How the solar production calculator works
The production chain
Annual kWh = kW × sun-hours × 365 × derate (0.78) × orientation × shading. Every factor is measurable, which is why honest quotes come with a production model (PVWatts or similar) rather than a round number. Demand to see the model and its shading assumptions — "we assumed minimal shade" is where optimistic quotes are born.
Orientation and shade do the damage
South-facing at a good tilt is 1.0; east/west lose ~15%; north-facing loses ~30%. Shade is worse and nonlinear — a single branch across the array for four afternoon hours can cost 25%+ of production, because panels (or string inverters) throttling in series drag whole sections down. Micro-inverters and optimizers mitigate but never erase it.
Sanity-checking a quote
Take the quote's promised production, divide by system kW: you should get roughly 1,100–1,600 kWh per kW per year depending on region. Below that range with a clear roof means the model is padded somewhere — or your site is shadier than you think. Above 1,800 is fantasy.