System Sizing

Array size is daily consumption divided by peak sun hours, divided again by a derate factor. The derate factor is where most estimates go wrong, and it is not a small correction.

0.77 is the PVWatts default from NREL. It bundles soiling, heat, wiring losses, module mismatch and inverter efficiency. Size an array without it and you will be short by roughly a quarter — which shows up as a system that meets its numbers in April and misses them all winter.

Peak sun hours is an annual average, and December is not average

Winter output across most of the US runs 40–60% of the annual average. A grid-tied system sized to the average is fine, because the grid covers the difference. An off-grid system sized the same way runs out in January.

Roof area is not panel area

The figures here give panel area. Add 15–25% for row spacing, fire setbacks and obstructions — and check the roof plane actually faces within about 45° of south. A due-east array is not a due-south array with a small penalty; it is a meaningfully different system.

The inverter is sized smaller than the array, on purpose

A DC-to-AC ratio around 1.2 is standard. Arrays hit their nameplate rating for only a few hours a year, so an inverter sized to peak DC output spends its life underloaded, which is both expensive and less efficient. Some clipping at midday in June is the correct trade.

What this does not size

Batteries, wire, and the loads themselves. Those are separate calculations, and each has a way of going wrong that array sizing does not reveal — a correctly sized array with undersized cable delivers a fraction of what it makes.