What this calculator does
Solar energy yield is an estimate of how much electricity an array will actually generate over a year, as distinct from its nameplate wattage. Nameplate wattage is measured under fixed laboratory test conditions that real rooftops rarely see for more than a few minutes a day, so a useful yield estimate needs two further pieces of information: how much usable sun the site actually gets, and how much of the panels' theoretical output survives the trip through the rest of the system.
Peak sun hours is the standard way of compressing a whole day of varying sunlight intensity into a single number: it is the number of hours of sunlight at a standard reference intensity that would deliver the same total energy as the day's actual, varying sunlight. It depends heavily on location, season and local weather, which is why it is left as an input here rather than assumed.
The formula
Multiply the array's rated size in kilowatts by the average peak sun hours per day, by 365 days, by the system efficiency expressed as a decimal. System efficiency rolls together inverter conversion losses, wiring resistance, dust and soiling on the panels, and the fact that panels run less efficiently when hot, all of which reduce output below the simple size-times-sun-hours figure.
| Term | Meaning |
|---|---|
| Array size | The panels' combined rated (nameplate) output, in kilowatts, under standard test conditions. |
| Peak sun hours | The average number of hours per day of sunlight at reference intensity that would deliver the same energy as the actual, varying sunlight received; it depends on location and season. |
| System efficiency | The share of the panels' theoretical output that reaches usable electricity after inverter, wiring, soiling and temperature losses, typically in the range of 75% to 85% though it varies by installation. |
The inputs explained
| Field | What to enter |
|---|---|
| Solar array size (kW) | The total rated (nameplate) capacity of the solar array, in kilowatts, as stated on the panel and inverter specifications. |
| Average peak sun hours per day | Average peak sun hours per day for the site and period being estimated. This varies by location, season, roof orientation and shading, and is best sourced from a local solar resource map or your installer's own estimate rather than guessed. |
| System efficiency (inverter, wiring, dust, heat losses) (%) | An overall efficiency figure covering inverter losses, wiring, dust and temperature derating. A well-installed system with a modern inverter often sits around 80% to 85%; a shaded or poorly maintained one can be lower. |
When to use it
Sizing an array against a known consumption figure
Comparing the estimated annual yield of a proposed array size against a household's actual annual electricity consumption shows roughly what share of usage the system is likely to cover.
Comparing two sites with different sun exposure
The same array size produces very different annual yield in a consistently sunny location compared with one prone to cloud cover or shading, which this calculator makes explicit by varying only the sun-hours figure.
Checking an installer's output promise against your own assumptions
Running the installer's quoted array size and stated efficiency assumption through the same formula is a quick way to sanity-check a yield estimate given in a sales quote.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How estimated annual yield changes with array size at fixed sun hours and efficiency
A fixed 4.5 peak sun hours per day and 80% efficiency, across a range of array sizes.
| Array size | Estimated annual yield | Estimated daily yield |
|---|---|---|
| 3 kW | 3,942 kWh | 10.8 kWh |
| 5 kW | 6,570 kWh | 18.0 kWh |
| 6.6 kW | 8,672 kWh | 23.8 kWh |
| 8 kW | 10,512 kWh | 28.8 kWh |
| 10 kW | 13,140 kWh | 36.0 kWh |
| 13.2 kW | 17,345 kWh | 47.5 kWh |
How estimated annual yield changes with peak sun hours at a fixed array size and efficiency
A fixed 6.6 kW array at 80% efficiency, across a range of average peak sun hours.
| Peak sun hours per day | Estimated annual yield | Estimated daily yield |
|---|---|---|
| 3 h | 5,782 kWh | 15.8 kWh |
| 3.5 h | 6,745 kWh | 18.5 kWh |
| 4 h | 7,709 kWh | 21.1 kWh |
| 4.5 h | 8,672 kWh | 23.8 kWh |
| 5 h | 9,636 kWh | 26.4 kWh |
| 5.5 h | 10,600 kWh | 29.0 kWh |
Questions
Why is the estimated yield lower than array size times 24 hours times 365?
Panels only produce their full rated output under specific test conditions equivalent to strong, direct, perpendicular sunlight, which does not last anywhere near 24 hours a day. Peak sun hours already compresses a real day's varying sunlight into an equivalent number of hours at that reference intensity, so it should not be multiplied by 24; it already accounts for the whole day.
What system efficiency figure should I use?
A typical range for a reasonably well-installed modern system is 75% to 85%, but shading, panel age, dust accumulation and inverter type all shift it. Your installer's own performance estimate, or output from an existing nearby system, is a better source than a generic assumption.
Does this account for panel degradation over the years?
No, this is a single-year estimate at the efficiency and sun-hours figures you enter. Panels typically lose a small percentage of output each year as they age, so a long-term projection would need to reduce the efficiency figure gradually over time.
How does this relate to the solar payback period calculator?
This calculator estimates how much electricity the system will generate; the solar payback period calculator takes the resulting bill savings and a system cost to work out how many years it takes to recover that cost.
Feed the estimated yield into the solar payback period calculator to see how it translates into bill savings and payback time, or into the off-grid battery sizing calculator if the system needs to run without grid backup.