Solar Output Calculator

How much electricity will my solar panels produce?

Solar output is how much energy your array actually generates — and it's reliably lower than the simple "panel watts × hours of daylight" that people first reach for. The gap comes from two things: sunlight strength varies through the day and year, and real systems lose a portion of what they capture to conversion and other losses. Estimating output honestly means using peak sun-hours rather than daylight hours, and applying a performance ratio. Do that and your projection will match reality, which matters because every savings and payback figure is built on it.

Output is the number that quietly determines whether a solar investment lives up to its promise. Two homes can install the same hardware and see very different results, because output depends on sunlight and real-world losses that have nothing to do with the panels on the box. That’s also why output is the figure most often quoted too generously — a salesperson with an optimistic sun-hours assumption or no allowance for losses can make any system look better than it will perform. Learning to estimate output honestly, with peak sun-hours and a realistic performance ratio, lets you check any projection against physics rather than marketing, and sets the foundation for trustworthy savings and payback figures.

This is one of the most important numbers to get right, because an optimistic output estimate quietly inflates every downstream calculation — savings, payback and return all depend on it.

The output formula

For a 5 kW system at 4.5 peak sun-hours and a 0.8 performance ratio: 5 × 4.5 × 365 × 0.8 ≈ 6,570 kWh a year. Note what each term does — capacity sets the ceiling, sun-hours set how much of that ceiling you reach, and the performance ratio trims it to a realistic figure.

Peak sun-hours, not daylight hours

This is the single biggest source of error in DIY estimates. A "peak sun-hour" is an hour of full-strength sunlight (1,000 W/m²). A location with 12 hours of daylight might deliver only 4–5 peak sun-hours, because morning and evening sun is weak and oblique. Using daylight hours instead of peak sun-hours can overstate output by two to three times. Peak sun-hour figures already fold in your latitude, climate and the seasonal average, which is why they're the right basis for an annual estimate.

What the performance ratio covers

• Inverter conversion — DC to AC costs a few percent.
• Temperature — panels lose efficiency when hot; a roof in summer runs well above the 25°C test condition.
• Wiring and connections — small resistive losses throughout.
• Soiling and shading — dust, leaves, snow and even partial shade reduce output, sometimes sharply.
• Degradation — panels lose roughly 0.5% of output per year, so year-25 production is ~88% of year-one.

Together these typically total 15–25%, which is why a performance ratio of about 0.8 is the standard realistic assumption.

Seasonal and daily variation

The annual figure averages large swings. Summer days can produce two to three times a winter day's output, and a clear day far outproduces an overcast one. For a grid-tied home with net metering, the annual total is what matters because summer surplus offsets winter shortfall. For off-grid systems, the worst month matters more than the average, since there's no grid to fill the winter gap — which is why off-grid arrays are sized to winter, not the yearly mean.

Turning output into savings

Output is only half the value story; what each kilowatt-hour is worth to you is the other half. A unit you consume yourself saves your full retail rate, while a unit you export is increasingly paid far less under modern tariffs. So two systems with identical output can deliver very different savings depending on how much of that output is used on-site versus exported. When you move from an output estimate to a savings estimate, split your generation into self-consumed and exported shares and value each correctly — this is the step optimistic projections skip, and it’s why our savings tools treat self-consumption and export rate as separate inputs rather than valuing everything at the retail rate.

Frequently asked questions

Roughly its wattage × peak sun-hours × performance ratio. A 400 W panel at 4.5 sun-hours and 0.8 ratio makes about 1.4 kWh on an average day — more in summer, less in winter. Multiply by your panel count for the array's daily output.

Because the rating is a lab figure under ideal conditions. Real output is reduced by inverter losses, heat, wiring, dust, shading and the fact that full-strength sun only occurs for part of the day. A ~0.8 performance ratio captures these, giving a realistic estimate.

The equivalent number of hours per day your location receives full-strength sunlight (1,000 W/m²). It's usually 3–6 and is much lower than daylight hours because weak morning and evening light doesn't count for much. It's the correct basis for output estimates.

Yes, slowly — about 0.5% a year, so after 25 years a panel still produces roughly 88% of its original output. This gradual degradation is why lifetime-savings estimates apply a small annual reduction rather than assuming constant output.

Solar irradiance maps and databases (such as NASA POWER or national meteorological sources) give peak sun-hours by location, and many are free. As a rough guide, very sunny regions get 5.5–6.5, sunny temperate areas 4.5–5.5, mixed climates 3.5–4.5, and cloudy northern regions under 4. Using your local figure rather than a generic one is the biggest single improvement you can make to an output estimate.