Start with your own consumption
Take a year of electricity bills, find the kilowatt hours used, and divide by 365. That daily figure is the only number in this calculation that is really yours; everything else is local climate and hardware.
A typical European household lands between 7 and 12 kWh a day, and a home with an electric car or a heat pump can be double that.
The formula
array size in kWp = daily kWh ÷ (peak sun hours × (1 − losses))
Then panels = array size ÷ panel rating.
A worked example
10 kWh a day, 4 peak sun hours, 20% losses, with 450 W panels.
- Array: 10 ÷ (4 × 0.8) = 3.125 kWp.
- Panels: 3,125 ÷ 450 = 6.9, so 7 panels.
What peak sun hours means
It is not daylight hours. It is the number of hours the sun would have to shine at full strength, 1,000 W/m², to deliver the same energy as the whole day actually does. Rough yearly averages: 2.5 to 3 in northern Europe and the UK, 4 to 5 in southern Europe and the southern United States, 5 to 6 in much of India and Australia. The winter figure can be a third of the summer one, which is why a system sized on the average runs short in December.
Where the 20% of losses goes
- Inverter: 2 to 4%.
- Cable and connectors: 1 to 3%.
- Heat: a panel at 60 °C gives perhaps 10% less than its rated output.
- Dust and dirt: 2 to 5%, more under trees or near a road.
- Orientation and tilt: nothing if due south at the right angle, 10 to 20% if east or west facing.
- Shade: not in the 20%. A shadow across one panel can drag down a whole string, and no amount of extra panels fixes it.
Battery size
battery kWh = days of backup × daily kWh ÷ usable fraction
Two days of 10 kWh with a lithium battery you can take to 80%: 2 × 10 ÷ 0.8 = 25 kWh. On a 48 V bank that is 25,000 ÷ 48 = 521 Ah. Lead-acid gives you only about half its capacity, so the same job needs twice the battery.
Will it fit on the roof?
A 450 W panel is about 1.9 m², so seven of them need around 13 m² of clear roof, plus edge gaps and walkways. Work out the real sloping area first: a 10 × 8 m footprint at a 6:12 pitch is 89 m², not 80.
Tools
The Solar Panel Calculator sizes the array and the battery from the figures above. Roof Area Calculator checks the space, kW to Amps Calculator turns the output into current, and the Cable Size Calculator sizes the run from the roof to the inverter.