How to Estimate Solar Panel Output

What you need

Your installer's quote (it states the system size), the peak sun hours for your location (from a solar map or your installer), and your electricity bill (to compare the output with what you use). The solar panel calculator turns those three numbers into daily, monthly and yearly output.

Step 1 — System size

The rated power of the panels, in kilowatts (kW). A 5 kW system is 5,000 W of panels — for example, 12 panels at 420 W each. The size is on the quote from your installer.

Step 2 — Peak sun hours

Peak sun hours are not daylight hours — they are the hours of full sun your location gets on an average day. A place with 4.5 peak sun hours gets the energy of 4.5 hours at full panel power. Find yours from a solar map or an installer; do not guess it.

Step 3 — Efficiency

Panels lose about 20% of their energy to the inverter, heat, dirt and wiring. An 80% efficiency figure is a common planning assumption. Enter your own if your installer gives you one.

Step 4 — Multiply

Daily output = system size × peak sun hours × efficiency. A 5 kW system at 4.5 sun hours and 80% efficiency gives 5 × 4.5 × 0.80 = 18 kWh per day, 540 kWh per month and 6,570 kWh per year. The solar panel calculator does this for any system.

Step 5 — What about storage?

Solar makes power in the day; most homes use it in the evening. A battery stores the daytime surplus — size it with the battery backup calculator from the load and the hours you need.

Worked example

A 5 kW system, 4.5 peak sun hours and 80% efficiency: daily output = 5 × 4.5 × 0.80 = 18 kWh. Over a 30-day month that is 18 × 30 = 540 kWh, and over a year it is 18 × 365 = 6,570 kWh. That is exactly what the solar panel calculator gives you.

Common mistakes

• Using daylight hours. Daylight is not peak sun. Using 12 hours instead of 4.5 triples the estimate — and the disappointment.
• Forgetting losses. 100% efficiency does not exist. Always allow for the inverter and heat.
• Ignoring shading. A shaded panel produces far less. Get a professional survey before buying.

Safety

Solar panels are fitted on roofs and wired into live direct current. Do not install them yourself: roof work is a fall risk, and DC wiring can arc and cause fires if it is done wrong. Only a certified installer or electrician should fit the panels, the inverter and the grid connection. If a panel needs cleaning, rinse it from the ground with a long-handled tool — never climb onto the roof to clean it.

When to hire a professional

Always. Solar installation is not a DIY job: it needs a certified installer for the panels and the electrical work, a shading survey before you buy, and approval for the grid connection. Get two or three installer quotes and compare the system size, the panel and inverter brands, and what is included — not just the price.

Frequently asked questions

• Do solar panels work on cloudy days? Yes, but at reduced output. Panels produce power from daylight, not just direct sun, so a cloudy day still generates electricity — just less than a clear one.
• Will solar power my home at night? Not on its own. Panels only produce in daylight, so night-time power needs a battery or the grid. Our guide to sizing a home battery shows how to work out the battery for your evening load.
• How do I know what size system I need? Start from your annual electricity use — it is on your bill. Our guide to reading your electricity bill shows where to find it, and your installer can size the system from that number.
• Do panels need cleaning? Rain usually does the job. In dusty areas, rinse them from the ground a few times a year. Never walk on the panels or climb onto the roof to clean them.

Try these calculators

• Solar Panel Calculator — daily, monthly and yearly output.
• Battery Backup Calculator — the battery for your load and hours.
• Electricity Cost Calculator — what the solar saves you.