Solar what?

Solar export or the ability for you to export excess energy from your solar array into your supply grid for money.

How do I know you’re doing it wrong?

Well I don’t. You might be doing it right but I suspect you are doing it wrong so let me explain.

If you have something like a hot water diverter for your solar system to heat water when there is no other electrical demand AND you usually use gas to heat your hot water, you are probably doing it wrong. If you normally use electricity to heat your water, you might be doing it wrong.

The basic problem is our instinctive view of “free electricity”. Our mind tells us that using spare electricity to heat our water is good because that electricity is free and therefore we only need to pay for the remainder needed to top it up.

This is not true. In the case of a grid-connected home, there is no “free electricity”, only electricity that costs you money and electricity that makes you money. In my case, I have 3 “import” rates:

  • 13p/unit off peak (02:00-05:00)
  • 22p/unit standard (05:00-16:00, 19:00-02:00)
  • 30p/unit peak (16:00-19:00)

I also have 3 export rates (these vary slightly depending on the time of year):

  • 4.5p/unit off peak (02:00-05:00)
  • 14.6p/unit standard (05:00-16:00, 19:00-02:00)
  • 22p/unit peak (16:00-19:00)

So back to my example. If I top up my battery between 02:00 and 05:00, which I do, I buy the cheapest electricity I can at 13p/unit. Most of the day in the summer, the solar will supply what we need for background usage and then towards the end of the day, or if we’ve used a lot more, the battery can provide up to 10 units. We usually also run a surplus from solar which initially tops the battery back up and on most sunny days, is already exporting by late morning.

There isn’t much difference between buying it off peak and selling at normal rate so at this point, all of my spare energy, once the battery is full, it paying me the same as what I paid to charge the battery but on most days, I get more back because the solar generates more than I buy to top up the battery minus my usage.

My gas is only 7p/unit so why would I NOT export the electricity in order to save 7p but lose the 14.6 I would get if I sold it? Yeah, electrical heating is slightly more efficient than a gas boiler but not 50% more. Also, if I am not using the hot water much during the day (which is typical) then the heat from the hot water is leaking anyway and being lost to the atmosphere.

So what if I use electricity to heat my water?

If you use an immersion heater, it depends on when you need the hot water as to what you should do. If you can heat it between 02:00 and 05:00 (in my case) then it would still be better to do that and export your spare electricity later. If you mainly use it in the evening then you don’t want to be heating it during the peak time, you might as well heat it with your spare electricity before the peak rate hits. Hot water does collect at the top of the tank so you don’t need to heat the entire tank to get a bath out of it.

What about if I use a heat pump?

The same maths holds as before. You will need to be running it for longer periods so it does make it a bit trickier but although you might be getting a 3x efficiency boost from your electricity, you would get the same boost whether you import it or get it from spare solar. Probably the 3 hours off peak (which you should definitely always use) won’t be enough to top up your hot water, so in this case, again, spare solar usage is fine but avoid running it during the peak times.

Bonus points - export at peak times

Since we are not a large energy user and we have a 10Kwh battery, most of the summer months, by the time we reach 16:00 we have a full battery and are still getting solar. What do I do? Dump as much as I can (roughly 3.4Kwh) for 3 hours back into the grid. It doesn’t matter if I empty the battery (I leave some to power our background stuff until at least 19:00) because I will top the battery back up at night for much less money. Again, think about the value of what you have, not just the cost of it.

It goes without saying: you need the right tariff

My father-in-law has solar, but he doesn’t have a variable tariff like mine so he can’t make very much money by trading electricity. In the UK it is free and easy to switch providers so there is no real excuse to stay with someone who offers e.g. a fixed 5p per unit exported. Go to someone like Octopus Energy who provide consumer and grid-friendly policies like these since by balancing the grid driven by supply and demand, we also reduce the need to build more power stations to satisfy maximum demand. Imagine instead, a country full of battery owners who all provide that energy for peak time usage and suck up the surplus over night? Cool eh?