How much power will I get from a 136 Watt solar panel?

You have to be realistic about the amount of power you can get from a solar panel. Solar panels only produce power when sunlight strikes them. So it's pretty obvious that solar panels don't give you any power at night, and that you aren't going to get as much power in overcast conditions, or when the sun is at an oblique angle to the solar panel, as you do in the middle of a sunny day when the panel is pointing directly at the sun.

Solar panels are rated under standard test conditions. All manufacturers are obliged to use the same conditions for testing, so by comparing the rated output you can compare the performance of different solar panels.

Output in summer

The figure you will be quoted by manufacturers is the 'peak power' of the panel. Essentially, it's the maximum you can expect to get from the solar panel in UK conditions - your 136 Watt solar panel will be giving about 136 Watts on a cloudless summer day at noon, if the panel is pointing directly at the sun. Of course, it isn't always sunny, and very few people have tracking devices to allow the panels to follow the sun. However, in the summer in the UK, you can expect to get on average about 5 hours of full power output each day. That means, your 136 Watt solar panel will give you about 680 Watt-hours of energy on a summer day. For a 12 volt system, that corresponds to 56.7 Amp-hours put back into your battery bank each day. On sunny days, you will get more; on overcast days you will get less.

Output in winter

In winter there are fewer hours of daylight, it's often more overcast, and the sun is lower in the sky - which means that sunlight has a longer path through the atmosphere before it hits your solar panel. All these factors combine to reduce the output from the panel. In the depths of a dismal winter you may only get around one hour of full power output on average. So, your 136 Watt solar panel will only be giving you about 136 Watt-hours of energy per day in the worst case. For a 12 volt battery, that corresponds to 11.3 Amp-hours.

Total annual output

If you have a grid-connected solar system, you may be more interested in how much power you can expect to generate over the course of a year. The annual output does vary from year to year depending on the weather, and on your location in the country (we have a map of the output you can expect from a 1000W array if you're interested). Looking at the map, you'll see that a typical return is about 900kWh for a 1000W array. Scaling that to your 136 Watt solar panel means that you can expect about 122 kWh each year from your panel.

• You may also be interested in our Solar Panel Calculator, which will work out how much power you use, and suggest an appropriate solar panel for your needs.