How to use this solar output calculator
- System size, enter your array size in kilowatts (kW). Or tick the box to enter the number of panels and the panel wattage instead, and the size is worked out for you.
- Peak sun hours, type your local daily average. Most places sit between 4 and 6.
- System losses, leave this near 20% unless you have a specific figure for your install.
- Read your daily, monthly and yearly production. They update as you type.
- Scroll to the monthly breakdown table for a seasonal, month-by-month split of that same yearly total.
How solar production is calculated
Solar output starts from a simple formula: daily kWh = system size (kW) × peak sun hours × performance ratio. When you enter panels instead of a kW figure, the system size is just panels × panel wattage ÷ 1000. So fifteen 400 W panels make a 6 kW array.
Peak sun hours are not the same as daylight hours. They count the hours where sunlight averages 1,000 watts per square metre, the condition panels are rated against. Most regions average 4 to 6 peak sun hours a day across the year, with sunnier climates near the top of that band and cloudier ones near the bottom.
The performance ratio is where reality meets the spec sheet. Panels rarely deliver their full rated power, so we apply a ratio of about 0.8 (roughly 20% losses) to cover heat, wiring resistance, inverter conversion, dust and minor shading. That ratio is just 1 minus your losses percentage, which is why a higher losses figure lowers your estimated output. Monthly production is the daily figure times 30.4 (the average days in a month) and yearly is the daily figure times 365.
The monthly breakdown table takes that same yearly total and spreads it unevenly across the twelve calendar months instead of dividing by twelve. It applies a fixed set of seasonal weights built for a typical temperate, northern hemisphere location, roughly 0.71 times the flat monthly average in December and January, rising to about 1.29 times the average in June and July. Those twelve weights always add up to 12, so the twelve monthly estimates always add back up to the same yearly total shown above. This is a general seasonal shape, not weather data for your specific address. Actual month-to-month variation depends on your latitude and local cloud cover, and southern hemisphere locations see the same shape shifted by six months.
Treat the result as an estimate
This calculator gives a realistic planning figure, not a guaranteed yield. Actual output shifts with roof angle and orientation, seasonal sun, temperature, shading from trees or chimneys, snow cover and the slow degradation of panels over their lifetime. A south-facing, unshaded array will beat one that faces east and catches afternoon shade, even at the same kW rating. Use this number to compare system sizes and set expectations, then confirm with a local installer who can model your exact roof.
Frequently asked questions
How do I calculate solar panel output?
Multiply system size in kW by peak sun hours per day, then by a performance ratio. A 6 kW system at 5 peak sun hours and 20% losses makes about 6 × 5 × 0.8 = 24 kWh per day.
What are peak sun hours?
The hours per day when sunlight averages 1,000 W/m². Most regions see 4 to 6, so use a number near 6 in a sunny climate and nearer 4 in a cloudier one.
Why is the performance ratio about 0.8?
Around 15 to 25% of rated power is lost to heat, wiring, inverter conversion, dust and shading. A ratio of roughly 0.8 reflects a typical well-installed system.
How much does a 6 kW system make per year?
At 5 peak sun hours and 20% losses, about 24 kWh a day, roughly 730 kWh a month and around 8,760 kWh a year, depending on your local conditions.
Why is winter production lower than summer in the monthly breakdown?
Peak sun hours are not the same every month. The calculator spreads your yearly total across the twelve months using a general seasonal curve for a typical temperate, northern hemisphere location, low around December and January and highest around June and July, instead of dividing the year evenly. It is a general shape, not weather data for your specific address.