Solar Panel Output Calculator

How do you calculate solar panel output?

Multiply the panel rating in watts by the number of panels and the peak sun hours, then by the derate for losses, and divide by 1,000 for kWh per day. PVWatts’s defaults, 14 percent system losses and a 96 percent efficient inverter, give a derate of 0.86 × 0.96 = 0.8256. Multiply by 30.44 for a month or 365.25 for a year.

What peak sun hours measure

The power of sunlight arriving on a surface is its irradiance, in watts per square meter. It climbs from nothing at dawn to a midday high, falls again toward sunset, and drops whenever cloud passes. Added up over a day it becomes the irradiation, in kWh/m². Panel ratings are measured at standard test conditions, 1,000 W/m² of sunlight with the cells at 25 °C, so a day that delivers 5 kWh/m² to the panels gives them as much energy as 5 hours of that standard sunshine would: 5 peak sun hours. The irradiation in kWh/m² per day and the peak sun hours are the same number.

The figure has to be for the plane of the panels. Sunlight on flat ground differs from sunlight on a panel tilted toward the sun, and a panel facing away from it collects less again. The IEC 61724 standard for monitoring solar systems calls this quantity the reference yield, and a system’s measured kWh divided by its rating and by the reference yield is its performance ratio, the measured counterpart of the derate used here.

Finding the peak sun hours for your address

The free PVWatts calculator, run by the US Department of Energy’s National Laboratory of the Rockies (formerly NREL), works them out. Enter an address, then the tilt of the panels and the direction they face, and its results list the solar radiation on the panels for each month and for the year in kWh/m²/day, calculated from typical-year weather data for that location. Those are the peak sun hours to enter here.

Sun hours change through the year, so use the yearly figure for the yearly output and a month’s figure for that month; a summer month’s value entered for a yearly total is that month repeated twelve times. PVWatts also gives its own monthly and yearly output, modeled hour by hour, and setting it beside the figure here shows how much the effects this calculator leaves out matter at that site.

Where the 14 percent comes from

PVWatts’s default system losses are 14 percent, from ten categories: soiling, shading, snow, mismatch between panels, wiring, connections, light-induced degradation, the nameplate rating, age and availability. They combine by multiplication, each one taking its share of what the others leave, so the defaults in the PVWatts Version 5 manual (Dobos 2014) add to 15 percent but come to 14, which leaves 0.86 of the output. The default shading loss assumes nothing close by casts a shadow, so raise the losses for nearby trees or buildings.

The inverter, which turns the panels’ direct current into the alternating current a home uses, sits outside the 14 percent: PVWatts gives it a separate efficiency, 96 percent by default, and so does this calculator. If you have one loss figure that already includes the inverter, such as the 0.77 derate of the first version of PVWatts, enter it as 23 percent losses with the inverter at 100, so the inverter is not counted twice.

What this estimate leaves out

Panels lose output as they heat up, and in sunshine their cells run hotter than the air. In PVWatts’s model a standard panel loses 0.47 percent of its power for every degree Celsius its cells are above the 25 °C its rating assumes. PVWatts also counts the extra sunlight the glass reflects when light strikes it at a glancing angle, and the inverter’s lower efficiency at part load, hour by hour. A daily total times a fixed derate includes none of these, so in hot, sunny months this calculator usually comes out above PVWatts for the same system.

From kWh to a month, a year and a bill

The month here is the average month, a twelfth of a 365.25-day year, so twelve monthly figures add up to the yearly one. The value readouts multiply the energy by your price per kWh, in whatever currency you type: 0.15 gives dollars and 15 gives cents. That value is a saving on the bill only where each kWh replaces one you would have bought at that price. Energy sent back to the grid may be credited at a different rate, so check how your utility credits it.

To set the output against what you use, the electricity cost calculator turns an appliance’s watts and hours of use into kWh per day and a cost at the same price. A panel’s rating is a power, as an appliance’s is, and the electrical power calculator works with P = IV: the rating on a panel’s datasheet is its voltage at maximum power times its current at maximum power.

Common mistakes

• Using hours of daylight as peak sun hours. The sun is below 1,000 W/m² for most of the day, so a day holds fewer peak sun hours than hours of daylight, and entering the hours of daylight overstates the output.
• Adding the loss percentages. Losses multiply: two losses of 10 percent leave 0.9 × 0.9 = 0.81 of the output, a loss of 19 percent, not 20.
• Treating kW and kWh as the same thing. A system’s kW rating is a rate. Ten 400 W panels make a 4 kW system, and at 5 peak sun hours with the default derate it delivers 16.51 kWh in a day.
• Multiplying by the panel area as well. The rating already includes the area and the efficiency: it is the area times 1 kW/m² times the efficiency. Multiplying the rating by the area again counts the area twice.

Converting units first? Use the power conversion table.