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Calculating power using current and voltage 

There are three ways of writing an equation for power, current and voltage:

Power = Current x Voltage so:
P = I x V 
I = P divided by V 
V = P divided by I

P = power in watts (W) P = power in milliwatts (mW)
V = voltage in volts (V) V = voltage in volts (V) 
I = current in amps (A) I = current in milliamps (mA)

A milliwatt is a 1000th of 1 watt
A milliamp is a 1000th of 1 amp

So, if an 80W – 12V Monocrystalline panel in peak light conditions will give approximately the following current using one of the above equations. We have the 2 variables, watts and volts so the following equation can be used. If the working voltage of the panel is 18 volts (to enable charging of the battery). I = P divided by V I = 80 divided by 18 = 4.44 amps So as the formula states that a 80 watt solar panel in peak light conditions will supply a current of 4.44 amps an hour.

Size Matters!

This depends on the load you have connected to the battery. You will need to calculate the watt-hour or amp-hours you are using in a period of time, then having a rough idea of the amount of direct sunlight your solar panel will receive each day. The solar panel needs to supply approximately 20% more than your needs to compensate for variables, such as cloudy days etc. Obviously the winter will have shorter sunlight hours per day than the summer, but you can work on an average of approximately 4 hours sunshine a day in the UK over a year.

Cloudy Day?

Yes, a solar panel does produce electricity even when it is not placed in bright sunlight. On a normal cloudy day there is always enough so-called diffuse light, by which the panel will produce electricity. However, the production of electricity is not as high as when the panels are placed in bright sunlight.


Clean using a non-abrasive cleaner. In the long term check the sealing especially in marine use and reseal with a silicon sealant if damage is suspected. Check battery connections periodically when you check battery levels. Fuse holders and connections should be kept dry and clean