Solar panel calculator

How many panels cover your electricity use, how much roof they take and when they pay back.

kWh
Letters on the drawing match the letters next to the inputs.
kWh
%
kWh/kWp per year
W
$
$/kWh
More settings
%
$/kWh
sq ft
Solar panels13 pcs
System size5.59 kWp
Payback time15.3 years

Shopping list

  • Solar panels13 pcs × 430 W
  • Inverter, about4.9 kW
  • System price, about$16,770

How we got there

Output per year
7,826 kWh
Share of your use
75 %
Roof area for the panels
273 sq ft
Savings per year
$1,096
Net gain over 25 years
$8,978

Results are estimates. Always check the coverage printed on the product and your local building code.

Calculated on planicalc.com · https://planicalc.com/us/solar-panel-calculator/

How it is calculated

The calculator takes the part of your yearly use you want to cover and divides it by the yearly output of 1 kWp of panels at your place. That gives the system size; dividing by the power of one panel and rounding up gives the number of panels. Savings count twice: the power you use yourself saves the full electricity price, and the rest is sold to the grid at a lower price. Payback is the system price divided by the savings per year.

FormulaSystem size (kWp) = yearly use × share to cover ÷ output of 1 kWpPanels = system size × 1,000 ÷ panel power (W), rounded upSavings per year = output × (own use × electricity price + rest × export price)Payback = system price ÷ savings per year

Example

A home uses 10,500 kWh a year and wants to cover 70 %. With 1,400 kWh per kW that is 5.25 kW, so thirteen 430 W panels (5.59 kW, about 7,800 kWh a year). If 40 % is used at home at $0.17 and the rest credited at $0.12, the panels save about $1,100 a year; at $3.00 per watt the system costs about $16,800 and pays back in about 15 years.

Tips from practice

  • Find the exact output of 1 kW for your address and roof angle in NREL’s free PVWatts calculator. A roof facing east or west gives about 10–20 % less than one facing south.
  • You use more of your own solar power if you run the washing machine, dishwasher, water heater or car charger in the middle of the day.
  • Ask for at least two quotes. The price per kWp falls for bigger systems, and roof work, scaffolding and the connection fee are often extra.
  • Shade from a tree or a chimney on even one panel can cut the output of the whole string. Optimisers or microinverters help on partly shaded roofs.

Questions and answers

How many solar panels do I need for a house?

Divide your yearly use by the output of 1 kW at your place, then by the power of one panel in kW. For 10,500 kWh, 1,400 kWh per kW and 430 W panels that is about 18 panels for the whole year, or 13 to cover 70 %.

How much roof does one panel need?

A modern 400–450 W panel is about 5.6 × 3.7 ft, close to 21 sq ft. Leave space at the roof edges as your installer and the fire code require.

Do solar panels work in winter and in the north?

Yes, but most of the output comes from April to September. In Finland or Scotland one kWp gives about 850–950 kWh a year, in Spain about 1,500–1,600 kWh.

Found a mistake in a formula? Write to [email protected]