What will this system actually generate?

Month-by-month output with every loss shown separately, degradation compounded properly, and a range rather than one falsely precise number.

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What will this system actually generate?

Month by month, with every loss shown separately and applied exactly once.

Check your module warranty — it states a guaranteed figure.

Solar resource

Peak sun hours a day, by month. Regional figures are published for most places.

Optional — shading by month, if trees or a neighbouring building block part of the year.

Losses

Month by month

Where the energy goes

A loss waterfall. Each row takes its percentage of whatever is left, which is why they do not add up to the total.

Output as the panels age

Does it pay back? → ← Back to sizing
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Planning estimate — not an engineering or installation plan. Results depend on the information and assumptions you enter. Output, savings and equipment behaviour vary with weather, shading, tariffs, temperature, ageing and installation. Verify equipment limits against current manufacturer documentation. Permits, structural capacity, electrical protection, conductor sizing, grounding, rapid shutdown, fire access, utility interconnection and code compliance must be reviewed by qualified professionals and the applicable authorities.
How this is calculated

monthlyProduction = arrayKw × peakSunHoursPerDay × daysInMonth × retainedFraction

retainedFraction = product(1 − eachLossFraction) — losses multiply, they do not add. Two 10% losses retain 0.9 × 0.9 = 81%, not 80%.

Degradation compounds: yearN = year1 × (1 − rate)^(N−1). At 0.5% a year, output after 25 years is about 88.7% of year one — not 87.5%, which is what subtracting 25 × 0.5% would give.

This is a peak-sun-hour estimate, not an hourly weather simulation. It cannot model a specific year's cloud cover, the exact effect of a tree's shadow moving across an array through the afternoon, or inverter clipping on the brightest days. For sizing and for deciding whether a project is worth pursuing it is adequate; before signing a contract, ask your installer for a modelled estimate and compare the two.

Why the band, rather than one number

Two identical systems on neighbouring roofs will not generate the same amount, and the same system will not generate the same amount two years running. Weather alone moves annual output by roughly ±10% around the long-run average. Quoting a single figure to the nearest kilowatt-hour implies a precision the underlying physics does not support, so this shows a range and tells you what the range represents.

What the loss waterfall is for

It shows where the energy actually goes, in order. Most people are surprised that inverter conversion is usually the largest single loss and that soiling matters more in dusty or pollen-heavy places than shading does. If one row dominates, that is the row worth spending money to fix — and often the only one you can change.

Limitations

Orientation and tilt are not modelled separately here: your peak-sun-hour figure is assumed to be the plane-of-array value for how the panels will actually sit. If you use a horizontal-surface figure for a steeply pitched or badly oriented roof, this will overstate output. Nothing here accounts for snow cover, inverter clipping, or a specific year's weather.