Annual PV yield estimate

What a production forecast is made of, and which term carries the honesty

This is the sizing calculation turned around. Sizing divides a load by sun and losses to demand an array; the estimate multiplies an array by the same sun and the same losses to promise an energy. The two share their vocabulary deliberately — same peak sun hours, same performance ratio — so an array sized on this cluster’s first page and estimated on this one tells a consistent story, and any change of assumption has to be made in both places or the inconsistency shows.

The product of nameplate and peak sun hours alone would be the IDEAL year: the energy the array would deliver if the field behaved like the laboratory. The performance ratio is the term that carries the honesty. Multiplying by it concedes cell temperature, inverter conversion, wiring, soiling and mismatch in one factor — so the estimate is only as credible as that single number, and quoting a yield without stating the ratio behind it is how solar forecasts mislead politely.

An annual figure smooths what a year actually looks like. Production concentrates in the bright months and thins in the dark ones, and the estimate deliberately averages that swing away: it is the right quantity for bills, offsets and payback arithmetic, which settle over a year, and the wrong quantity for asking whether the lights stay on in December. The pack carries monthly and seasonal siblings for that question.

The relation is linear in every input, and linearity cuts both ways. It makes the estimate transparent — double the array, double the year — and it means every input error passes straight through to the answer undamped. An optimistic sun figure or a flattering performance ratio does not average out; it propagates, and then it propagates again through the payback calculation this estimate feeds.

When this calculation is used

• Forecasting the production of a proposed array before purchase, as the number a quotation’s promises are tested against.
• Feeding the economics: the annual yield is the quantity the payback and savings calculations in this pack consume.
• Solving backwards for the array a production target demands — the declared reverse workflow, for targets set in energy rather than load.
• Comparing candidate sites at a fixed array size, where only the peak-sun-hours figure differs between runs.

Worked example

Run the cluster’s reference system forwards: the 5 kWp array the sizing page’s scenario called for, at the same site with 5.5 peak sun hours, keeping the assumed performance ratio of 0.8.

What returns is a year of production in kilowatt-hours. Read it as the central estimate for an average year — the number to set against annual consumption to see how much of the bill the array displaces, and the number the payback page will convert into money. It is not a floor, not a promise for any particular month, and not resilient to a wrong performance ratio.

Nudge only the performance ratio downwards, as a dusty site or a hot summer would. The year shrinks in exact proportion — no other input softens it — which is why the difference between an assumed ratio and a measured one is the first place to look when production disappoints.

What each input represents

Assumptions and limits

• One annual-average peak-sun-hours figure stands in for the whole year, so the estimate says nothing about seasonal distribution.
• The performance ratio is constant across the year, although the temperature losses inside it are largest exactly when the sun is strongest.
• The figure is a first-year, degradation-free estimate. Module output declines slowly with age; the pack’s degradation calculators handle later years.
• Losses beyond what the performance ratio bundles — extended outages, inverter clipping under a deliberately undersized inverter — are not modelled here and have their own siblings in the pack.

What the guards protect against

• The performance ratio must lie strictly between zero and one — the pack declares the refusal at zero as a test vector. A zero ratio describes a system that delivers nothing, and a ratio of one or more describes one with no losses; neither is an estimate worth returning.
• Peak sun hours are bounded to the band real sites occupy, which refuses annual totals, irradiance in other units and daylight-hours figures instead of folding them silently into a wildly wrong year.
• The array size must be positive and below a utility-scale cap, so an entry in watts rather than kilowatts — a thousandfold slip — lands outside the band and is refused.

Provenance

Standard simplified PV yield form, annual kWh = kWp × peak sun hours × performance ratio × 365

Product of nameplate array size, daily peak sun hours and performance ratio, scaled by 365 days, returning the estimated annual energy in kilowatt-hours; the declared reverse workflow solves for the array size a target annual yield demands.

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.