Calculating the requirements for an off-grid solar power system

It is very important to make good calculations when you design a solar system mainly because it is not possible to mix old batteries with new ones so you have to get it right first time.

Before you use this calculator you will need to work out how many kWh you use each day. A kWh is the use of 1000 watts for 1 hour. kWh is the unit of energy consumption which appears on electricty bills. One way of finding your daily kWh is to find out how much power each of your electrical devices uses. To calculate the daily kWh use this formula. (hours used X watts) divided by 1000 = kWh. It is possible to buy devices which measure electrical useage which can help you. Click here for info

It is important that we take into account the maximum and minimum usage. There may be some flexibility in your power usage. For example if it is cloudy you may choose to not use your washing machine and turn off all devices that are not absolutely necesary.

The following letter system will explain all the values to put into the form.

A Maximum usage. The amount of power you would like to use if there is plenty of sun and you can use your high power appliances such as pool pump, washing machine, chain saw etc This probably happens in the summer.

B Minimum usage. The lowest amount of power you can use if it is cloudy and you are reducing your use to a minimum.

C Independance days. The amount of days the batteries need to last if there is no sunlight. It depends on how mission critical you are. If you are a mountain hospital in a cloudy country then you need a lot of days. If you have a weekend cabin and you can go home if it is cloudy then you don't need many.

D Maximum power input. Everywhere on Earth receives a certain amount of sun power per metre per day. The best place to find the kWh/m2/day of your location is using online calculators. Click here for a list of links. Here you put the figure for the sunniest month which will be in the summer.

E Minimum power input. This is the same as above but we put the figure for the least sunny month which will be in the winter.

F Fudge factor. Most experts aggree that the figures need to be modified by about 1.5 due to inefficiencies.

G System voltage. Most domestic solar systems are either 24 or 48 volts. Panel wattages between 300 watts and 1000 watts use a 24 volt battery. Panel wattages higher than 1000 up to 4000 use 48 volt

FIRST WE PUT DOWN INFORMATION ABOUT OUR SYSTEM

A . Maximum usage kWh per day in summer

B . Minimum usage kWh per day in Winter

days of independence (for when there is no sunlight)

D . kWh/m2/day = Maximum sun power at my location in Summer.

E . kWh/m2/day = Minimum sun power at my location in Winter.

F . Fudge factor due to inefficiency and 50% max depth of discharge

G System voltage