The question people actually ask is not how many kilowatts of solar they need. It is whether solar can run the fridge, or the air conditioning, or the well pump. This section answers that appliance by appliance, showing the arithmetic each time so you can substitute your own numbers.
The method: daily kWh divided by (panel kW × peak sun hours × 0.8). A 400 W panel at 4.5 peak sun hours delivers about 1.44 kWh a day after losses. Divide your appliance daily energy by that and round up.
What the derate factor is doing
A 400 W panel does not produce 400 W. It produces 400 W under standard test conditions, which involve a specific irradiance and a cell temperature of 25 degrees, and your roof is not a laboratory. Real arrays lose output to heat, dust, wiring resistance, inverter conversion, imperfect orientation and the fact that the sun moves.
Twenty percent is a reasonable blanket allowance for a decent installation. A shaded or poorly oriented array loses considerably more, and shading is the one loss that is worth solving physically rather than accounting for. Our shading loss calculator and temperature loss calculator break those out separately.
Peak sun hours, not daylight hours
Peak sun hours is the number of hours of full-strength sunlight a day is equivalent to. A long summer day with weak morning and evening sun might total five peak sun hours across fourteen hours of daylight. Most of the continental United States falls between 3.5 and 5.5 as an annual average, and the winter figure can be half the summer one.
Sizing on an annual average works for a grid-tied system, where the grid covers the shortfall in December. It does not work off-grid, where you size on the worst month or you sit in the dark in January.
Every appliance, with the numbers
Each page shows daily energy, panel count at five panel sizes, panel count across seven sun-hour bands, battery capacity for overnight and multi-day autonomy, and inverter continuous and surge requirements. All of it recalculates from one central set of assumptions.
[hed_appliance_index]When to stop calculating and size the system
Appliance-level sizing is useful for understanding and for genuinely isolated loads such as a well pump or an off-grid cabin. For a house, add the loads together and size once, because appliances rarely run simultaneously and sizing each in isolation produces an array far larger than you need.
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