Calculator
Roof Area and Panel Count Calculator
Works both ways. Tell us the system size and we give the roof space needed. Or tell us your roof size and we tell you what will fit.
The formula
Panels = System size in watts / Panel wattage
Roof area needed = System kW x area per kW
AREA PER kW
Polycrystalline 110 to 120 sq ft
Mono PERC 90 to 100 sq ft
High efficiency 75 to 85 sq ft
Only about 80% of a roof is usable. Water tank, stairs,
parapet shadow and walking space take the rest.
What this assumes
- Only about 80% of your roof can actually hold panels. The rest goes to access paths, water tank and shadows.
- Assumes panels are flat or on a low frame. A high raised structure lets you keep using the roof below.
- Any shade at all matters. Even one shaded panel can pull down a whole string.
- A sloping roof gives less usable area than a flat one.
- Panel sizes keep changing. Check with your installer what they actually supply.
Common questions
How much roof do I need for 3 kW?
About 300 sq ft with normal mono PERC panels. With high efficiency panels around 250 sq ft. Remember only 80% of your roof is usable.
I have 500 sq ft of roof. What can I fit?
Roughly 4 kW with normal panels. Enter your exact area in the calculator for a proper figure.
Can I still use my roof after installing solar?
Yes, if you use a raised structure. It costs more but you keep the space below for drying clothes or sitting. Many people regret not doing this.
My neighbour built a floor and now there is shade. What now?
This is a real problem and it is common. Even partial shade for a few hours cuts output badly. Talk to your installer about panel level optimisers, or move the array. Check for future construction around you before installing.