There are three things to consider in order to choose a Solar panel or create a Solar system.You need to know how much energy your battery can store and then select a Solar panel that can replenish your ‘stock’ of energy in the battery in line with your pattern of use.
How much energy can your battery store?
Battery capacity is measured in Amp Hours (eg 17AH). You need to convert this to Watt Hours by multiplying the AH figure by the battery voltage (eg 12V).
For a 17AH, 12V battery the Watt Hours figure is 17 x 12 = 204WH
This means the battery could supply 204W for 1 hour, or 102W for 2 hours i.e. the more energy you take, the faster the battery discharges.
How much energy will your appliance(s) use over a period of time?
The power consumption of appliances is given in Watts (eg 21" fluorescent light, 13W). To calculate the energy you will use over time, just multiply the power consumption by the hours of use.
The 13W light fitting, on for 2 hours, will take 13 x 2 = 26WH from the battery.
Repeat this for all the appliances you wish to use, then add the results to establish total consumption.
How much energy can a Solar panel generate over a period of time?
The power generation rating of a Solar panel is also given in Watts (eg STP010, 10W). To calculate the energy it can supply to the battery, multiply Watts by the hours exposed to sunshine, then multiply the result by 0.85 (this factor allows for natural system losses).
For the Solar 10W panel in 4 hours* of sunshine, 10 x 4 x 0.85 = 34WH. This is the amount of energy the Solar panel can supply to the battery.