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How much power will I get from a 68 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 68 Watt solar panel will be giving about 68 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 middle of summer in a good location in the UK, you can expect to get on average about four hours of full power output each day. That means that your 68 Watt solar panel will give you about 272 Watt-hours of energy on a summer day. For a 12 volt system, that corresponds to 22.7 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 68 Watt solar panel will only be giving you about 68 Watt-hours of energy per day in the worst case. For a 12 volt battery, that corresponds to 5.7 Amp-hours.

These figures assume that the panel is facing south, at the optimum angle, and is unshaded. You may get less if your conditions aren't perfect. They also assume the use of an MPPT regulator - with a standard regulator you will typically get between 10 and 20% lower output. Also, further north in the UK the difference between summer and winter will be greater.

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, but a typical return is about 900kWh for a 1000W array. Scaling that to your 68 Watt solar panel means that you can expect about 61 kWh each year from your panel.