How design your solar system

How To Get Free Electric…

The title may a little bold but its almost true.
But before we get to the situation of free electricity we need to get a pen and paper and a calculator.

You will have read lots and everyone has a view on solar panels, the size on an inverter and more. But when you are off grid the first thing to think about is always how much power do I need?
This is quite easy to establish as this information as it is found on the information tag on all products. Then decide for how long you are going to use the product for. 

Here are my products for my planned garden shed. You will see I have a laptop that uses 100w and in my shed I work on it 5 hours a day. So we take the 100w and times it by 5 hours to give is Watt Hours or Wh Then below are more products in the shed. 

Laptop – 100w x 5h = 500Wh
LED – Light 8w x 5h x 2 Bulbs = 80Wh
Phone – 6w x 1h = 6Wh (Phone would be charged in 1h)

This totals to 586Wh

Now, not every day is going to be a sunny day and these are known as Days of Autonomy. These are days that the Solar Panels don’t fully recharge the Battery.
So that we can still go and hide in the shed after a rainy day or two we need to add some Days of Autonomy.
Today we are going to go with 2 days of no sunshine and this means we have simply multiply by 2.

We now require 1172Wh of power.

But now we need to convert the Watt hours to Amp hours.
This is 1172Wh divide by 12v as we are going to use 12v Battery’s and that equals to 97Ah

Now we could just buy a 100Ah battery but a Lead Acid / AGM Battery can only be discharged by 50%. Any more and they get upset and will fail early in their life. So we need to double the size of the Battery required. 

97Ah x 2 = 195Ah 

The calculations are done and you would need to purchase…
4 x 50Ah Deep Discharge AGM Battery’s…
2 x 100Ah Gel Batteries

And not forgetting a Charge Controller

gel battery 100ah 12v Gel Deep Cycle Battery 

Solar Panels
Now we have to established the power required to re-charge the Battery in one day.
This calculation is quite simple. In the UK we get approx. 4h of good sunshine each day. This is not when the sun comes up and goes down. Its about good sunshine on the Solar Panels. Work 4h for good results. Go lower and you may end up in the dark. 

Take the 1172Wh and divide by 4 = 293Wh This is the solar power required per hour. 

We need a Solar Array, or collection of Solar Panels to be a little over this number.
I would go with 3 x 120w Solar Panels as this gives you extra charge capacity of 360w. 



Charge Controllers
Next up are Charge Controllers, again this is quite easy

The Battery is 1172Wh
The Solar Panels are 360W    

So we take the 360w and divide it by the 12volt Battery we plan to use.
This gives us 30Amp

I would use a 30Amp MPPT Charge Controller

Now you get the math’s play around with 24v systems or even 48v.
And as a side note, the more volts the less Amps so the smaller the wires required. 
But all of that will soon be on another technical page…


I hope this helps and we are always here to help you select the right one for you.