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How to calculate the solar power I need?

To ensure your household battery is properly charged, the capacity of the solar module must be calculated according to the expected daily energy consumption in Ah.


A television with power consumption of 45W is used for 2 hours on a 12v battery. The total energy consumption then is 2h x 45W = 90Watt/Hour. In Ah this means 90Wh : 12V= 7.5Ah.
The calculation of needed solar power is most easy by multiplying the amount of Ah with factor 3, in above case. Rule of Thumb: 7.5Ah x 3 = 22.5Wp (Watt peak).

If you know the current consumption of your devices in Ah, you can use the following method to
approximate the value of a necessary solar power: Daily consumption (Ah) x 12.6 V : 4 = ....... Wp.
This is the solar power (Wp) you would have to store up, based on our experience, to compensate for the daily consumption between spring and fall.

To compare both methods on the above example:

Rule of Thumb:    7.5Ah x 3= 22.5Wp (Watt peak).
Daily consumption: 7.5Ah x 12.6 V : 4= 23,6 Wp (Watt peak).

Calculation example of an average daily consumption:

Device: Power input:   Runtime:  
TV 45 W: 45 W : 12 V = 3.75 A x 1.5 h   = 5.6 Ah
Light 16 W: 16 W : 12 V = 1.30 A x 5.0 h   = 6.6 Ah
Water pump 25 W: 25 W : 12 V = 2.00 A x 0.5 h = 1.0 Ah
Radio 15 W: 15 W : 12 V = 1.25 A x 2.0 h   = 2.5 Ah
Total       15.7 Ah

NOTE: of course, there will always be vehicles where a necessary solar power cannot be fitted for roof space reasons. In that case there is no other choice than to install the maximum possible solar power and compensate the missing energy with additional battery capacity.

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