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Solar panel & battery sizing
There are three things to consider when choosing a solar panel & battery combination:-
1) How much energy can be stored in a leisure / solar battery?
Battery storage capacity is measured in Amp Hours (eg 20 Ah)
This figure can be converted into Watt Hours by multiplying the Amp hours by the battery voltage
For a 20AH, 12V battery the Watt Hours figure is 20 x 12 = 240WH
In other words the battery could supply 240Watts for 1 hour, or 120Watts for 2 hours
It's not a good idea to totally drain a battery so leaving about 50% reserve power in it will lengthen it's life. 
For the same 20AH, 12V battery the better Watt Hour figure is 20 x 12 x 1/2 = 120WH
2) How much energy will the appliances you want to run from the kit consume over a given period of time?
Power consumption of household appliances is given in Watts (eg 21" fluorescent light, 13W)
Calculate the energy used over a period of time by multiplying the power consumption by the hours it's switched on.
A 15W light fitting, on for 2 hours, will take 15 x 2 = 30 Watt Hours from the battery.
Repeat the same process for all the appliances you want to run using renewable energy and add the results to calculate total consumption.
3) How much power can a solar panel produce in a given period of time?
The power rating of a solar panel is also given in Watts (eg 20W) To calculate the energy it can produce and store in the battery, multiply the Watts by the hours exposed to sunshine then multiply the result by 0.75 (factor to allow for performance and power losses within the kit's components).
A 20 Watt Solar panel in exposed to 4 hours of sunshine is 20 Watts x 4 Hours x 0.75 = 60Watt Hours
60 Wh (Watt hours) is the amount of power the solar panel in this example can supply the battery or inverter (if it's a grid tie system)

When choosing the right components for a solar power or lighting kit it's also important to take into consideration a number of other important factors:-

a) Always size the components for the months when solar irradiation is at it's lowest (Normally the winter months)

b) Some appliances consume more power than they are rated for. Use a power/consumption meter to monitor the power used over a 24Hour period. This is especially the case for fridges and freezers.

c) Running cookers or heaters using solar power is invariable unrealistic. If you want to heat up water use solar panels that are designed for that purpose. Converting solar energy into electricity then into heat is extremely inefficient process.
For some more help and advice please just contact us
We are always pleased to help!
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