How solar panel output is calculated
Panels × wattage × sun hours × derate. A single 400W panel at 4.5 peak sun hours produces about 1.44 kWh a day — about 43.8 kWh a month and 525.6 kWh a year — after the 0.8 system derate for inverter, wiring, temperature, soiling and shading losses. Ten of them produce about 14.4 kWh a day, 438 kWh a month and 5,256 kWh a year.
Output swings with the seasons. In the US, July output can be roughly double January output for the same panels, because days are longer and the sun is higher. The monthly chart on this page shows that swing for your system.
A peak sun hour is one hour of sunlight at 1,000 watts per square metre — the standard test condition for panels. A place with 4.5 peak sun hours a day gets the same energy as 4.5 hours of perfect noon sun, even though the real sun is weaker in the morning and evening.
Work backwards from your bill with the solar panel calculator, or check how many panels you need with the how-many calculator.