Skills books › Solar Backup Power for Your Home

What size backup power system do I need?

Before you ask for a quote on a power station, an inverter or a solar system, work out two numbers: how much energy you need for an outage, and how much power you need at one moment. Fifteen minutes with your appliance labels and a calculator is enough.

Step 1: Make a load list

List every appliance you want to run during an outage. For each one, write down:

1. the power on its label, in watts, and how many of them you have;
2. the hours it will run during the outage;
3. the energy: watts × number × hours = watt-hours (Wh).

Be honest about the hours. A TV that is on for three hours of a four-hour outage counts three hours.

Step 2: Keep the heaters off backup

Size the system for your essentials, not the whole house.

• Essential: lights, the fridge and freezer, Wi-Fi, phone and laptop charging, the TV and your security system.
• Nice to have: a second TV, a microwave for a few minutes, a fan.
• Keep off backup: anything that heats. Geysers, stoves, kettles, irons, hairdryers, heaters and air conditioners each use one to three kilowatts or more. They would empty a home battery in an hour or two.

Step 3: Add up the energy

Add the watt-hours of all the loads. Two refinements make the total more accurate:

• For a fridge or freezer, use the energy per day on its label. A fridge labelled 1.0 kWh per 24 hours uses about 170 Wh in four hours, because its motor is off much of the time.
• Routers, alarms and gate motors run all the time, so count every hour.

Then add about 15% for losses in the inverter and cables. Multiply your total by 1.15.

Step 4: Check the peak power

Appliances with motors draw a short surge when they start, often three to five times their running power. A fridge that runs at 150 to 200 W may surge to 600 to 900 W. If the backup cannot supply the surge, the fridge will not start, or the inverter will trip.

Your peak power is all the running loads together, with the motor's starting surge in place of its running power.

A worked example

Lindiwe wants to get through a four-hour evening outage with a fridge-freezer, three LED lights, a TV, a laptop and two phones.

• Her load list adds up to 683 Wh.
• With 15% for losses, she needs about 785 Wh from the battery.
• With everything running, she draws 379 W.
• When the fridge starts, the peak is about 870 W.

So she needs a backup with about 0.8 kWh of usable energy that can handle about 900 W for an instant.

Step 5: Decide how many hours

Four hours covers most load-shedding blocks. Think also about how often outages come. If two outages can fall in one day, the battery must recharge in the gap, or it starts the next one half empty. In that case, size the battery for both outages, or add solar panels to recharge it during the day.

This article is based on Chapter 3 of Solar Backup Power for Your Home, which goes on to cover batteries, inverters, solar panels, installers and the law.