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How many solar panels
Here's how to calculate how many solar panels you will need for your solar power or solar lighting kit:-

Let's start by creating an example project to provide solar lighting for a workshop. The workshop is located near the town of Manchester and has no mains electricity. The requirement is to provide lighting all through the year for a workbench and general room lighting. The workshop bench light will be used for 2 hours every evening and the general lighting will be on for 1/2 an hour, each night 7 days a week as well.

Come rain or shine we need lighting available so the batteries should be capable of supplying power for at least 4 days without being recharged by the solar panels. This is commonly know as the autonomy of the solar lighting or solar power system. 

Here are our energy requirements:-

Bench light - 2 Watt LED/ Halogen style spot bulb

This light is on for 2-3 hours so the power requirement is (3 Hours X 2 Watts) = 6 Watt hours

Room light - 10 Watt, 12Volt CFC Lamp

This light is on for 1/2 an hour so the power requirement is (1/2 Hour X 10 Watts) = 5 Watt hours

We need 11 Watt hours each day for 4 days (autonomy) which is 44 Watt hours in total (4 days X 11 Watt hours)

With this figure we can calculate the size of our battery for this solar lighting kit. As a rule of thumb it's never a good idea to regularly drain a battery more than 50% as doing so will dramatically reduce it's life so we need a better twice as big as our requirements or 88 Watt hours.

Batteries are sized in Amp hours and not Watt hours so how do you calculate the Amp hour requirements? That's easy, you simply divide the Watt hours figure by the battery voltage to get the Amp hour figure:-

88 Watt hours divided by 12Volt battery = 7.33 Amp hours

So if we have the misfortune of many many days without sunshine and the battery is getting very flat (not 50%) we'll need to be able to put 7.33 Amp hours of power back in it in one days worth of sunshine. This is a good figure to use to calculate the size of the solar panels for this solar lighting kit.

A 50 Watt solar panel unfortunately doesn't produce 50Watts unless it's in absolutely idea conditions. We would use a factor of about 0.7 for Manchester so a 25Watt solar panel will only produce 25 X 0.8 = 20Watts. If the panel is in sunshine for 5 hours it will produce 5 X 20 Watts = 100 Watt hours or power. 

We need 88 Watt hours to that's fine if not a little too large. In our experience a little too large is a good thing because you should always consider your future requirements for the solar lighting or power kit. 

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