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Energy storage vs heat batteries: what's the difference and which is better?

  • 17 December 2021
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Energy storage vs heat batteries: what's the difference and which is better?
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Storage batteries for your home - which is best?

 

Our content team have written a fantastic guide on Battery energy storage: what it is and how it all works. Within that article they outline the different types of domestic storage, which we’ve copied below:

 

Lithium-ion batteries

Most home batteries in use right now are powered by Lithium-ion, the same substance that’s been used to make many commercial batteries. 

  • Pros: They’re lighter and more compact than other electrochemical batteries, and will also last longer.
  • Cons: As mentioned above, Lithium-ion is expensive, and it’s also a finite resource – so it’s not the most environmentally-friendly option. 

Lead-acid batteries

These work similarly to Lithium-ion batteries, but there are a couple of key differences. 

  • Pros: These are a slightly cheaper option than Lithium-ion.
  • Cons: They have a shorter lifespan than Lithium-ion batteries, while being less environmentally-friendly than heat and saltwater batteries.

Saltwater batteries

The new kid on the block of home battery tech, saltwater batteries don’t use any heavy metals. Instead, they rely on – you guessed it – saltwater, and its natural electrolytes.

  • Pros: These are very green, and easily recycled (unlike Lithium-ion or lead-acid).
  • Cons: As they’re so new to the market, saltwater batteries are still untested long-term. 

Heat batteries or thermal stores

These are both ways of storing energy as heat. While thermal stores are more like your trusty old hot water tank, heat batteries are their cool younger sibling: they’re sleek, small, and hold on to heat for longer. See our guide on thermal storage here.

Sunamp UniQ heat batteries are perhaps the most compact example out there. 

  • Pros: These clever systems can take the place of a regular hot water tank – and in the case of heat batteries, they’ll take up less space, too! They’re super-environmentally-friendly, and won’t lose heat to their surroundings. Oh, and they’re cheaper than electricity batteries, too.
  • Cons: These can get pretty hot to the touch, so you have to be careful. But, all things considered, they’re a great option for your home.

 

We’re a community of green tech adopters, and on top of that, OVO are involved in trials of low carbon, green tech, including some involving Lithium-ion batteries and heat batteries. We’re keen to hear from trialists with first hand experience of this tech!

 

If we start with Lithium-ion batteries, @Transparent @jenthomson @Heatherd @dnshorto @chrisjw37 @mrmhf @Llamedospete @Mark C @malc_smith @Graham110011 @Sean T @JoeC @f0rtune @timciren @osborner @Bev - I’d like tap into your first hand experience by sharing what you feel are the advantages and disadvantages of this storage technology? Imagine I’m trying to choose between which type of storage to invest in, or whether to invest at all. What what you advise me? 

 

Let me reach out to our Heat battery trialists are ask you the same question. @hambrook @Nicllb @M.isterWWould you recommend your type of heat battery to me over an alternative? 

 

Share your knowledge, share your experience, and help us to debunk some myths! 


12 replies

Userlevel 2
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hi I have a Lithium-ion solax 5,8kW battery as part of my solar array. I chose this type of battery because it is proven and its light weight. I fits in the loft space on heavy duty loft boards close to the solar panels. I am very happy with the performance. In the summer months it is charged by my solar panels. In winter months it is charged off peak electricity from the grid. fits really well with the heat pump. I did avoid the energy dealing solar model. The so called stockbroker in the attic that buys  electricity cheap then sells it when demand is high. Having no control over the cycles the battery would be put through was a concern.

    

Userlevel 2
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i Have 4 enphase batteries. each 1.kw linked to my solar edge panels. They have been in since 2017. Losing 5% a year. They do very well in the summer and I charge them off peak during the winter. with the ASHP during the winter.

I chose Enphase so it all communicated. I could add more at 2 at a time. Not may systems for batteries in then without  spending over £10k. ASHP is on 60% and 40% gas. 

Now i would go for higher battery storage.  They say the should last 10 years. 

Userlevel 6
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Today my specialist subject is.... Heat batteries.

We have four Sunamp batteries, two providing our hot water and two connected to our central heating.

How do they work? They contain a material that changes from solid to liquid when you heat it up. It remains a liquid until it is triggered to change back to a solid, releasing the stored heat. This means that once it is charged it loses very little heat (far less than a hot water cylinder). There are other types of heat battery that use different materials to store the heat but the Sunamp batteries all work the same way and don't get hot to the touch.

Sunamp batteries can be charged in different ways. One of ours has an electric heater and the others are charged by a heat pump. You can also hook them up to a gas boiler. They use different phase change materials for different applications so a battery with an electric heater will charge at a higher temperature than a battery linked to a heat pump.

 

What's good about a heat battery?

For hot water they work just like a hot water tank but they take up less space.

The phase change materials are non-toxic and don't have the environmental issues associated with the metals used in electricity batteries.

They last a long time. Sunamp have tested them over more than 10,000 cycles with no degredation.

