What Size of Solar Panel Do I Need?
Adding solar panels to your home
Harvesting solar energy can be an ideal way to supplement or replace your electricity supply from the National Grid with sustainable energy. The size and amount of panels required will depend on how much wattage you want to achieve and on the position of your house in relation to the sun. The guide below will help you determine approximately what your requirement would be, although this should be used as a guideline only.
How to calculate your electricity usage
Using a recent electricity bill, work out your daily watt usage by taking the total monthly watts used and dividing it by the number of days in that month. If the monthly watt usage is not shown on your bill, call your electricity supplier who should be able to give you this information. Alternatively, walk around your home and add up the wattage of all the appliances that will be used, taking into account the number of hours they will be used for.
Then you'll need to calculate the average amount of sunlight your panels will receive throughout the year. Only use peak hours; morning and evening sunlight will be too weak to generate anything other than a low amount of energy. So, if you think that the panels will receive 5 hours of peak sunlight in summer, 4 hours in spring and autumn and 3 hours in winter, add these together (16) and divide by four to obtain the average (4 hours). You do need to know the average peak sunlight hours in your location but there are online calculators that show this for each area of the UK.
From this you can work out the panel size you will need. Divide your daily watt usage (for example 450 watts) by the average peak sunlight in hours that the panels will receive (4 hours from the example above) to give the amount of wattage you will need your panel to produce each hour (112.5 watts); you should always round up to the nearest 10 so in the above example you would need one or more panels capable of generating 120 watts per hour.
Factors to consider
The area in which you live will determine the amount of peak sunlight you can expect, and this will impact on the number or size of panels you require. Similarly, the efficiency of the panels depends on the angle of your roof in relation to the sun. Panels facing south will provide the most efficient supply of electricity and all calculations are made based on full sunlight, cloudy or hazy days will decrease the efficiency of the panels. As such, the above calculation is a GUIDELINE ONLY and professional advice should be sought before installing solar panels.