What Happens to Excess Solar Energy You Don’t Use?

So, you've got (or are thinking about getting) solar panels on your roof…but what happens to the electricity they make when you aren't aren't using it all? Solar panels create power whenever the sun is out, and that's often at times when you aren't actually at home running things. Knowing how this extra solar power is handled will help you decide on the best way to set up your system and get the most benefit, both financially and for the environment.

Where does this spare power go?

Solar panels make ‘direct current’ (DC) electricity, and your inverter changes this into ‘alternating current’ (AC) - the kind you use in your house. When your panels are making more electricity than you’re currently using, that extra needs somewhere to go.

In most British homes with solar panels, one of three things happens to it: it goes into the national electricity grid, it fills up a battery in your house, or it's used for something else in your home, like warming your water. Most of the time, it's sent back to the grid, though that's not necessarily the most profitable thing to do with it.

The Smart Export Guarantee (SEG) has been in place since January 2020 and makes bigger energy companies in Great Britain offer you a price for the renewable electricity you send to the grid. It took the place of the Feed-in Tariff (which closed to new people in March 2019).

With the SEG, your energy supplier will give you money for each ‘kilowatt-hour’ (kWh) of surplus solar power you send back. The amount they pay varies hugely, from as little as 1p per kWh to more than 15p per kWh. Some companies even tie their rates to how much electricity is costing to buy on the wholesale market and those prices change throughout the day.

To get the SEG, you’ll need a smart meter that can measure how much you’re sending to the grid in thirty-minute blocks. Your solar panels must be officially approved by the Microgeneration Certification Scheme (MCS), and shouldn't have a total capacity of more than five megawatts.

And not all SEG deals are the same. Fixed-rate tariffs guarantee a certain price per kWh, whatever time you send the power. Or, variable/agile tariffs pay different amounts depending on the time of day, giving you more when wholesale electricity costs are high.

If you're able to change when your panels make power or when you use it, agile tariffs can be very good for you. But if you like to know exactly what you're getting, a fixed rate is likely to be a better choice. It's worth looking around at different options; you don't have to use the same company for the SEG as you do for the electricity you buy.

One of the best ways to get the most out of your solar power is to get a battery for your home. These charge up in the day when your panels are making more than you need, and then provide power in the evening when the sun's gone down but you’re still using electricity.

With a battery, you can avoid buying electricity from the grid at peak times, and those are usually when it’s most expensive if you’re on a time-of-use tariff. Most lithium-ion batteries for homes have a capacity of five to fifteen kilowatt-hours.

However, while batteries will definitely let you use more of the power your solar panels make, they are a significant expense. A typical battery system for a house will cost between £3,000 and £8,000 to install, depending on how much it can store and who makes it.

Whether a battery is a good idea financially depends on several things: how much more expensive it is to buy electricity than to sell your excess solar power, how much electricity your household uses, the size of your solar panels, and whether you can get a tariff that gives you cheaper electricity when you use it at certain times.

For lots of people, batteries become a more sensible choice as the price of electricity goes up and the price of batteries slowly goes down. If your solar panels are making more electricity than you’t using, a really popular thing to do is divert that extra power to your hot water tank - these are often called immersion controllers. They keep an eye on your electricity coming and going and automatically send the surplus from the sun to your immersion heater, warming up the water in your tank.

Instead of selling that electricity to the grid for only a few pence per kilowatt hour, you’re then using it in place of gas or electricity you’d normally pay a lot more for to get your water hot. This is a lovely compromise for homes who aren't quite ready for batteries.

These immersion controllers work by constantly checking your meter’s electricity flow. When they realise your solar is making more than your house is using, they slowly increase the power going to the immersion heater, and carefully adjust it to make sure you don't have to buy electricity for your hot water.

This system is especially good in spring and autumn, when the sun's producing a good amount of power but you aren't needing as much heating. In the summer, you might find you're making more hot water than you need, and in the winter, the sun may not generate enough to cover all your hot water use.

Now, some really forward-thinking projects let you share the extra solar electricity you generate with your neighbours, or join a local energy community. Through these person-to-person trades, you can sell your spare power directly to people nearby - and potentially at a better price than the standard SEG tariff, though still cheaper than what they'd pay their supplier.

These are still being developed in the UK, but point to an exciting future for how we get and use energy, building resilience in the area and making sure the benefits of renewable energy stay within the community.

To really get the best from your solar panels, try these things:

Use your electricity when you can! Run things like washing machines, dishwashers, and tumble dryers during the day when the sun is shining. Even moving them an hour or two can really boost how much of your own power you’re using.

Keep an eye on how much you're making. Knowing when your panels are at their best helps you adjust your habits. Many inverters and clever home systems have apps for your phone which will show you exactly how much electricity you’re generating and consuming in real-time.

Think about how you heat things. If you have a heat pump or are thinking of one with the Boiler Upgrade Scheme, running it when the sun is at its strongest can cut your costs a lot. Some heat pumps can even be set to warm up your house or water tank with the sun's power.

Check your SEG tariff regularly. Export prices change, and new companies arrive. Having a look each year makes sure you're getting a fair price for the electricity you send back to the grid.

Be ready for the future. If you're not up for batteries right now, make sure your solar setup can have them added later. This usually means a ‘hybrid’ inverter that you can connect a battery to without a massive amount of extra stuff.

In the end, any surplus solar isn't wasted, but how you handle it makes a big difference to how much value you get from your solar investment. The Smart Export Guarantee gives you a basic income from sending it to the grid, but batteries and hot water diverters are ways to use more of your own power and save money.

The best thing to do depends on your household, how much electricity you use and your budget. For some people, doing a bit of both is best - perhaps starting with a hot water diverter and then getting batteries later as they become cheaper.

However you choose to do it, understanding these options means you can get the most from your solar panels, and help the UK move towards a more sustainable, locally-controlled energy system.