Product Categories

Newsletter Subscription
  • Get our FREE guide "How To Get The Most From Your Solar Panel" plus the latest SelectSolar news via email.
  • * Indicates a required field.
    (Please note: we value privacy & will not share your details with any third party

How To: Size A System


Sizing a system


The aim is to balance the power going in from the solar panel with the power going out of the battery over a period of days or weeks (depending on how it is being used). A 10W panel will give 10W (0.6A @ 16.5V) over an hour under standard test conditions (1000W/m sq and 25oC). In the UK allow around 4 hours equivalent sunshine in summer and 1 in winter. In Winter a 10W panel will give 10W over a whole day, whereas in summer it will give 40W. These are fairly conservative figures – some companies use up to 6 hours in summer. You can do the same calculations with the Amps (people are often more familiar with Amps).


Charging appliances eg mobile phones – the bigger the panel the faster the charge. Beware, modern laptops are power hungry and often cannot be charged directly from a solar panel. We always recommend using a small 12V battery.



Some simple steps for sizing a 12V system:


  1. Find the Wattage of your appliances. List all the 12V electrical appliances you’ll use in a typical day, and find out how many Watts they each consume. Usually this is on the appliance or in its handbook. If you can only find a figure for Amps, simply multiply this by 12, to convert it to Watts.

  2. Calculate your daily total Watt-hour requirement. Estimate how many hours you would use each appliance for over a typical week, then divide by 7 for a daily rate. Multiply each appliance’s wattage by the hours you’ll use it for in a day. Then add all the totals together to get the final daily total Watt-hours you require



  1. Next calculate your panel size. Simply divide the daily total Watt-hours you require by the hours of usable light you expect in an average day. This will give you your minimum panel wattage. In the UK, allow 1 hour of light in winter, rising to 4 hours by mid-summer.



  1. Then your battery size… Multiply your daily Watt-hour requirement by 7 to create a weekly requirement, and divide this by 12 to convert back to Amp Hours, which batteries are rated in. Multiply by two to give the correct battery size.

  2. And finally, your charge controller. Size your charge controller according to the Amps produced by your panel. Calculate the Amps produced by dividing the panel wattage by 16.5.



  1. A worked example. In one week you want to run a 65W television for 4 hours, and an 8W light for 5 hours. Your daily Watt-hour requirement for the TV is 65 x (4/7) = 37Wh; and for the light you require 8 x (5/7) = 6Wh. Your total daily requirement is thus 43W. You only intend to use the system in summer, so you need a panel that is 43/4 = 11W or more. Your battery size needs to be (43 x 7 x 2)/12 = 50Ah. And you need a charge controller suitable for a solar input of at least 11/16.5 = 0.7A