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How to Calculate what size 12v Panel you need

How do I calculate the amount of Watts I require?


For an On-Grid system it is down to budget and space available. 


Off-grid, firstly you need to calculate the amount of power you will require. This is done by finding the watt rating of all the devices you intend to run. Then times this value of each device by the time you intend on running it for. This will give you a total watts draw value. 


eg. You have a shed light that you wish to run on solar. The bulb uses 11w. You wish to use it for 20 hours a week. 


11 x 20 = 220w


To make this value into the amount of power you will draw from the battery you divide by 12 (since the battery is 12v)


220 / 12 = 18.33 round up = 19


You then need to take into acount when you plan to use it. If it is all year round then you need to make the calculation based on the winter value of 0.05ah /w/day. If only during the summer then 0.7ah/w/day. Times the necessary value by 7 to account for the full 7 days of the week.


For only summer use:


0.7 x 7 = 4.9 ah / w / week


Then take the AH figure you require for the weeks power. ie 19ah and divide this by the generation value. ie 4.9


19 / 4.9 = 3.87watts PV required


Winter use or all year round:


0.05 x 7 = 0.35 ah / w / week


19 / 0.35 = 54.3 watts PV required 


As you can see there is a fair difference between winter and summer values in the UK. Please be sure to take this into account when calculating. Please contact us by email info@sunstore.co.uk or on 0845 1800 654 if you are unsure or wish to discuss your project further.

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