Because they have minimal heat loss you can charge them at any time during the day, to take advantage of excess solar, cheap rate electricity, low carbon electricity etc.

 

What's not so good about a heat battery?

It isn't flexible when compared to storing electricity. You can use electricity for all sorts of things. You can only use heat for.... well, heating!

They're expensive compared to using a hot water tank, although they are cheaper per kWh of storage than an electricity battery.

The control system on Sunamp batteries isn't user friendly. There's no user interface so I can't tell if they're charged and how much heat they're holding.

 

Would I recommend a heat battery?

That isn't an easy question to answer. I would say... Maybe!

If you don't have space for a hot water tank then they're a great option for a hot water supply.

They are a cheap and long lasting way to store energy but within 10 years most of us will have cars with a large electricity battery and that's a much better storage option (using vehicle to grid chargers).

If Sunamp improve the control system to include a user interface and easy integration with an electricity management platform, like Kaluza, then I would be more likely to recommend them.

Userlevel 3
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We have a Sonnen Eco 9.43 battery, only had it installed since september so not had a chance to test it out fully though the summer!
 

It’s very simple product - charges up with the sun and discharges when we need it, from that point of view it works very well!
 

If i was planning a new system now I would use a battery with a built in solar inverter or a ‘dc coupled’ battery - because the solar panels are generating DC electricity, and the battery stores DC electricity, at the moment we go:

 

DC Solar → Solar Inverter → AC House energy → Battery Inverter → Battery storage → Battery Inverter → AC Home Usage. 

 

So - that’s a lot of inversion!! And that will incur heat generation and electricity losses so not ideal from that point of view, it’s probably only 2% each time but still worth considering. 

 

With a DC coupled system, the system diagram would go:

 

DC Solar Generation → DC Battery Storage → Combined Solar/Battery Inverter → AC Home Usage

 

Overall, with the cost of electricity heading skywards, the combination of Solar, battery storage and cheap off-peak energy is a winner, and will probably pay for itself in 3-5 years, amazing. 

 

Userlevel 4
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We have a 7.3 kWp of Solar, Tesla Powerwall 2 with Backup Gateway 2… if I break those two down;

First the Battery - a 13.8kWh all in one battery with inverter, liquid cooling all in one unit that can be floor mounted, wall mounted, even outside as fully waterproof. Install cost was circa £7.5k. Did I consider other batteries? Yes the Solid State Pylontech modular batteries is an ideal way to cheaply enter battery storage and then expand BUT the app and support looked patchy. 

The App - is almost perfect. It's the same Tesla app to manage a Tesla car which for me worked well. There are some incredible data and usage charts to really dig into the detail of what energy you have used from which source. It works with great with Time of Use tariffs where you can set cheap off peak and it will charge the battery using cheap off peak electric. 

What’s Backup Gateway 2? So strictly speaking this is NOT a UPS but it works just as a UPS does. You can run totally off-grid either by clicking button in the app, or it will just kick in when there is a power cut. It is insanely quick to cut in. It even has a storm mode so if there is a major storm approaching it fully charge the battery to provide continuity of power, though that doesn't happen to often here on the South Coast! 

Is it perfect? Nearly. Tesla use their own Algorithm to choose how much the battery needs to charge using off-peak until solar power kicks in the next day (we have 7.3kWp of Solar). It routinely gets this wrong. It has no idea of you have a house full at the weekend and needs loads of cheap stored power, so there is a couple of manual solutions; 1) I have a Raspberry Pi running a custom app that allows me to set charge level, times, it even integrates with Octopus Agile (back in the day when that use to be cheap). Before switching to OVO I used to fully charge the PowerWall on the Octopus Go and routinely paid 6p per kWh over the course of a month - brilliant !  2) Home Assistant can also control Powerwall and perform the same Tesla open API. 

Heat Storage - Mixergy - those who have read my other posts know I am a huge fan of Mixergy. It’s a hot water cylinder born out of a University Project, made in the UK and works  like no other. I use this as a secondary heat store in the height of the summer. On a very sunny day our Solar PV system can produce ~50kWh of free electric, once the Powerwall is full the MyEnergi Eddi then diverts the free electric into the Mixergy storing that energy as free hot water. WAY cheaper than battery storage to install. A brilliant smart phone app giving complete control and loads of data. As part of the OVO heat pump trial they disabled my ability to use use this Solar Diverter feature and cut the cable from the Eddi to the Mixergy. Meanies.  As soon as the trial is over the Mixergy will once again be the primary heat source and the ‘dumb’ energy wasting Sunamp Heat Batteries will be ditched.  Mixergy Customer Service and technical team are UK based and utterly brilliant.  

