# How can I calculate from peak power (kWp) to energy per day (Wh/day)?

According to this Wikipedia article (Dutch) there is a rule of thumb that the energy output of a solar system is 850 to 1000 kWh per year per Wp. However, this takes the grey climate of the Netherlands into account. So I cannot just divide by 365 to get the maximum energy possible per day.

How could I calculate the theoretical maximum solar output per day, when only the Wp number of the system is known?

I know that there will be a difference between summer and winter because of the sun's angle, but I would be helped already with a rule of thumb, or some winter/summer factor.

• Take a look at this answer, which includes info about PVWatts, an easy-to-use tool for location-specific modeling of solar arrays.
– LShaver
Feb 2 '19 at 21:29

This ratio of average power to peak power is called the capacity factor or load factor.

It will vary by location, by day, and by technology.

It varies by location: some places are just sunnier than others. In the UK, 10% is a decent capacity factor; so an installation of 10 kWp will, averaged out over several years, yield 1 kW of power on average. In the tropics, the same technology might have a capacity factor of 20%, yielding 2 kW of power on average.

It varies by day: in winter, the capacity factor on a given day might be one tenth of what it is on a summer day, at a given location for a given technology.

It varies by technology: a PV installation that uses tracker motors so that the panels tilt during the day to follow the sun, will have a higher capacity factor than panels which are in one permanently fixed position, at any given location on any given day.

On units: Note that Wh/day is just another unit of power (it is energy per unit time). Wh/day is not a common unit of power, but you do sometimes see kWh/day. You can convert from kW to kWh/day easily: note that h/day is a constant (24 hours per day), so to get from kW to kWh/day, just multiply by 24. i.e. an average power of 1 kW is equivalent to an average power output of 24 kWh/day.

As LShaver wrote in a comment, tools like PVGis and PVWatts will help you calculate how the output of a given installation varies by location and over the year.

• I just input my own numbers in PVWatts and it gives quite a good estimate. I found a REST interface as well, so I'll have some coding to do! Feb 4 '19 at 13:42
• @BartFriederichs Yes they do have an API: developer.nrel.gov/docs/solar/pvwatts/v6
– THelper
Feb 4 '19 at 13:46
• May want to check tht 10% capacity factor. That's even worse than Germany, which is about 800 Wh/W/year Feb 6 '19 at 17:15
• @SherwoodBotsford there are 8760 hours in a non-leap year, so 10% capacity factor = 876 Wh/W/y Feb 6 '19 at 17:21
• Oops. You're right. Feb 7 '19 at 21:57