How much solar power does my home need? A step-by-step sizing guide

Most people start by asking how many panels they need. That is the wrong first question. A home solar system is sized from the appliances you want to keep running, and how long you run them. Get that list right and the inverter, batteries and panels follow from it.

This guide walks through the method we use, so you can sanity-check any quotation you receive, including ours. If you would rather get a quick first answer, our solar calculator runs the same steps from your appliance list.

Step 1: write down what you want to keep on

Go room by room and list every appliance you want solar to run. For each one, note:

• Its power in watts (W). This is printed on a label on the back or underneath, or in the manual. If the label shows amps, multiply amps by 230 to get an approximate wattage.
• How many hours a day it runs, and how many of those hours are after dark or during outages.
• Whether it has a motor or compressor. Fridges, freezers, pumps and air conditioners draw several times their running power for a moment when they start.

Be honest about hours. A fridge is plugged in all day, but its compressor cycles on and off, so it does not draw full power for 24 hours.

Step 2: work out the most you will run at once

Add up the watts of everything that could be on at the same time. This is your peak load, and it sets the size of the inverter.

Allow headroom on top of this for motors starting and for the appliances you will inevitably add later. Inverters are usually rated in kVA. Our inverters guide explains how kVA relates to watts, and why the two numbers are not the same.

Step 3: work out the energy you use

Energy is power multiplied by time, measured in watt-hours (Wh) or kilowatt-hours (kWh). For each appliance, multiply its watts by its daily hours of use. Add the results together to get your daily energy use.

Then split that total into two parts:

• Daytime energy, which the panels can supply directly while the sun is up.
• Night-time and outage energy, which must come from the batteries.

Step 4: size the batteries

Battery capacity must cover your night-time and outage energy, with a margin. Two things matter:

• Usable capacity. You cannot safely drain most batteries to empty. Lithium batteries let you use far more of their rated capacity than tubular lead-acid batteries, so a smaller lithium bank can deliver the same usable energy. Our battery guide compares the two.
• Days of backup. Decide whether you need to get through one night, or longer stretches of grid outage and cloudy weather. More backup means more batteries and more cost.

Step 5: size the solar panels

The panels have to do two jobs on an average day: run your daytime loads and recharge the batteries. Panel output depends on how much sun your location gets, which way the roof faces, shading, heat and dust.

Your installer should base the panel count on these site conditions, not on a fixed rule of thumb. Ask them to show the assumptions they used for sun hours and system losses.

A worked example

The figures below are assumptions for illustration only, not a quotation. Your own appliances and hours will be different.

In this example the peak load, if everything ran at once, is about 725 W before allowing for the fridge starting. That points to an inverter with comfortable headroom above that figure. If roughly half the energy is used after dark, the batteries need to deliver about 2.6 kWh of usable energy each night, plus a margin for outages.

Now add one 1.5 horsepower air conditioner running for six hours a night. Depending on the model, that could add several kilowatt-hours of night-time energy and a large starting load, which can more than double the battery bank and push the inverter up a size. This is why air conditioners deserve a separate conversation with your installer.

Loads to think twice about

Some appliances turn heat into electricity bills. They are possible on solar, but they make the system much bigger:

• Electric cookers and hot plates
• Instant and storage water heaters
• Pressing irons
• Kettles and microwaves used for long periods
• Large air conditioners, especially older non-inverter models

Many households keep these on the grid or generator, or use them only in full sun when the panels have power to spare. If you do want to run them on solar, say so at the start, so the system is sized properly.

Common sizing mistakes

• Buying a package before listing loads. A “5 kVA system” means little unless you know what it will actually run.
• Ignoring starting loads. Pumps and compressors can trip an inverter that looks big enough on paper.
• Forgetting usable capacity. Two batteries with the same rating can deliver very different usable energy.
• Too few panels. Batteries that never fully recharge wear out early.
• No room to grow. If you plan to add appliances, ask for an inverter and mounting layout that can take more panels and batteries later.

Checklist before you request a quotation

• A room-by-room appliance list with watts and hours
• Which appliances must stay on during outages
• Recent electricity bills or meter top-up records
• What you spend on generator fuel each month
• Photos of your roof and your distribution board
• Any plans to add appliances, such as an air conditioner or borehole pump

If you would rather have us do the sums, our home solar service starts with exactly this load assessment, and every quotation shows the assumptions behind it.