Solar Panel Output Calculator

How to use the solar panel output calculator

Enter the system size in kilowatts, the peak sun hours for your area, a system efficiency factor, and your electricity rate, and the tool returns daily, monthly and yearly production along with the bill offset that production is worth. The system size is the DC nameplate figure, which is your panel count times each panel wattage divided by 1,000, so twenty-one 400 W panels is 8.4 kW.

The rate sets the dollar figure and nothing else, and it varies enormously by state, so take it from your own statement rather than a national average. Everything else on the page is energy, and energy is the part of a solar estimate that is genuinely predictable, unlike the tariff arrangements that decide what those kilowatt-hours are eventually worth to you.

Peak sun hours is the input most people misread. It is not daylight hours and it is not hours of sunshine. It is the number of hours of full-strength irradiance, defined as 1,000 watts per square metre, that would deliver the same total energy your site actually receives across the whole day.

A location with fourteen hours of daylight and thin winter sun can easily have fewer peak sun hours than one with nine hours of hard summer light. The figure bundles latitude, climate, cloud cover and season into a single planning number, which is why annual state averages across the lower 48 run from around 3.8 in Washington to about 6.5 in Arizona and New Mexico, with an unusually wide spread inside big states.

The efficiency factor exists because a panel nameplate is measured under standard test conditions no roof ever reproduces. Real arrays lose output to inverter conversion, wiring resistance, module mismatch, soiling, snow, shading, and heat, since cell output falls as temperature rises above the 25 C test condition at a rate typically quoted around 0.3 to 0.4 percent per degree.

NREL PVWatts uses a default total system loss of about 14 percent on the DC side, built up multiplicatively from categories including 2 percent soiling, 3 percent shading, 2 percent mismatch, 2 percent wiring and 3 percent availability, and a default nominal inverter efficiency of 0.96. Combined, that is roughly 0.83 DC to AC. The 0.80 default here is deliberately a shade more conservative.

Be clear about what the money column is. It values production at your flat rate, which is a ceiling rather than a promise. What you actually save depends on your net metering arrangement, whether you are on a time-of-use tariff, the fixed monthly charges that stay on the bill regardless of production, and how much of the energy you consume on site instead of exporting.

In many jurisdictions exported kilowatt-hours are credited at less than the retail rate, which pulls the real number down. Use the yearly production against a year of your own bills to judge coverage, and get an installer to model your specific roof, shading and utility before committing.

Location beats hardware

Change only the location and the answer moves hard: at the 6.5 peak sun hours that Arizona and New Mexico average, the identical array makes 31.2 kWh a day and 11,388 kWh a year, worth $1,992.90, while at the 3.8 hours Washington averages it makes 18.2 kWh a day and 6,658 kWh a year.

Nothing about the array changes between those figures, and yet the annual production nearly doubles, which is why the reference table above walks the same 6 kW system through fourteen states before any question of panels or inverters comes up.

The formula, worked line by line

Production is the array nameplate scaled by how much usable sun the site receives and then reduced by everything that goes wrong between the cells and the meter.

Peak sun hours already carries the units that make this work: because a peak sun hour is defined as an hour at 1,000 watts per square metre, and panel nameplate ratings are measured at that same irradiance, kilowatts multiplied by peak sun hours comes out directly in kilowatt-hours with nothing left to convert.

The monthly and annual figures are simple multiples of the daily one, using 365 days for the year and a twelfth of that — 30.42 days — for the month. That matters more than it looks: a flat 30-day month multiplied by twelve lands five days short of the year printed beside it, so the two figures would quietly disagree by 1.4 percent.

It is still a planning convention rather than a seasonal model. A real array produces far more in June than December, so the annual total is the honest number, and the monthly figure is best read as a twelve-month average rather than a forecast for any particular month.

daily kWh = system kW × peak sun hours × efficiency
monthly kWh = daily × 365/12      annual kWh = daily × 365
bill offset = annual kWh × rate
6 kW × 4.5 h × 0.80 = 21.6 kWh per day

The efficiency term is the second big lever and the one worth understanding rather than guessing. NREL PVWatts builds its default from individual loss categories multiplied together, which compounds to about 14 percent, leaving roughly 0.86 on the DC side.

Apply the PVWatts default nominal inverter efficiency of 0.96 and the DC-to-AC figure lands near 0.83. Raising this tool 0.80 default to 0.86 lifts the daily output from 21.6 to 23.2 kWh, so the choice is worth a few percent either way.

Questions people ask