Should every house have a battery?

Increasingly, homeowners are adding solar panels to the roofs of their houses to help offset their bills. The problem is that - during the day when it’s sunniest and their system is generating most of its output - most people are not at home. So they can’t so easily run the washing machine, charge up the car or put the hot water on. And although you get paid to export your homemade electricity onto the grid, the rate of payment is much lower than you have to pay when you buy that electricity back off the grid at night when you are at home and need the lights on. One way around this is to install a battery system that stores up what you generate during the day and then powers your home with that energy through the night. But how practical is this, what’s involved, and do the numbers stack up? Chris Jardine is a founder of JoJu Solar, who supply these systems, and previously he ran a research group at Oxford University looking into solar energy and microgeneration...

Julia - How do these systems work?

Chris - It's comparatively simple. A battery system will be installed alongside the solar panels in your house, connected into your main fuse board. And when it detects that there's an excess of solar energy being produced by the solar panels, it will charge up that battery. And similarly, if it detects that there's a higher load in the house and that the home wants to import energy from the grid, it will discharge that battery to balance that off to zero.

Julia - And how much energy can be stored in these batteries. Say it's a really, really sunny day and you've got sun on those panels all day. How much can you store up?

Chris - Batteries come in a variety of sizes. But let me give you an example of the Tesla power wall, which is probably the most popular battery being installed in the UK. The Tesla power wall stores 13 and a half kilowatt hours of electricity, which compares to the demand of an average home of around about 10 kilowatt hours. So Tesla power will be installing you roughly a day's worth of electricity in that battery. Smaller batteries might be around about half the size, perhaps around about five kilowatt hours.

Julia - And what is the output? So say you've got a family of four and it's nighttime. So there's no sun, we're relying solely on these batteries, and everyone's got their gadgets out. Is this going to be a doable situation with this type of battery?

Chris - It depends on the size of the battery that you purchase, but certainly the Tesla power wall, which has a five kilowatt power output would be capable of meeting lighting, TV loads in the home, as well as major heating loads, like a kettle or an oven. Smaller batteries would have a smaller power output, maybe two or three kilowatts, and they may struggle to meet those peaks in demand that come from high powered heating appliances.

Julia - A problem with batteries is their lifetime. We have terrible phone batteries, which have got better over the years, but what is the lifetime of these batteries that will be installed?

Chris - We're seeing the typical warranties on these batteries being around about 10 years, which means that we probably expect their useful lifetime to be extending out around maybe 15 to 20 years. And certainly the data that's coming off some of these early battery systems that have been going in over the past five years or so, is that actually the batteries themselves seem to be holding up very well and not degrading too much.

Julia - And there's going to be an upfront cost for getting one of these batteries installed. So what is the payback period for this?

Chris - If you had asked me this question about a year ago, I would've said that the payback of a solar system on its own compared to a solar and battery system in the home would've been roughly equal. The battery itself would've had very minimal impact either way on the payback of that overall home energy system that you'd be putting in. The reason for that, as we've discussed, is obviously the benefit comes from the export payment for solar electricity, which is around about 5p per kilowatt hour, and the import price for electricity, which historically was sitting at around 15p per kilowatt hour. That difference is really what's provided the benefit, the financial benefits, of installing a battery, but obviously that has to be offset against the capital cost of the battery system itself. So historically we weren't seeing a lot of difference there, and we were seeing people installing batteries really for much more emotive reasons based around wanting to be more self-sufficient, wanting to be independent from the dirty electricity grid. Clearly the pictures changed over the last year, and now we're seeing electricity import prices up around 25 to 30p per kilowatt hour, then I think there is now a financial case for installing batteries, and that you would see solar systems with batteries incorporated into them giving improved payback periods compared to just installing solar on its own.

Julia - And just in the last 30 seconds here, with the current energy crisis has demand for these batteries at home gone up?

Chris - Demand for both solar and battery storage has gone through the roof really since last October, when people started getting worried about electricity prices. There was another big bump in March when the Ukraine crisis broke as well. And certainly at times we've seen demand for solar and battery systems running about 10 times normal levels at the minute.