Guides

How to size a home battery from the load profile

Published 8 October 2026. About 7 minutes to read.

A battery that is too small runs empty every evening. One that is too large never fills in winter and takes years longer to pay for itself. The right size starts with when the household uses electricity, not how much it uses in total.

Start with the night-time load

For a battery that stores daytime solar for later, the useful question is how much electricity the household uses between the time solar production drops off in the late afternoon and when it picks up the next morning. For many homes that window runs from roughly 17:00 to 07:00, and it contains the evening peak of cooking, lights, television and often water heating.

A smart-meter export or a load profile for a typical day gives this directly. Without one, a rough split is to assume a home with an evening peak uses 50 to 60 % of its daily electricity outside the solar hours.

From usable energy to battery size

A battery's nominal capacity is not all usable. Lithium iron phosphate batteries are typically set to use 80 to 95 % of their capacity (the depth of discharge), and some energy is lost on the way in and out (the round-trip efficiency, often around 90 to 95 %).

nominal capacity ≈ night-time load ÷ depth of discharge ÷ round-trip efficiency

A home using 8 kWh between 17:00 and 07:00, with a 90 % depth of discharge and 92 % round-trip efficiency, needs about 8 ÷ 0.9 ÷ 0.92 = 9.7 kWh of nominal capacity. A 10 kWh battery is the natural fit.

Can the solar array fill it?

A battery only helps if there is surplus solar to charge it. On a winter day the array may produce barely more than the daytime load, leaving little to store. Compare the surplus on typical summer and winter days with the battery size. A large battery paired with a small array spends most of the winter partly empty.

Power, not only energy

The inverter and the battery both limit how fast energy can move. A 51.2 V battery with a 100 A discharge limit can deliver about 5.1 kW. If the evening peak is a 3 kW kettle, a 2 kW stove plate and a 1.5 kW heater at the same time, the rest has to come from the grid however large the battery is. Check the battery's charge and discharge current against the inverter's battery port limits, and check the battery's voltage range against the inverter's.

Backup is a different question

Sizing for power cuts is a separate exercise. You decide which circuits must run, for how long, and at what power, and size for that. A battery sized for daily self-consumption may be too small for long outages, and one sized for long outages is often oversized for everyday use. Be clear with the client which one the design is for.

Sizing a battery in Lekker Sun

On the Finance page, enter monthly consumption and pick a daily usage pattern, or import meter data. Lekker Sun simulates the battery half hour by half hour, charging from solar surplus and discharging into the load within the battery and inverter limits. It shows typical summer and winter weeks with the battery's charge level, and how self-sufficiency and payback change. Try two or three battery sizes and compare.

This guide is general information to help you plan and compare options. It is not engineering, electrical or financial advice. Always follow the equipment manufacturer's instructions and your local wiring rules, and have a qualified installer check the design.