Use a simple inventory format (on paper or a computer spreadsheet) to list how many amps each 12-volt device requires. Look at the data plate on each device; you'll find that some items are listed in watts, and some in amps. Convert watts to amps with this simple formula: amps = watts/volts. So, if you have a 60-watt light bulb on a 12-volt system, it uses 5 amps in one hour.
Estimate how much time you’ll use each device in 24 hours, so you’ll know approximately how many total amps your devices will consume in a typical day. It takes some guesswork to determine how long a device will operate each day. For example, if your water pump is rated for 4 amps, it would use 4 amps each hour. But the pump doesn’t run continuously. You might estimate that your water pump is on for a total of 15 minutes during a day, including time for washing dishes, cooking and flushing the toilet several times. Express that time in hours or a fraction of an hour. That means your water pump will use 4 amps x 1/4 of an hour (15 minutes), or 4 x .25, or 1 amp-hour each day.
List each powered device in your inventory on its own line. For example, each light bulb should be recorded separately, because you probably won't use them equally. The bathroom light is only going to be used occasionally, while a reading lamp by your bed might be on for an hour each evening. If there are two lights in a single fixture, you can group them on one line. As an example, a fixture holds two 40-watt bulbs, for a total of 80 watts per hour. You estimate that the lights will burn for two hours each day. Convert the watts to amp-hours: 80/12 = 6.6 amps. Multiply this by 2 hours to obtain an answer of 13.2 amp-hours.
Sum the total number of amps required by the devices you'll use in a typical 24-hour period and triple this number. This tripling is a rule of thumb with solar-powered systems. Let's say your devices total to 34 amp-hours. Triple this and you will see that you should plan on a battery pack of at least 102 amp-hours. This allows for occasional higher usage or cloudy days without exhausting the batteries.
Determine how many watts your solar panel will need to generate to keep the batteries charged. A typical panel with 32 photovoltaic cells generates around 48 watts at 14 volts, or about 3.5 amps each hour. This is in direct sun, which the panel will probably receive for no more than five hours each day, for a total charge of about 17 amps into the battery pack.
For your example system above, using approximately 34 amp-hours each day, two 48-watt solar panels, each generating 17 amps, could replace the energy to the batteries that you would use in an average day. Of course, solar panels that produce more watts are available; this example gives you the information you need to size your own system.
