Solar Panel Calculator
How Does Solar Sizing Work?
Calculating the size of a solar system isn't just about how much electricity you use; it's about how much sun your specific roof receives.
The formula works backwards from your monthly bill:
- Usage: Convert your bill ($) into Kilowatt-Hours (kWh).
- Sunlight: Determine your local "Peak Sun Hours."
- Inefficiency: Add a buffer for real-world losses (heat, dirt, wiring).
What are "Peak Sun Hours"?
This is the most confusing concept for beginners. A "Peak Sun Hour" is not just an hour where the sun is in the sky.
A Peak Sun Hour is defined as one hour where the intensity of sunlight reaches 1,000 watts per square meter.
The sun might be up for 14 hours in the summer, but early morning and late afternoon sun is weak. You might only get 4 to 5 "Peak" hours worth of usable energy in that entire day.
- Phoenix, AZ: ~6.5 Peak Hours (Excellent).
- New York, NY: ~4.0 Peak Hours (Average).
- London, UK: ~3.0 Peak Hours (Low).
The "Derating Factor" (Real World Loss)
If you buy a 400-watt panel, it will rarely produce 400 watts. Physics gets in the way.
Solar engineers use a "Derating Factor" (usually 0.75 or 75%) to account for losses:
- Heat: Solar panels lose efficiency as they get hotter. On a scorching July day, they actually produce less power than on a cool sunny spring day.
- Inverter Loss: Converting DC (Panel) to AC (House) loses about 5-10%.
- Dirt & Wiring: Dust and resistance account for another 5%.
To get 100% of the energy you need, you actually need to build a system that is 125% of the theoretical size to cover these losses. Our calculator handles this buffer automatically.
Panel Wattage: 300W vs 450W
In 2015, a standard solar panel was 250 Watts. Today, standard residential panels are 400 Watts.
Does wattage matter? Yes, for roof space. Higher wattage panels allow you to generate more power in a smaller area.
| Panel Wattage | Panels Needed (6kW System) | Roof Space |
|---|---|---|
| 300 W | 20 Panels | ~350 sq ft |
| 400 W | 15 Panels | ~260 sq ft |
| 450 W | 13 Panels | ~230 sq ft |
Net Metering: The Grid as a Battery
Most home solar systems are "Grid-Tied." This means you don't need expensive batteries.
During the day, your panels produce more energy than you use. This excess goes back to the grid, and the utility company gives you credits (Net Metering). At night, when panels are off, you pull energy from the grid using those credits.