Peak Sun Hours Explained: Sizing a Solar Array for Your Location

"How many panels do I need?" is the second question every off-grid builder asks. The answer depends less on the panels than on the sunlight available where you are, and on how honestly you account for losses. The key concept is peak sun hours (PSH).

What a peak sun hour is

Solar panels are rated under Standard Test Conditions (STC): irradiance of 1,000 watts per square metre with a cell temperature of 25 °C. One peak sun hour is the amount of solar energy equal to one hour at that 1,000 W/m² intensity, which is 1 kWh per square metre.

Real sunlight ramps up in the morning, peaks around solar noon, and falls in the afternoon. If a location receives 5 kWh/m² over a whole day, that is equivalent to 5 hours at full intensity, so it has 5 peak sun hours, even if the sun was above the horizon for 14 hours. This is why daylight hours and peak sun hours are not the same thing, and confusing them is one of the most common sizing errors.

The useful property of PSH is that a panel's daily energy is roughly its rated wattage multiplied by PSH. A 400 W panel in 5 PSH produces about 2,000 Wh per day before losses.

Where to get reliable data

Do not rely on a single national average. Use a free solar resource tool that gives month-by-month values for your coordinates and panel tilt:

• NREL PVWatts: the US Department of Energy's calculator, with worldwide coverage.
• PVGIS: the European Commission's tool, excellent for Europe, Africa, and much of Asia, with a dedicated off-grid mode.
• Global Solar Atlas: World Bank maps for quick comparisons between sites.

Look for "monthly in-plane irradiation" or "solar radiation (kWh/m²/day)" for your tilt angle. That daily figure is the peak sun hours value.

Design for the worst month you need to cover

Annual averages hide large seasonal swings. In central Europe or the northern United States, a winter month can deliver one-quarter of the energy of a summer month. Pick the design month based on when you use the system:

• Year-round home: design for December/January (or June/July in the southern hemisphere), and plan a generator for the darkest weeks if that month gives an impractically large array.
• Summer cabin or van, April to October: design for April or October.
• Mostly summer use: design for the lowest summer month.

The calculator's slider runs from 2.0 PSH (winter or high latitude) to 6.5 PSH (sunny desert summer); 4.5 PSH is a reasonable temperate spring or autumn figure.

Panel tilt and orientation

PSH values depend on panel angle. As a rule of thumb, a tilt roughly equal to your latitude maximizes annual output; latitude plus 10–15° favours winter, when the sun is low; and latitude minus 10–15° favours summer. Panels should face the equator (south in the northern hemisphere). Flat-mounted panels on a van roof lose a lot in winter because the low sun hits them at a steep angle.

Shading is far more damaging than a slightly wrong angle. A shadow over part of one panel can cut the output of a whole series string. Check the site at midday in midwinter, when shadows are longest.

System losses: the derate factor

A 400 W panel almost never delivers 400 W. Several losses stack up between panel and battery:

Loss source	Typical loss
Cell temperature above 25 °C	5–12%
Dirt, dust, pollen, snow	2–5%
Cable resistance	1–3%
Charge controller conversion	2–4%
Panel mismatch and ageing	1–3%
Battery charge efficiency	2–5% (LiFePO4) / 10–20% (lead-acid)

Multiplying the efficiencies together gives a combined factor of about 0.75–0.82 for a lithium system. The 6th Star calculator uses 0.78. Lead-acid systems in hot climates can be closer to 0.70.

The array sizing formula

Array size (W) = Daily energy (Wh) ÷ (Peak sun hours × Derate factor)

Example: a cabin uses 2,000 Wh per day and is used through the autumn, when the design month has 3.0 PSH.

2,000 ÷ (3.0 × 0.78) = 855 W. Three 400 W panels (1,200 W) give a comfortable margin for a run of poor days. In July, at 5.5 PSH, the same array produces roughly 1,200 × 5.5 × 0.78 ≈ 5,150 Wh per day, far more than needed. That summer surplus is normal for a system sized for the shoulder seasons.

Recharging after cloudy days

If your battery is sized for several days of autonomy, the array also needs spare capacity to refill it while still covering daily use. An array sized exactly to daily consumption will never recover a depleted bank. Adding 15–30% to the result above, or relying on a generator after long storms, keeps the battery from sitting at a low state of charge, which is especially harmful to lead-acid batteries.

Next, choose a charge controller that can handle the array: see the MPPT sizing guide.