Heat Storage - Sunamp Heat Batteries - I have the same configuration as Mister W above with 4 batteries acting as heat stores for heating and hot water instead of the buffer tank and hot water cylinder you normally get with a heat pump install. The marketing looks great, clever phase change materials storing energy that can create instant hot water and heating. In reality ‘dumb’ batteries with no real controls or data just absorbing huge amounts of energy keeping them hot just in case you need the heat. You can’t even see how full they are, or how much energy they have used..  And for the hot water side it has an immersion that brings the hot water up to a useable temperature but that immersion has used almost 40% of the electric that the whole heat pump system used to heat the batteries. Can’t wait to ditch Sunamp and install a Mixergy and Buffer tank. Sunamp Customer service non existent having not replied to any emails, phone calls. And that’s before you look at the price point with just one battery costing TWICE the cost of a Mixergy Cylinder.  Avoid. 

What upgrades am I considering? A second Powerwall, already have the DNO approval.

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What's not so good about a heat battery?

It isn't flexible when compared to storing electricity. You can use electricity for all sorts of things. You can only use heat for.... well, heating!

They're expensive compared to using a hot water tank, although they are cheaper per kWh of storage than an electricity battery.

The control system on Sunamp batteries isn't user friendly. There's no user interface so I can't tell if they're charged and how much heat they're holding.

Fast forward to the heady days of next Summer and it’s 32 degrees .. how do we stop the Sunamp heat batteries infinitely heating to 55 degrees C. Remember when we used to turn the gas boiler off for the summer? Yeah how do we turn off heating heat batteries ONLY leaving the hot water ones still charging?

 

Userlevel 6
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I don't think the fact that we have no control over our heat batteries should be held against Sunamp. In a conventional installation the user would, I hope, be able to switch them off so they don't charge in the summer.

 

For those who aren't aware... @hambrook and I have heat batteries as part of a trial. Our systems are managed remotely so we don't control when the batteries are charged.

Userlevel 4
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I don't think the fact that we have no control over our heat batteries should be held against Sunamp. In a conventional installation the user would, I hope, be able to switch them off so they don't charge in the summer.

 

For those who aren't aware... @hambrook and I have heat batteries as part of a trial. Our systems are managed remotely so we don't control when the batteries are charged.

Thats fair. But the missing data; How charged are the Sunamp batteries? Whats the current temperature of the Sunamp battery? How much kWh of energy did it take to charge the sunlamps today?Why is there no app or web interface to control the batteries? I’ve already had a temp sensor replaced, how can we see this failure with a light , alarm, notification? 

Those to me are very basic questions. 

Userlevel 3
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I don't think the fact that we have no control over our heat batteries should be held against Sunamp. In a conventional installation the user would, I hope, be able to switch them off so they don't charge in the summer.

 

For those who aren't aware... @hambrook and I have heat batteries as part of a trial. Our systems are managed remotely so we don't control when the batteries are charged.

Thats fair. But the missing data; How charged are the Sunamp batteries? Whats the current temperature of the Sunamp battery? How much kWh of energy did it take to charge the sunlamps today?Why is there no app or web interface to control the batteries? I’ve already had a temp sensor replaced, how can we see this failure with a light , alarm, notification? 

Those to me are very basic questions. 

Is this because of the trial or are Sunamp just missing some of the basic control/monitoring software?
I’ve an interest in these in the long term but having fine control over knowing how “full” they are and when they are heated would be crucial to me.

Making all these things play nicely together is going to be a long term challenge.

In 5 years I expect to add to my existing PV and home battery (OVO trial) with a ASHP, heat battery, V2G from an EV and more solar/battery. Goal is net zero but also to move to an off peak tariff for almost all my consumption. Getting all these to cooperative will be...interesting.

Userlevel 6
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Sunamp don't currently provide any means of monitoring most of their heat batteries. You can't tell how full they are (except the ones charged with an electric heater, which have a simple light display). They must be providing some feedback to the system but I don't know how sophisticated that is. I could be as basic as a two state indicator showing some charge or no charge.

 

It is a fairly significant failing, if they're to be used more widely in domestic systems.

Userlevel 3
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I know they’re relatively new. Perhaps are light on the software development function at the moment (we ain’t cheap). I know they got some investment recently so they really should be spending on that area pretty soon or, as you imply, they’re making a significant strategic error.

Userlevel 3
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Ok… so relatively new to Lithium Battery storage for the house. Have a Sonnen 5Kwh system (and a 75kwh Tesla car but that’s a different story). 

Advantages - Electricity is the most usable form of energy for cooking, heating, lighting, run computers etc. I guess it’s pretty efficient at returning most of the power stored.

Disadvantages - Electricity is relatively expensive compared to primary energy (Solar heat, or gas), so unless you are charging from Solar PV it does not make sense to use it for heat.

Pretty much the battery is maintenance free. It just sits in the garage and supplies electricity when it needs to. To actually find out if it’s done anything I have to look at the app!

Future plans may change that balance of cost if OVO offered lower cost off peak electricity to fill the battery (e.g. like Drive plus anytime), where they that potentially surplus wind/solar energy from other sources and push it into the battery to offset at a later time (e.g. like the old storage heaters overnight).  That could both offer the owner of the battery much cheaper electricity and potentially also keep the peak costs down, reducing the average prices as well.

 

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