Every solar estimate on this site — and most honest ones anywhere — hangs on one number: peak sun hours, the hours per day your location averages at full-strength sunlight. Get it wrong by a fifth and every downstream figure (kWh made, dollars saved, years to payback) is wrong by a fifth too. It’s also the most misquoted number in residential solar, so it’s worth understanding properly.

What one peak sun hour actually is

The sun’s strength at Earth’s surface tops out near 1,000 watts per square meter — “one sun” in lab language. A peak sun hour is the energy equivalent of one hour at that full strength: 1 kWh/m².

Your location doesn’t get a row of identical full-power hours, of course. Morning sun is weak, noon is strong, haze steals what geometry gives. So PSH compresses the whole day’s varying irradiance into an equivalent count of full-strength hours:

PSH = daily solar energy (kWh/m²) ÷ 1 kW/m²

A day that delivers 5.3 kWh onto each square meter is a 5.3-peak-sun-hour day — whether that energy arrived in 5 blazing hours or 11 diluted ones. That’s why PSH is not “hours of sunshine”: the two numbers can differ by a factor of two, and confusing them is the oldest trick in the misleading-quote playbook.

How PSH turns kW into kWh

The production chain on this site is:

daily kWh = system kW × PSH × orientation factor × performance factor

A 7 kW array at 5.5 PSH with ideal orientation and ~14% losses produces 7 × 5.5 × 1.00 × 0.86 ≈ 33.1 kWh/day — call it 12,100 kWh/year. That PSH middle term is where all the geography lives: the same array makes ~60% more in Arizona (6.4) than in Michigan (4.0), before you spend a cent on hardware.

US ranges — where the states sit

Annual-average PSH across the 50 states spans roughly 2.9–6.5. The spread is wide enough to be the difference between an 8-year and a 19-year payback at the same price:

Region	Typical PSH	Feel
Southwest (AZ, NM, NV, interior CA)	5.8–6.5	the benchmark desert sun
Texas, Florida, Colorado, Utah	5.2–5.8	strong, some summer-cloud trade
Mid-South, Plains, California coast	4.7–5.4	solid solar country
Midwest, Mid-Atlantic, New England	3.9–4.7	four real seasons, winter dips
Pacific Northwest, Alaska	2.9–3.9	cloudy-west vs. sunny-interior split

Two cautions on any state-level number. First, states aren’t points: western Washington sits near 3.5 while eastern Washington tops 5; west Texas deserts and Gulf-coast humidity share a flag, not a sky. Second, these are annual averages — a 4.5-PSH state still sees ~6.5-PSH Julys and ~2.2-PSH Januarys. The calculator presets are midpoints of these ranges; your address can sit a half-hour off in either direction.

World cities — same measure, wider spread

The calculator includes major international cities because PSH is a climate property, not a property of any country’s rebate program:

City	≈ PSH		City	≈ PSH
Dubai	6.1		Cape Town	5.5
Cairo	6.2		Johannesburg	5.7
Mexico City	5.4		Mumbai	5.3
Sydney	4.8		Madrid	4.9
Tokyo	3.9		São Paulo	4.6
Paris	3.3		Berlin	3.0
London	2.9		Stockholm	2.9

The pattern rhymes with the US map: desert belts and Mediterranean climates punch high, northern maritime and monsoon-humid climates punch low. Germany’s famous rooftop-solar buildout happened at ~3 PSH — proof that incentives and electricity prices matter as much as photons, which is also the whole point of the ROI-by-state guide.

Why annual PSH is the right default (and when it isn’t)

For a payback estimate, annual-average PSH is correct by construction: it multiplies by 365 to reproduce the year’s total production. Where a single number misleads is anything seasonal — sizing a battery, predicting a January bill, judging whether winter output covers a heat pump. Those need monthly irradiance (PVWatts reports all twelve months), not the average.

Tilt is the other caveat. Published PSH maps usually assume a horizontal or latitude-tilt surface; the calculator handles your real geometry through the separate orientation factor rather than baking it into the sun number. Keep the two apart and each stays honest.

Getting a better number for your roof

1. NREL PVWatts (free, US + growing international coverage): enter your address, array size, tilt and azimuth — it returns monthly and annual kWh built on decades of measured irradiance. This is the reference standard the presets approximate.
2. Global Solar Atlas for international addresses — same idea, World Bank’s data.
3. Your installer’s proposal — legit proposals print the assumed irradiance and yield (kWh/kWp). Compare it to the map, not to the promises around it.

Then come back and type your real number into the PSH field — the presets are the starting point, not the ceiling of what this calculator can model.

Frequently asked questions

Is 5 peak sun hours the same as 5 hours of sunshine?

No. PSH counts strength, not duration. A location with 5 PSH gets the equivalent of 5 hours at full 1,000 W/m² irradiance — which might arrive as 3 bright midday hours plus 7 weaker morning/evening hours totaling the same energy. A hazy 12-hour day can deliver fewer PSH than a crisp 8-hour desert day.

Why does the calculator use one number for the whole year?

Because annual-average PSH already folds the seasons in: June's long bright days and December's short dim ones average out across 365 days. Multiplying daily output by 365 then gives the right annual total — which is what matters for a payback estimate. Just know your monthly reality swings: a 4.5 PSH average might mean ~6.5 in July and ~2.2 in January.

Where do these preset numbers come from?

They're midpoints of NREL PVWatts-style annual-average irradiance ranges — the same dataset family installers use for first-pass estimates. Honest approximations: real roofs vary by several tenths of a PSH inside a single state (eastern vs western Washington is a whole category of difference). For a decision-grade number, run NREL's free PVWatts tool with your address and roof pitch.

Does tilt change my PSH?

It changes how much of that irradiance your panels capture — which is why the calculator treats it separately as the orientation factor. PSH is measured on a horizontal or latitude-tilt reference; a panel aimed at the equator near its latitude tilt captures close to 100% of the reference, a flat panel in a northern winter or a wrong-facing slope captures less.

Can I just use a higher number to be safe?

You can — the field is editable — but optimism is the classic way solar quotes go bad. If a sales proposal implies 6.5 PSH in a 4.5 PSH climate, that single tweak inflates projected savings ~45%. Match the map, not the brochure.