Review: Moixa 4.8kWh Solar Battery


I have upgradeitis. If something newer and shinier comes out, my stupid monkey-brain compels me to buy it.

Seven years ago, we installed a solar battery. It was part of an experimental project which looked at creating a community power-grid, so it came at a subsidised price.

As I explained to BBC Click, the 2kWh capacity was reasonable - but I expected the future would bring higher capacity, cheaper costs, and smaller sizes1.

But, after 7 years the battery was starting to show its age. The little Raspberry Pi inside it needed more frequent reboots, the fans were grinding a bit, and it would occasionally drop off our network.

So Moixa made me an offer I couldn't refuse. Upgrade the battery to the newest 4.8kWh model. Total cost for supply and installation? £2,700.

Ooof! That's a lot.

We did the maths. The previous subsidised battery had paid for itself over the 7 years. And with electricity prices rising, it made sense to be able to use more of our home-grown supply. So we went for it. Also, I like buying new gadgets.

Installation

Someone comes along with a big drill and lots of electrical flex. The whole thing is mounted on the wall like this:

Large industrial battery with lots of cables and wires.

And then covered up like so:

Large box attached to the wall. A white metal case with blue polka dots covers it. There is a small black and white screen in the middle.

The old battery was sent to be recycled.

A few minutes after it was commissioned, the website was showing power flowing from the solar and going into our battery and home - with enough left over to sell to our neighbours.

The Good

It can store nearly 5kWh of sunshine. That's about £1.60 worth of electrons at today's capped prices.

Smart profiles - it can monitor how much energy the house is using and charge / discharge as necessary. Or, it can look at realtime energy prices and make a decision about whether to charge. We'll move onto a smart-tariff soon and it will charge up from the grid when prices are low and allow us to sell our excess when prices are high.

It can connect via WiFi and Ethernet. Moixa supply some homeplugs for the Ethernet connection. That allows it to look up energy prices and means Moixa can monitor the battery's health.

It has a modular design. Inside are two US2000 batteries. They're of a fairly standard design and - in theory - could be swapped out for something bigger and cheaper in the future.

Oh, also, there's an app! So I can check what the battery is doing while on the go. A bit silly, but lets me know if the battery is full - which might mean I decided to switch on a high-energy device like a tumble-dryer.

The Bad

It's a big brute! There's been a bit of effort made to soften its industrial appearance. But there's no getting away from the fact it is a big box with wires coming out of it. It needs a bit of airflow around it for the fans, and a sturdy wall to be mounted on.

It's a bit noisy. There's the hum of the inverter, the whirr of the fans, and the occasional click of the relays. Unlike a gas boiler, it is on constantly. Moixa recommend installing it out of the way if possible. Ours is in the porch, so we only notice it when entering or leaving the house.

The information screen doesn't say what the state-of-charge is. All you get is whether it is connected to the Internet and whether it is charging, discharging, or idle. A few more stats on it might have been nice.

If the power goes out, so does the battery. The UK has strict rules on "islanding". If the electricity went out but the battery kept feeding back into the grid it could injure people working on the wires. So it can't be used as an Uninterruptible Power Supply.

The battery's network connection doesn't expose any open ports. I mean, I suppose that's good for security - but I wanted to play! The website is great - but I don't think it offers an API.

Economics

*sigh* Money, eh?

What's the return on investment for your new kitchen? Or for getting your garden landscaped? Sometimes it is nice to have cool things. And, as we head further into a climate emergency, being able to reduce our personal dependency on polluting forms of electricity is a good thing.

But let's talk money anyway!

On the first day, the battery charged up to 95% on solar. At about 8pm, it started discharging. At 8am the next morning it was down to about 35%.

60% of 4.8kWh is 2.88kWh. Divided by 12 hours is 240 Watts. That's how much our house uses over night. We paid £0.00 for electricity during those hours.

The average price of electricity is about £0.35 per kWh - so we saved £1.01.

£2,700 / £1.01 / 365 days = 7 years, 4 months.

But… of course, it is a little more complicated than that. Here's what our solar panels do on a rainy winter's day:

Hardly any solar generated, not enough to even half fill the battery. So the system is useless, right?

Nope!

We're moving to a smart-tariff from Octopus (join and we both get £50). When electricity prices are cheap, the battery will charge from the grid. When electricity prices are high, the battery will discharge. We effectively become small-time arbitragers. We buy low and sell high.

And, if electricity prices go higher, our payback time will be shorter.

Come back to this blog in the year 2030 and I'll tell you how the battery performed.

Verdict

Solar batteries are expensive. There's no VAT to pay if you get them installed at the same time as your solar panels - but the Government doesn't seem keen on making it cheaper for people to retrofit energy-saving measures.

Battery tech is still in its relative infancy. We don't have Moore's Law increasing capacity and reducing prices. So for now we're stuck with big expensive boxes.

If you have the space and money for one, I think solar batteries are a reasonable investment. They're the ultimate in "think global, act local" when it comes to electricity.


  1. The new unit is cheaper and has more capacity. But it is physically bigger. Well two-out-of-three ain't bad! 

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