A solar panel only makes power when the sun hits it. Obvious so far.
Here's what's less obvious. The same panel in Arizona makes nearly twice as much power as the same panel in Scotland. Same brand. Same wires. Same install.
Why? Because the sun isn't equal everywhere. To plan a solar system, you need to know your number.
What Is A Peak Sun Hour
Solar people don't count regular daylight hours. They count "peak sun hours".
One peak sun hour means one hour where the sunlight is at full strength. About 1000 watts per square meter, if you want the precise number.
A weak winter morning isn't a peak sun hour. The sun is up, but it's giving you maybe 200 watts per square meter. That counts as one-fifth of a peak hour.
So peak sun hours mash all of a day's sunlight (weak in the morning, strong at noon, weak in the evening) into a single equivalent number.
A day with 5 peak sun hours might have 12 hours of daylight, but the total sunshine adds up to 5 hours' worth at full strength.
→ How sunlight becomes electricity, the full story
How Many Peak Hours Each Region Gets
A few rough yearly averages.
Africa. 6 to 8 peak hours a day. Some of the sunniest places on Earth.
Australia. 5 to 7 peak hours. Lots of sun, low cloud cover.
Southern Spain, Portugal, Greece. 5 to 6 peak hours. Why they're solar farm hotspots.
Texas, Arizona, California. 5 to 6 peak hours.
Northern Europe (Netherlands, Belgium, UK). 2.5 to 3.5 peak hours.
Germany. About 3 peak hours.
Northern Canada, Scandinavia. 2 to 3 peak hours, and very uneven across seasons.
So a roof in southern Spain catches roughly twice as much sun as a roof in the UK. That's why a 10-panel system in Spain might cover everything, but the same 10 panels in Manchester might only cover half.
The Seasonal Catch
Yearly averages hide a brutal truth. Solar isn't even close to balanced across the year.
In northern Europe, June might give you 6 peak hours a day. December might give you 1. That's a six-times swing.
In tropical regions near the equator, the swing is much smaller. Maybe 4 to 5 hours all year round.
This matters if you're going fully off-grid. In summer you have too much power. In winter you might run out. You need batteries, or a backup, or both.
→ What to think about before going off-grid with solar
How To Get Your Own Number
The yearly averages above are rough. Your exact spot can be different by 20% in either direction.
Two ways to get a more accurate number.
Use an online solar map. Sites like SunCalc, PVGIS (for Europe), or NREL's PVWatts (for the US) take your address and give you accurate monthly peak sun hours, based on years of weather data.
Ask a local installer. They've already done the math for your region. Most will give you a free quote that includes the number.
Check your location and solar hours
What Hurts Your Number
A few things can drop your real sun hours below the regional average.
Shade. A tree, a chimney, a hill, or a tall neighbour can block the sun for hours a day. Even partial shade on one corner of a panel can drag down a whole array.
Roof angle. A flat roof catches less than a properly-tilted one. In northern Europe, panels should sit at about 35 degrees, facing south.
Direction. South-facing roofs (in the northern hemisphere) catch the most. East and west still work but give you about 20% less. North is mostly a no.
Local weather. A coastal town with daily fog gets less than the regional map suggests. A high-altitude town with clear skies gets more.
→ How many sun hours does your area get?
Why The Number Matters For Sizing
Once you know your peak sun hours, sizing a solar array becomes simple.
Daily power needed ÷ peak sun hours = solar array size
If you use 10 kilowatt-hours a day in a region with 4 peak hours, you need a 2.5 kW system.
If you use 10 kilowatt-hours a day in a region with 2 peak hours, you need a 5 kW system. Twice as many panels.
That's the rule. Your sun hours tell you how big your roof needs to be.
→ How many panels do you actually need?
→ Use the solar panel calculator
→ Use the yearly kWh calculator
A Note On The Three Panel Types
How much sunlight a panel catches also depends on the type. Monocrystalline panels catch about 20%. Polycrystalline catches 15%. Thin-film catches anywhere from 10 to 19% depending on the type.
So the math is really: peak sun hours × panel area × panel efficiency = power per day.
A small mono array in a sunny place can beat a huge thin-film array in a cloudy place. The panel quality matters as much as the sun.
