Solar panel daily energy:
formula and worked example

How a panel rating becomes daily energy

A solar panel's rating in kilowatts is its output under standard test conditions, which are bright, cool and rarely met on a roof. What matters for the bill is energy over a day, which depends on how much sunlight actually arrives and how efficiently the system turns it into usable power.

The calculation turns a rating into an expected number of kilowatt-hours per day by applying two correction factors: the equivalent hours of full sunshine at the site, and a performance ratio for the losses in the system.

It is useful for sizing an array against a household's daily usage, for comparing quotes that quote different panel capacities and for checking whether a generation claim is plausible.

The formula and its inputs

• Panel kW is the total DC rating of the array, the sum of the panel ratings in kilowatts.
• Peak sun hours is the daily solar energy on the panels expressed as hours of 1 kW per square metre, which is the intensity of the test condition.
• Performance ratio is a decimal between 0 and 1 for the overall system efficiency after temperature, soiling, wiring, mismatch and inverter losses.

Daylight of ten or twelve hours is not the same as peak sun hours, because early morning and late afternoon sun is weak and counts for a fraction of an hour.

Worked example: 4 kW, 4.5 sun hours, 0.78 ratio

A homeowner is considering a 4 kW rooftop array at a site that receives about 4.5 peak sun hours per day on average, with a performance ratio of 0.78.

1. 1Array rating: 4 kW.
2. 2Ideal daily energy: 4 x 4.5 = 18 kWh.
3. 3Apply the performance ratio: 18 x 0.78 = 14.04 kWh.
4. 4Monthly estimate over 30 days: 14.04 x 30 = 421.2 kWh.

The 22% lost to the performance ratio is a sizeable share, which is why the factor must not be left out.

How to judge 14 kWh a day

Compare the daily output with the household's daily consumption to see how much of the load the array could cover, and whether a battery is needed to shift the energy to the evening.

Seasonal averages hide a wide spread. Winter and monsoon days produce far less than the annual mean, and a clear spring day produces more. Choose a sun-hours input appropriate to the month if you are planning for a season rather than the year.

• Orientation and tilt influence the sun hours that reach the panel, so check the number used is for your roof, not for a horizontal surface.
• Shading from a tree or a water tank can cut the output more than the ratio suggests.

What a larger array does

A 10% larger array, 4.4 kW instead of 4 kW, with the same sun hours and performance ratio, produces 15.444 kWh per day instead of 14.04.

The output scales in direct proportion to the panel rating. The same percentage improvement in performance ratio, by cleaning and fixing shading, would have exactly the same effect without more panels.

Mistakes that inflate solar estimates

• Using the hours of daylight as the peak sun hours, which overstates the output by two or three times.
• Leaving out the performance ratio and expecting the full rated output.
• Using a clear-day figure as the average, rather than a season-weighted average that includes cloudy days.
• Ignoring panel degradation over the years, which lowers output each year by a small fraction.

A quick check is to divide the result by the array kW: 3.51 kWh per kW per day is the implied yield.

From daily energy to a sizing decision

Take the household's daily consumption and divide by the per-kW daily yield from the formula to find the array size that matches it. Then adjust for the part of the load that occurs at night or on cloudy days.

Keep panel surfaces clean and the array free from shade; both are cheaper than adding capacity.