[leonidasrup]

It has lot to do with geographic latitude and weather patterns. The amount of electric output per amount solar installed strongly affects the profitability of solar installation (if you don't count of government subsidies).

You get the following output on average each year

Denmark 1000 kWh/kWp

South Germany 1200 kWh/kWp

South Spain 1700 kWh/kWp

Egypt up to 2000 kWh/kWp

[Jensson]

And summer isn't when you need the power anyway, so its very inefficient since northern winters has barely any sunlight at all, its close to 0 from solar power then. In warmer countries you want power in the summer for AC during the day, so there it matches usage, but in northern countries solar isn't very useful at all.