Solar System Size Calculator
Enter how much electricity you use and how much sun you get. The calculator estimates the size of the solar system in kW and the number of panels needed to cover your use.
How to use this Solar System Size Calculator
- Find your monthly electricity use in kWh on a recent bill.
- Enter the peak sun hours per day for your location.
- Keep or change the system losses. 14 percent is a common planning figure, so check what your installer uses.
- Enter the rating of the panel you plan to use and its area.
- Set the share of your use you want to cover, then press Calculate.
How to size a solar system
The system must produce your daily energy from the sun hours it sees, after losses.
- Daily energy = monthly kWh x 12 / 365 x share covered
- System size (kW) = daily energy / (sun hours x (1 – losses))
- Panels = system size x 1,000 / panel watts, rounded up
- Roof area = panels x area of one panel
Worked example
A home uses 900 kWh a month, which is 29.6 kWh a day. With 5 peak sun hours and 14% losses, the system needs 29.6 / (5 x 0.86) = 6.88 kW. With 400 W panels that is 17.2, so 18 panels, for an installed size of 7.20 kW. At 20 sq ft each the panels take 360 sq ft of roof.
If you want to cover only 80% of your use, the system shrinks to about 5.5 kW. Many people size a system a little below their use to keep the cost down, or a little above to allow for an electric car or heat pump later. The sun hours figure has the biggest effect on the answer, so check it for your own location, and ask installers for a production estimate for your roof, since shade and direction change the result.
Assumptions & limits
- Peak sun hours and losses are editable inputs. Check a solar resource map and ask installers for the figures they use.
- The calculator uses one yearly average. Output changes by season and your use may too.
- Roof direction, tilt, shading and panel degradation are not modelled.
- It does not include costs, tax credits, net metering rules or battery storage.
- It is a planning estimate and not an engineering design.
Frequently asked questions
How do I calculate what size solar system I need?
Divide your daily electricity use by the peak sun hours, adjusted for losses. That gives the system size in kW.
What are peak sun hours?
They are the number of hours per day when sunlight is strong enough to give a standard rating of power. It varies by location and season.
How many panels do I need?
Divide the system size in watts by the rating of one panel and round up.
What are system losses?
They are energy lost in the wiring, the inverter, dirt, heat and shade. Many planning tools use around 14 percent, but yours may differ.
Should I cover 100 percent of my use?
Not always. It depends on cost, roof space, local rules and future needs.
Does this include batteries?
No. Use the Solar Battery Size Calculator for storage.
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Last updated: November 2026