Enter your daily electricity use and peak sun hours to size your solar system — in kilowatts and in number of panels.
What Is a Solar Panel Calculator?
A solar panel calculator sizes a rooftop solar system from your electricity use: daily kWh divided by peak sun hours gives the system size in kW. A home using 30 kWh per day with 5 peak sun hours needs a 6.00 kW system — fifteen 400 W panels. Peak sun hours are not daylight hours: they are the equivalent hours of full 1,000 W/m² sunshine, typically 4–6 in sunny regions like Pakistan. Real quotes add a 15–25% buffer for losses, shading and future growth — size with the tool, then round up with your installer.
How to Use the Solar Panel Calculator
- Enter your average daily electricity use in kWh (check your bill: monthly units ÷ 30).
- Enter your area’s peak sun hours (5 is typical for sunny regions).
- Pick the panel wattage.
- Click “Size My System” for kW and panel count.
Solar Panel Calculator Formula
System size (kW) = daily kWh ÷ peak sun hours. Panels = ceil(system kW × 1000 ÷ panel watts).
Worked Example
30 kWh/day ÷ 5 sun hours = 6.00 kW. Panels: 6,000 ÷ 400 = 15 panels.
How It Works, In Plain Words
The tool works backwards from your bill. The first input is your average daily electricity use in kilowatt-hours, and the How to Use list shows where to find it: take the monthly units from a bill and divide by 30. The second input, peak sun hours, is the clever part of the method. The What Is section defines them as the equivalent hours of full 1,000 W/m² sunshine — not how long the sun is up, but how much full-strength sun the day adds up to, typically 4–6 in sunny regions. Dividing use by sun hours gives the system size: 30 ÷ 5 = 6.00 kW, because a 6 kW system enjoying 5 full-strength hours produces roughly the 30 kWh the home uses. The last step turns kilowatts into hardware: multiply by 1,000 to get 6,000 watts, divide by the panel wattage you picked, and round up — the Formula writes ceil — since panels come whole. At 400 W that is exactly 15 panels; the 450 W and 550 W options simply divide the same 6,000 watts into fewer, larger panels.
Real-World Uses
First scenario: sizing before you shop. A homeowner with a monthly bill works out the daily figure the How to Use way — monthly units ÷ 30 — lands on the worked example’s 30 kWh, and learns they are shopping for a 6.00 kW system of 15 panels, with the FAQ’s roof check of about 280–300 sq ft to pace out before anyone visits. Second scenario: keeping quotes honest, the job the Benefits list names. With your own 6.00 kW base figure in hand, a quote can be read properly: the buffer FAQ explains why an installer legitimately lands above it — 15–25% for losses, shading and future load growth — and anything far beyond that range is a question to ask, not a number to accept. Third scenario: fitting a tight roof. The Mistakes list warns that 15 panels need roughly 300 sq ft of clear roof; switching the wattage list to 550 W covers the same 6,000 watts with fewer panels, which is exactly the comparison to run when roof space, not sunshine, is the limit.
Benefits of Using the Solar Panel Calculator
- Sizes from your actual bill, not a guess
- Peak sun hours explained — the key concept
- Panel wattage selectable
- The installer buffer note keeps quotes honest
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device
Common Mistakes to Avoid
- Using daylight hours instead of peak sun hours — that halves the real answer
- Sizing exactly with no buffer: shading, dust and inverter losses eat 15–25%
- Ignoring roof space: 15 panels need roughly 300 sq ft of clear roof
More Common Mistakes to Avoid
Three more deserve care. First, typing the monthly units from the bill straight into the daily box: the How to Use list’s divide-by-30 step exists precisely because that one slip multiplies the system size enormously. Second, sizing from a single unusual day — a house full of guests or a quiet week away — instead of the average the first How to Use step asks for; the tool can only be as typical as the day you describe. Third, treating the tool’s 6.00 kW as a finished quote rather than a base: the What Is section is explicit that real quotes add the 15–25% buffer on top, so compare installers against the base figure plus an openly stated buffer, not against the base alone.
Frequently Asked Questions
What size solar system for 30 kWh per day?
6.00 kW at 5 peak sun hours — fifteen 400 W panels.
What are peak sun hours?
Equivalent hours of full-strength sunshine per day — typically 4–6 in sunny regions; not the same as daylight hours.
How many panels do I need?
Divide the system kW by panel watts and round up: 6,000 ÷ 400 = 15.
How much roof space do I need?
Roughly 18–20 sq ft per panel: 15 panels need about 280–300 sq ft of clear, unshaded roof.
Should I add a buffer?
Yes — installers typically add 15–25% for losses, shading and future load growth.
What happens if I choose a bigger panel wattage?
The system size stays 6.00 kW, but the panel count falls, because the same 6,000 W is divided by a bigger number: at 450 W the count is 6,000 ÷ 450 rounded up — 14 panels — and at 550 W it is 6,000 ÷ 550 rounded up — 11 panels. Bigger panels mean fewer units on the roof for the same total watts.
Does the tool include the 15–25% buffer?
No. The tool gives the base size from your figures — 6.00 kW and 15 panels in the worked example. As the What Is section explains, real quotes add a 15–25% buffer for losses, shading and future growth on top, applied by the installer when quoting. Size with the tool first, then let the buffer be added openly so you can see it.