Home batteries – hope or hype?

If you’ve got PV panels, the case for home battery storage seems obvious: bottle up all that free power generated during the day when you can’t use it, so it’s still available for when the sun goes down and you need it most. Batteries are often marketed as the perfect complement to PV, so much so that the two are increasingly sold together.

All of which means it’s been a surprise to discover that, in a grid-connected home, a battery is more red herring than obvious next step. Here’s why:

THE FINANCIAL ARGUMENT

For too long we’ve assessed the way we live purely according to financial criteria, so I wouldn’t take this as the ultimate deciding factor. But from the perspective of a simple return on investment:

IN FAVOUR

At face value, it seems clear that using free solar power we’ve generated ourselves will be cheaper than buying it in from the grid. Better yet, even when the sun stops shining it’s possible to maximise on your investment by charging from the grid at an off-peak rate, then selling it back when the price goes up. But the reality is a little more complex…

AGAINST

Where to start? Unfortunately there are multiple issues often unmentioned by the firms punting home batteries, all of which chip away at the claim that they’re a financially savvy investment.

• Depending on your set-up at home, installation costs can be an extra £500 to £2,000.
• Unless you install a battery at the same time as your PV panels, you’ll pay the full 20% VAT.
• Worse, your PV inverter might not be compatible – another big expense.
• If you’re already benefiting from the Feed in Tariff or the Smart Export Guarantee, you’re paid for every exported unit, regardless of the inefficiencies of the grid. By contrast, a battery never operates at 100% efficiency, meaning you’ll have to put in more energy than you’ll ever be able to take out.
• Solar panels might last 25 years, but a home battery’s life expectancy is about half that, at best.
• And its efficiency will decrease significantly over time. Estimates put the Tesla Powerwall (considered by many the Rolls Royce of home batteries) at 70% capacity after ten years.
• As mentioned, you could play the market and charge your battery off-peak then sell the power back when it’s more expensive. But that requires a smart meter, which will then override the Feed in Tariff’s ‘deemed’ export rate. In other words, if you’re currently benefiting from the FiT, you’ll end up being paid far less because they’ll know for sure that you’re exporting far less.
• Forget those misleading claims that a battery can make you independent of the grid. Unless you’ve an enormous PV array (in which case you’ll want to be grid-connected anyway to sell the vast summer surplus), you’ll still need mains power through the winter. So a battery won’t obviate the need to remain connected to the grid, meaning you’ll still have to pay your daily standing charge, a sizeable chunk of the bill.

Batteries are as varied as our needs, and the technology is improving fast. As the cost continues to fall and electricity prices continue to surge, perhaps the financial calculations will improve. But it was only two years ago that, taking some of the above into consideration, the Centre for Alternative Technology calculated the cost per unit of home-generated power drawn from a battery at almost twice the cost of the same unit drawn from the grid.

OTHER ADVANTAGES?

But it’s not all about the money. I’d still consider the outlay if the non-financial arguments were strong enough. So what are they?

All of 25 miles from a capital city, we still experience all-too-frequent power cuts. Some batteries – the more expensive – can automatically kick in to cover outages. But not for long. And if I have to forever maintain enough back-up in my battery for that eventuality, I’d be significantly reducing its capacity for day to day use.

Then there’s the real cost of a home battery. Although increasingly recycled, the fact remains they come with an environmental price tag, from the ecological destruction of mining to the massive amounts of energy required in their production and transportation. If we have to built lots of batteries stuffed with hazardous substances, better we keep them for when there’s no alternative – like in our electric cars.

In fact, when it comes to claims that home batteries can help buffer the grid, my car will soon do a far better job. In the near future we’ll be able to sell the power in our car batteries to the grid at peak times – within preset parameters, of course, that will still allow us to get to work the next day. And with a 40kWh battery, even our Leaf has a capacity three times that of the Tesla Powerwall.

Undeniably, a home battery would make me less reliant on fossil fuel power from the grid. But here, for me, is the most damning argument against home batteries: we need a good quality, low carbon grid for everyone, whether that’s a rural homeowner with an acre of south-facing roof, or someone renting a north-facing basement flat in the city (only more crucial as we all turn to heat pumps and EVs.) And we cannot achieve that by going alone.

The solution – as so often – is interconnection, so when the sun’s not shining in Stafford, they can draw from the solar in Stirling, and when the wind drops in Blackpool they can get power from the turbines in Ullapool. On a macro scale, that’s precisely why we’re laying cables beneath the North Sea, to share energy for mutual benefit.

Meanwhile excess power can be stored at far greater levels of efficiency and with far less environmental damage on a grid scale, diverting surplus into green hydrogen production, or thermal storage. Or it can simply pump water up a hill, ready to generate power when it runs back down.

At the moment, there’s just no point installing a home battery to save money. And doing it to reduce your carbon footprint and contribute to global sustainability is highly questionable. Far better to encourage renewable growth by signing up for a genuinely green supplier, and to think of the grid itself as your battery. Right now it needs all the help it can get to reduce its reliance on fossil fuels. A shiny, hi-tec box on the wall might salve the conscience of those able to afford one, but it does little for the wider world. Isolation is not the answer. Withdrawing from the world is no way to fix it.