Peak sun hours: why your dullest month decides your solar size
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Two homes with identical appliances can need very different solar arrays, simply because of where they are. The number that captures this is peak sun hours.
What a peak sun hour is
Solar panels are rated under standard test conditions: sunlight of 1,000 watts per square metre. One peak sun hour is one hour of sunlight at that strength, or 1 kWh of solar energy per square metre.
Real days aren’t like that. The sun is weak in the morning, strong at midday and weak again in the evening, and clouds come and go. Peak sun hours add all of that up and express it as the number of full-strength hours it equals. A place with 5 peak sun hours a day receives the same energy as 5 hours of full-strength sun, even though it is light for 12 hours or more.
That makes the arithmetic simple: a 400 W panel in a place with 5 peak sun hours produces at most 400 × 5 = 2,000 Wh a day. In practice, heat, dust, wiring and conversion losses take about a quarter of that, so plan on about 1,500 Wh.
How much it varies
These are long-term monthly averages from NASA’s POWER project (satellite data, 2001–2025), the same source the calculator uses:
| City | Yearly average | Dullest month | Sunniest month |
|---|---|---|---|
| Phoenix, United States | 5.85 | December, 3.08 | June, 8.50 |
| Kano, Nigeria | 6.00 | December, 5.50 | April, 6.56 |
| Karachi, Pakistan | 5.61 | December, 4.13 | May, 7.15 |
| Johannesburg, South Africa | 5.58 | June, 3.95 | November, 6.83 |
| Manila, Philippines | 5.10 | December, 4.15 | April, 6.67 |
| Mumbai, India | 5.29 | July, 3.79 | May, 7.09 |
| Lagos, Nigeria | 4.59 | July, 3.82 | April, 5.03 |
| Houston, United States | 4.65 | December, 2.64 | June, 6.29 |
| London, United Kingdom | 2.94 | December, 0.66 | June, 5.30 |
Two patterns stand out:
- Near the equator, sunshine is fairly steady through the year, and the dullest month is often the rainy season rather than winter: July in Lagos and Mumbai.
- Far from the equator, winter is the problem. London’s December gives about an eighth of its June sunshine.
Why the dullest month decides the size
A solar array has to refill the battery every day. If you size it on the yearly average, it will keep up for about half the year and fall short for the rest, often in the very weeks when outages are worst: storm seasons, monsoons and winter.
So the calculator plans for the month with least sun by default:
panel watts needed = daily energy to replace ÷ (dullest month’s peak sun hours × 0.75)
In Kano the dullest month is only slightly below average, so the difference is small. In London, sizing for December would need roughly four to five times the panels that the yearly average suggests. Few homes there do that. Instead, they size panels for spring to autumn and top up batteries from the grid or a generator in midwinter. Both approaches are valid as long as you choose deliberately.
Getting more from the sun you have
- Tilt and direction. NASA’s figures are for flat ground. Panels tilted towards the equator at roughly your latitude usually collect more over a year, and tilting more steeply helps in winter.
- Shade. Even partial shade on one panel can cut a whole string’s output. Keep panels clear of trees, tanks and neighbouring buildings between mid-morning and mid-afternoon.
- Heat. Panels lose output as they get hotter. Leave an air gap behind them.
- Dust. In dry, dusty seasons a dirty panel can lose a noticeable share of its output. Rinse panels regularly.
Find your city’s month-by-month figures on our locations pages, or search any town in the calculator.