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How To: Calculate Power Requirements

How to calculate your solar power requirements:

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.

Battery condition and weather conditions affect performance.

How much energy can your battery store? 
How much energy will your appliance(s) use over a period of time?
How much energy can a Solar panel generate over a period of time?

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.

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.

The power generation rating of a Solar panel is also given in Watts (e.g. 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.
The bigger the battery, the more energy you can store so you can draw more power for longer.
Look for energy-efficient appliances (low power consumption) so the battery lasts longer.
Large panels can collect more energy in less time, but a small panel that is permanently exposed to the sun may be suitable.

*Note: 4 hours of sunshine per day is the average measurement in the UK between March and May - based on the information from Sandia National Laboratories - Albuquerque - USA.

Which Solar panel is right for you?

Choosing the right Solar panel for your needs is like choosing a battery. In the same way that a bigger battery will provide more power for longer, a larger Solar panel will collect more energy in less time.

The right size of panel will depend on variables such as the power required by the appliance, the length of time you want to use it and how much sunshine you get at the time of year.

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