1) How much power do you use?

Appliance	Watts	Hours	Wh	To calculate approximately how much electricity you use every day in watt-hours (Wh), enter the power of each appliance in watts and the average length of time it is on for each day, in hours. The first couple of lines are given as an example; just click in the boxes to replace the values with an estimate of your own power consumption. Clicking the button will add the total Watt-hours for the individual appliances together to give a daily total. Power ratings can often be found on information plates on appliances, or instruction booklets. Note however that these are normally the maximum power draw - a radio may be rated at 50W for example, but only when it is blasting out at full volume. Fridges draw a lot of power, but usually the pump switches on every few minutes rather than runs continually. You may have to make a guess at the amount of time it is actually running for each day.

2) Correcting for system voltage

Battery banks sizes are normally quoted in amp-hours (Ah), which are simply the watt-hours calculated above divided by the system voltage. To calculate how many Ah the above appliances use every day, enter the system voltage and click the button.The vast majority of systems on boats and caravans run at 12V, although there are a few that run at 24V. Trucks normally have 24V systems.	System voltage	Amp hours per day

3) Compare to power from solar panels

Watts				This section calculates the approximate daily power output of a solar panel array so you can compare it to the amount of power you consume. Simply enter the panel power in watts in the 'W' box, and press the button. 64 watts is one of the most popular solar panels we sell so we have put that in as an example. The power output is given in both amp-hours and watt-hours. An average value is given, as well as the values for typical summer and winter days. Remember that there will be a lot of variation from these values - on sunny days you will get more power than the amounts quoted, and less on cloudy days.

	Average	Summer	Winter
Wh
Ah
		Max A

4) So what does this mean?

The figure you have calculated in section 2 is how many amp-hours you are using from your battery bank each day. The calculation in section 3 shows, on the lower row, how many amp-hours we expect a solar panel of a given rated power output to return to your batteries each day. So, if you want to get all your electricity from solar power, this number must be greater than the amount you are using each day.

You may want to enter different panel ratings in the box to see how the output compares to what you are using every day. If you want your solar panel system to run all your power requirements even in the winter, you will probably find you need quite a high solar panel power rating to get the winter amp-hours higher than your daily usage. For most people this is impractical and uneconomic. On the other hand, a relatively small array of panels should be enough to contribute significantly to the power you use in the summer.

The 'Max A' figure that is also given in section 4 is the maximum current that a solar panel array will produce at the voltage you entered in section 2. You need this to size the regulator you need for your system. For example, if the Max A is 5 amps, you need a regulator that can cope with a 5 amp load. The SunGuard regulator we sell is only rated to 4.5 amps, so is not big enough. The SunSaver 6 regulator, rated at 6 amps, would be ideal.