Ohms Law

Ohms law can be useful for calculating your battery storage limits and the power delivered to it from the solar panel(s) against the power needed to run the lights and the electrical equipment you require.


P (watts) = Amps x Volts

I (Amps) = Watts ÷ Volts

V (Volts) = Watts ÷ Amps


Example 1

Lets assume you have an 100 amp hour leisure battery with one 30 watt solar panel charging it, to find how many amps the solar panel will deliver in a full sunshine hour we need our Ohms Law above. We need to convert the solar panel watts to amp hours, so to find the amps we need to divide watts by volts:-

i.e. 30 watts (solar panel) divided by the panels nominal voltage17.5 Volts (approx) = 1.7 Amp's per hour. This would be under exceptionally good circumstances (brilliant sunny day, not to hot or cold) and the likely average would be closer to 1.25 Amp's per hour.

If your total energy requirement is say a total of 200 watts, if we convert this to amps (Watts ÷ Volts) = 16.67 amp's per hour we can now work out how many hours we will get from our storage battery. We can see clearly that if using the 100 amp hour leisure battery mentioned above that we can maintain our power requirement for between 5 and 6 hours. This of course would not be recommended as it would bring the battery to a very low point and may mean permanent damage has been done.


Example 2

Often domestic electrical equipment is stated in amps (or milliamps) especially when the consumption is low. Ohms Law can be useful to convert all the milliamps into watts to match the other electrical equipment you are trying to calculate your usage.

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