A homeowner's experience: why I chose a Sunamp heat battery over a hot water cylinder

Michael Eve – homeowner with a Thermino heat battery

At the end of 2021, software developer Michael Eve moved into a 1970’s four-bedroom detached house near Brighton in the south-east of England with his wife and three-year-old daughter. After doing his research on how to achieve the low carbon, high performing system he wanted, he set about upgrading his heating and hot water. Michael shared his experience with us:

“We knew there was a whole bunch of renewable improvements we wanted to make to the house.

The first thing we did was to install solar panels and heat battery storage which was a no brainer for us as, with rising energy bills, we’d worked out it was easily to going to pay for itself.

We were on a vented heating system that had been put in when the house was built, and we noticed we had very low water pressure. The available hot water was just about enough to fill a bath – very slowly. Although we were heating the water from solar, we weren’t seeing a huge benefit from it.

Getting heat pump ready

We had been looking at what we needed to do to become heat pump ready which required us to replace hot water systems anyway, so I started looking at options. I’d originally looked at a smart hot water cylinder because that seemed to be a like-for-like replacement for the old hot water cylinder that was in situ. I had seen others talking about Sunamp heat batteries online, and I liked the idea of decoupling our hot water system from our central heating system.

I started looking into the pros and cons of a hot water cylinder compared to a heat battery. The cylinder is obviously a more familiar technology, the only problem is that hot water cylinders still suffer from loss of energy over time, which from solar was a big problem for us. We didn’t want to put loads of energy in and by the time we got round to using it, we’d lost half of it.

Sunamp’s Thermino heat battery became the more obvious choice. We took advice from our installer who agreed it would meet our needs. Lower heat losses and its smaller size compared to the hot water cylinder alternative were the final deciding factors.

Simple pipework and more free space

We fitted the heat battery in what used to be the airing cupboard – so called because the inefficiencies of hot water cylinders resulted in enough heat loss to dry clothes! It’s much slimmer than the hot water cylinder it replaced and has freed up space in the cupboard. It’s a really neat installation all round, with much simpler pipework.

We bought the solar panel compatible Thermino 300 ePV – the equivalent of a 300l hot water cylinder – which is probably slightly larger than we need on a daily basis.

Stored energy lasts us a week

On sunny day lots of energy goes to the battery from our solar array and it doesn’t matter if the next few days are cloudy, because the energy stored in the heat battery lasts. Based on an estimated heat loss of 0.5kwh per day, if we didn’t use it, I reckon we would still have enough hot water for a shower 20 days later. Once fully charged it will last us for a week of normal usage before needing recharged.

The installation process took three days all in. Day one was spent removing the old system which involved taking tanks out of the loft, draining down and putting in new pipework. The heat battery was fitted on day two, along with two new power showers.

Hot water at mains pressure

Having mains pressure hot water is the most amazing thing, I’d forgotten how great it is. It used to take up to an hour to fill a bath, now it takes five minutes. The main difference the Thermino ePV 300 makes is that we have hot water to spare. Previously we were pushing cold water into the cylinder to get hot water out, which cooled down the hot water in the cylinder. By the time the bath was filled, the water would not be as hot as when you started.

With a heat battery I can let the bath run knowing that it flows at a consistent temperature, and I know it’s not going to get any colder. We can set the temperature we want using the blending valve and I can rely on it coming out at that same temperature all over the house. We used to have to faff with mixer taps that to get the correct temperature.

We now have our hot water sorted with our Thermino ePV. However, following the recent heat wave I’m now considering an air-to-air heat pump which would allow us to have air blown into the house providing both heating and cooling. So, as it turns out, by keeping hot water separate from our heating system has opened up a few different options.

There have been a few things to get used to. For example, the indicator interface is not the most granular, which is a bit frustrating but not the end of the world. It’s still more of an indication of how much water we have than the old tank gave us.

Reducing fuel bills

As a software developer I would be keen to see an Application Programmable Interface (API). It would certainly be a big benefit to people like me who like to hook things up to a home assistant device, and to have an automation where I can decide to charge it overnight using off peak electricity if there’s not going to be much sun the following day. It would mean that we would never use peak electricity. Being on the Octopus Go tariff from energy provider Octopus means electricity is the same price as my gas at the moment, so it makes no difference to me in terms of cost, but I’m cutting reliance on fossil fuels by using electricity.

I reckon we will save £200 a year on our fuel bills, plus there are no maintenance costs. We still have boiler for our central heating, but it’s now turned down as a result of the Thermino heat battery.”


Michael is a software developer and renewable energy enthusiast. You can follow him on Twitter at @ungrim97

There’s more information about finding UK installers who fit Thermino heat batteries in our homeowners section.

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