How to Calculate Your Home or Workshop Power Needs

Every solar system starts with one number: how much energy you use in a day. Get that right and sizing the panels, battery and inverter becomes simple. Guess it and you'll either run flat by dinner time or pay for gear you don't need.

1. Watts, hours and watt-hours

• Watts (W): how hard something draws power while it's running. It's on the rating plate or in the manual.
• Hours (h): how long it runs each day.
• Watt-hours (Wh): the energy it uses. Watts × hours = watt-hours. 1,000Wh is 1kWh, the same unit on your power bill.

A 40W light running for 4 hours uses 40 × 4 = 160Wh.

2. List everything you run

Walk through the house, shed or workshop and write down each item that will run off the system, with its watts and the hours it runs on a normal day. Include the small things: phone and tool battery chargers, radios, routers and lights all add up.

• Rating plates show the maximum. Many items use less than their plate rating most of the time. If you can, measure with a plug-in energy meter for a day or two.
• Fridges and freezers cycle on and off. Don't use the plate watts × 24 hours. Use the energy rating label instead: divide the kWh per year by 365. A fridge rated 300kWh per year uses about 820Wh a day.
• Power tools run in short bursts. A drop saw might draw 1,800W but only cut for 15 minutes in a day, so count 0.25 hours.

3. Add it up

Multiply each item's watts by its hours, then add the results. Here's an example for a small workshop:

• LED lights: 40W × 4h = 160Wh
• Bar fridge: 300kWh per year ÷ 365 = about 820Wh
• Laptop: 60W × 3h = 180Wh
• Drop saw: 1,800W × 0.25h = 450Wh
• Tool battery chargers: 100W × 2h = 200Wh
• Radio: 10W × 6h = 60Wh
• Total: about 1,870Wh a day

4. Add a margin for losses

No system is 100 percent efficient. The inverter, battery charging and cables all lose some energy. Add about 20 percent to cover it.

In the example: 1,870Wh × 1.2 = about 2,250Wh, or 2.25kWh a day. That is the daily energy the system has to supply.

5. Work out your peak load

Daily energy sizes the battery and panels. The inverter is sized by what runs at the same time. Add up the watts of everything that could be on together.

In the example, the drop saw, lights, fridge and radio could all run at once: 1,800 + 40 + 100 + 10 = 1,950W. A 2,000W inverter would be at its limit, so a 2,500 to 3,000W unit leaves headroom. Motors also draw a surge when they start, often two to three times their running watts. Running Power Tools Off Solar explains how to allow for it.

6. What to do with your numbers

• Battery: multiply your daily energy by the number of days you want to run without sun, then allow for how much of the battery you can actually use. Check the manufacturer's specification for usable capacity.
• Panels: divide your daily energy by the peak sun hours for your area in winter, then add a margin for losses. For 2.25kWh a day with 3 winter sun hours, that's 750W before losses, so around 1,000W of panels is a sensible starting point.
• System voltage: bigger loads suit a higher voltage. See 12V, 24V or 48V?
• Inverter type: see Pure Sine vs Modified Sine Inverters.

These are planning estimates. Real results depend on your location, weather, shading and the gear you choose.

Safety: in Australia, any work on 240V AC wiring, including connecting an inverter, battery or solar system to a home's switchboard or fixed wiring, must be done by a licensed electrician. Grid-connected systems also need approval from your electricity network.

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