PV array size from daily load

Why an array is sized in peak sun hours rather than daylight hours

The answer arrives in kilowatts-peak, and the suffix is doing real work. A kWp is the output a module family produces under Standard Test Conditions — irradiance of 1,000 watts per square metre on a cell held at 25 °C — which is a laboratory reference point, not a promise about a roof. An array almost never delivers its nameplate in service; nameplate is simply the common currency in which arrays are bought, quoted and compared, and it is the currency the rest of this cluster trades in.

Peak sun hours compress a whole day of varying sunlight into an equivalent number of hours at full laboratory strength. A site quoted at 5.5 PSH receives, over a typical day, the same energy it would receive from five and a half hours of STC-grade sun and darkness the rest of the time. That compression is what makes the sizing a single division — but the figure is an ANNUAL AVERAGE, and averaging is where it bites: a mid-latitude site can see winter days at a fraction of its summer figure, so an array sized on the average will overshoot the bright months and fall short in the dark ones.

The performance ratio concedes that a real system is not the laboratory. Cell temperature above 25 °C, inverter conversion, wiring resistance, soiling, mismatch between modules — each takes its share, and the performance ratio is the fraction of the ideally available energy that survives all of them together. It sits in the denominator, so a lower ratio grows the array: the machine is deliberately oversized to cover its own losses.

The structure of the division is worth reading before any values go in. Load sits alone in the numerator, so the required array scales directly with demand; sun and performance sit together underneath, so halving either doubles the array. The pack also declares the reverse workflow: fix an array size — a roof or a budget already decided it — and the same relation, run backwards by the engine, returns the daily load that array can support.

When this calculation is used

• Turning a year of electricity bills into a first array size, before any installer has visited the site.
• Comparing candidate sites or roof orientations, where the peak-sun-hours figure is the only input that changes.
• Pre-sizing an off-grid system, where the array must carry the whole daily load before battery sizing even begins.
• Sanity-checking a quotation: an installer’s proposed size, the household’s stated consumption and a plausible performance ratio must be consistent with one another.

Worked example

Take the system this pack uses as its own anchor: a household averaging 22 kWh of consumption a day, at a site with 5.5 peak sun hours, assuming a performance ratio of 0.8 for a well-designed grid-tied installation.

The figure that comes back is the nameplate array size, in kWp, that covers the stated load at the stated sun over an average year. It is the number the rest of this cluster follows: the yield page runs it forwards into an annual production, the measured page audits the ratio assumed here, and the payback page prices the result. Read it as a screening size to set against roof area and budget, not as a completed design.

Now hold the load and drop the peak sun hours to a winter-grade figure. The required array grows in inverse proportion — which is the arithmetic behind a familiar trade-off: size on the annual average and accept a winter shortfall, or size on the worst month and overproduce the rest of the year.

What each input represents

Assumptions and limits

• Peak sun hours enter as one annual-average figure, so seasonal swing is invisible: the sizing balances the year as a whole, not its worst month.
• The performance ratio is a single constant, although the losses it bundles — temperature above all — genuinely vary through the day and the year.
• The load is an average energy figure. Nothing here sizes for peak power, so the inverter and any battery need their own calculations.
• The relation knows nothing about the roof: whether the resulting array physically fits, and how many modules it takes, are separate checks this pack carries as sibling calculators.

What the guards protect against

• The performance ratio must be strictly below one — the pack ships that refusal as a declared test vector. A ratio of one describes a lossless system, which does not exist, and the same ceiling catches the commonest slip: a percentage entered where a decimal fraction belongs lands far above the bound and is refused rather than sizing an array that is wrong by orders of magnitude.
• Peak sun hours are bounded to the range real sites occupy. An annual irradiance total, or a daylight-hours figure for a summer day, falls outside the band and is refused as a unit mistake rather than silently sizing against the wrong quantity.
• The daily load must be greater than zero and is capped at a utility-scale ceiling. A zero load has no array to size, and a figure beyond the cap is far more likely an annual total entered as a daily one than a real household.

Provenance

Standard PV sizing relation, kWp = daily load / (peak sun hours × performance ratio)

Direct division of the average daily load by the product of peak sun hours and performance ratio, returning the required nameplate array size in kWp; the declared reverse workflow solves the same relation for the supportable daily load at a fixed array size.

Screening and reference material, to be checked against the governing standard and a qualified engineer; not a design determination. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.