought.

A typical EV charge can be much larger than a home battery capacity. This means batteries often help most with:

covering household usage outside solar hours

topping up small EV charges

smart charging strategies that align charging to solar generation

If you plan to add an EV, tell your installer early. The solar system size, inverter type, and smart controls can be designed to work together.

Find out more about battery storage here

Should you install a battery now or later?

Many customers choose to install solar panels first and add a battery later, once they have monitoring data showing:

how much they export

how much evening electricity they use

whether a battery would cycle consistently

If you already know your usage is evening heavy and you expect regular surplus, installing both together can make sense, especially if you want a battery ready inverter setup.

Simple way to choose the right battery size

If you want the simplest approach, use this process:

Identify your typical evening and overnight usage

Confirm how much surplus solar you are likely to have in an average day

Choose a battery capacity that will regularly charge and discharge

Make sure the battery power rating suits the loads you want it to support

Avoid oversizing based on best case summer performance

A good installer should be able to explain why they have recommended that specific size, based on your real usage.

Frequently Asked Questions

There is no one best size. It depends on evening usage, solar system size, and how much surplus you generate. A survey and usage review provides the most accurate answer.

Often yes, but it depends on system design, inverter compatibility, and available space. Planning ahead makes future expansion easier.

Usually yes, because more surplus is stored and used on site. This can increase self consumption and reduce grid imports.

If your electricity use is mostly evenings and your solar system generates surplus during the day, a battery can still be a strong option because it shifts that surplus into your peak usage hours.