Design a solar array: Calculate yield realistically

The rated output of a solar module in watt-peak is measured under laboratory conditions: perpendicular irradiation, defined amount of light, 25 °C cell temperature. None of these conditions prevail on a roof or deck. Those who calculate with Wp regularly plan for half of the necessary area.

From Watt-Peak to Daily Yield

A useful calculation is one that concludes from the rated power to a daily yield in watt-hours. The factor for this depends on the season, latitude, orientation, and shading.

Guideline values for fixed modules in Central Europe. Tracked or deployable modules are higher; modules on a bimini or under a mast are often significantly lower due to shading.

System Design in Four Steps

1. Determine daily consumption. From the consumer list in ampere-hours, then convert to watt-hours using the onboard voltage: 90 Ah at 12 V is 1,080 Wh.
2. Define objective. Should solar cover consumption completely, or only standby current and a portion? Both are legitimate goals with very different area requirements.
3. Calculate back to rated power. Required watt-hours divided by the yield factor for the target season. For 1,080 Wh in summer at 450 Wh per 100 Wp: around 240 Wp.
4. Add reserve and losses. Controller, cabling, and aging reduce yield. A ten to twenty percent surcharge is realistic.

If you want to operate the same system autonomously year-round, you don't calculate with the summer value, but with that of the worst season — and end up with many times the area. In practice, this is the point where the decision is made between "solar covers everything" and "solar extends the time between two charging cycles."

Shading: The Biggest Single Error

A solar module consists of cells connected in series. If a mast, railing, or superstructure shades even a part, the weakest cell limits the current of the entire string. The yield loss is therefore not proportional to the shaded area — a narrow shadow across a module can cause the yield to drop disproportionately.

This leads to three rules for placement: Consider shading over the course of the day, not at the time of installation. Regulate several small modules separately instead of one large partially shaded one. And prefer a smaller, clear area over a larger one in partial shade.

MPPT or Simple Regulation

A simple controller connects the module and battery, allowing the module to operate at the battery voltage. This is rarely the point of highest power: A module with 36 cells delivers its maximum power at about 17 to 18 volts — on a 12-volt battery bank, it will inevitably operate at 13 to 14 volts.

An MPPT controller continuously searches for the operating point with the highest power and converts the excess voltage into current. The gain is greatest in cold conditions, with high module voltage, and with changing irradiation; in warm weather and with a small system, it is lower.

Two other points favor MPPT as the system grows: you can connect modules in series and thus work with higher voltage and thinner cables, and modern controllers offer multi-stage charging curves for each battery type instead of a fixed cutoff voltage.

NoteTwo module values are crucial for controller selection: the open-circuit voltage at low temperature — it must never exceed the permissible input voltage of the controller — and the short-circuit current. Both are in the datasheet, and both increase, not decrease, when it gets cold.

Cabling and Fusing

The cable from the controller to the battery is a charging cable like any other: it should be fused near the battery, not at the controller. On the module side, a fuse is required if multiple strings are connected in parallel — in this case, a faulty string can be back-fed by the others.

The cable cross-section follows the same calculation as in the rest of the onboard electrical system, based on current, double cable length, and permissible voltage drop. For solar, the permissible drop should be chosen more narrowly than for a consumer: what is lost here permanently reduces the yield.

Typical Mistakes

• Calculating with watt-peak instead of daily yield. Reliably leads to an undersized system.
• Considering only summer. The system will then not contribute for exactly half of the year.
• Underestimating partial shading. A narrow shadow costs more than its area suggests.
• Not checking open-circuit voltage in cold weather. The controller sees more voltage in winter than the summer value in the datasheet suggests.
• Not setting the charge termination voltage for the battery type. The system charges, but never fully.
• Fusing at the controller instead of the battery. Protects the wrong half of the cable.