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Should you buy a battery & how do you choose the right one?

Posted by Ollie Gibbs on 20 July 2017 at 5:06 pm

Why should you buy a battery?

If you have a well-designed PV system you’ll probably have noticed that on some days your system is exporting large amounts of energy back to the grid. This is because your system’s generating more energy than your home is using at that particular time. For instance, if you’re out of the house on a sunny day, your home probably won’t be using all of the energy your system generates and so energy will be exported straight back to the grid.

At other times in the evenings you’ll be buying energy back from the grid because your system isn’t generating at that time. So, sometimes you’re producing more energy than you’re using and at other times you’re not producing enough - there’s a mismatch in supply and demand!

This can be very frustrating especially from April to November, when your PV system is working at its best and probably producing enough energy overall to cover the majority of your home’s energy demand.

This is the problem that a battery helps to solve. It can store the solar energy that’s generated but not used at the time, so you can use it later on when your system isn’t generating.

So, for most PV system owners, the simple answer to “should you buy a battery?” is “yes you should!”

If you’re not sure how much you’re exporting or you think you might not export much energy it’s sensible to buy a device like the Owl Intuition-PV. This device will monitor your generation, export and overall energy consumption, so you can see exactly what’s going on.  

There are a few vital matters to consider before buying a battery and we have noted these below.

8 key matters you should consider when choosing a battery

1.      Usable Capacity

This refers to the amount of energy the battery can store, which can then actually be re-used in your home. 

Be aware that the advertised “total capacity” of a battery may not be the same as the “useable capacity”. It’s important to be aware of a battery’s usable capacity as a lot of batteries are mis-sold by advertising total capacity, without making clear that the usable capacity is different.

For instance, the new Tesla Powerwall 2 has a 14kWh total capacity, however it is actually sold as 13.5kWh as this is its usable capacity.

2.      Power Output

This figure is hidden by some companies selling the cheaper battery systems, but it’s probably the most important thing to look out for when choosing a battery!

What is it? It’s basically the amount of energy you can actually take (draw) from the battery at any one point. This is not the same as capacity. If you compare the battery to a bottle, the capacity is the total amount that fits inside the bottle. The power output is the size of the spout through which the contents of the bottle can be poured out. If the bottle has a spout that’s too small you might not be able to get enough out of it fast enough to quench your thirst!

Regardless of the capacity of your battery, if the output is too small you may not be able to power more than one appliance at any given time, even if the battery is full of power. For instance, some battery systems will have a maximum power output of 900 watts, which is rather pointless if you want to boil a kettle or do anything other than watching a bit of TV with some lighting on!

The principle reason to have a battery is so you don’t import and pay for as much energy from the grid. But, if your battery has a low power output you will still be buying energy from the grid if you turn anything significant on in the house.

Our general advice would be to make sure the power output is always above 3.6kW. The LG and Tesla Range of batteries have a very good power output rating.


3.      Life Span

This can be quite hard to understand as some battery units offer cycle warranty and others offer years warranty.

What on earth is a cycle? A battery doesn’t have wheels! A cycle is when a battery goes through a complete discharge and then one full recharge.

Basically, any warranty for around 5000 cycles is fairly decent.

We’d advise that when it comes to warranties you don’t go for anything less than 10 years or 5000 cycles.


4.      What chemistry is the battery?

The main ones you will come across are either lead acid or lithium ion. Our advice is to stick with the lithium ion! Quite simply, it lasts longer and will deliver a lot more power over its lifetime.


5.      What’s the cost?

The costs involved with the installation of a battery can determine which battery is most suitable. The best performing battery might not always be the best option for you if it costs much more to install, due to location or installation difficulties.

From a practical point of view, you do need consider whether you have a suitable location for the installation of a battery.

In terms of the cost-benefit for you, it’s sensible to consider the following:

1.      How long do you intend to stay at your home? Are you likely to stay long enough to reap the benefits of the battery you choose?

2.      How much are your bills after the solar covers some of the demand?  Therefore, what potential further savings might be available for you on your energy bills?

3.      What future financial benefits and income might the battery offer you? Please see more on this at 6 & 7 below

4.      What are the costs involved for the battery you are considering? How does this compare with the answers to questions 1, 2 and 3 above?

These simple questions will help determine which battery offering is of best value to you, which ones are worth considering and which ones aren’t. Do also take into account the other key points raised here. For instance, a cheaper battery may prove a false economy if it doesn’t last as long!

Generally you tend to get what you pay for and actually the more expensive battery options are often more flexible to install so usually offer better all-round value. Remember this is always going to be a long term investment. Do it once and do it properly!

We have set out a very transparent battery comparison further on in the blog to help you compare the options.

6.      Power cuts!

Will your battery kick in when there’s a power cut and the lights go out? Most battery options have the facility to take over when the power from the grid drops out, but this may come as an optional extra.

Make sure you check if this is important to you.


7.      Grid charging – buying in cheaper electricity

It sounds strange but you are going to want to do this in the future! Basically, as well as charging from your solar PV system, some batteries will in future be able to buy in energy when it’s cheaper (e.g. overnight). You can then use this energy later when you need it rather than buying it from the grid at a time when it’s more expensive.

Not all batteries will be capable of doing this and this is where a larger battery with the software to enable grid charging can offer a huge advantage in the winter months. For instance, the Tesla Powerwall 2 will be releasing a software update shortly to allow this in their models. We understand that this is the first domestic battery to offer this.  


8.      Grid services – get paid for having a battery!

You’ll probably have heard about the strain the grid is under at certain times when demand for energy suddenly increases, for example at half time in a football match when everyone gets up to put the kettle on.

In future, it is likely that the grid, via a service provider, will pay domestic battery owners a sum to have access to some of their battery that the grid can call on when demand is high, to help supply peaks in demand at short notice.

Unless your battery is set up to be able to do this you will miss out on this potential income stream. Also, if the capacity of your battery is too small, you won’t be able to offer some of its spare capacity in this way.


Is now the right time to get a battery?  

We’re often asked this question. The battery market has improved quite significantly over the last few years. Prices have come down a little as better products have emerged on to the market.

Currently, there are some fantastic options available and there are many people already taking advantage of these.

In our opinion, if it’s affordable then the time is right. But, go for the best you can afford as it’s a long term investment. Energy prices are starting to increase fairly rapidly now so those that can may as well start saving now!

However, if buying a battery is really stretching budgets or requires a loan then we’d suggest you simply wait, save a little more and hopefully prices will come down a little.

A battery is as much a lifestyle choice as it is an investment. Those that have the right battery are often rather smug and desperate to show you their bills or energy monitoring!

Our prediction is that within 5 years no PV system will be sold without a battery and the vast majority of system owners will have already adopted some sort of storage technology. As energy rates creep higher each year the transition to this way of thinking is accelerating.

Why buy energy in if you are producing enough on your roof not to?

So, you want a battery, who should you get to install it?

Our advice would always be to go to a reputable, accredited installer and ask for evidence of other battery systems they have installed.

If they’re any good they should have some happy customers that you can speak to. If not, get back on the internet!


Sample battery comparison of some of the battery storage systems currently being offered

Battery Storage System Power Output (KW)  Usable Storage Capacity (KWH)  Warranty (years)  Back up available 
Growatt 2 4 5 no
Powervault 4KW Lead Acid 1.2 4 5 yes (limited )
Tesla Powerwall 2 5.5 13.5 10 Yes
Sonnen 8.8 2.5 8 10 Yes
LG Chem RESU  5 8.8 10 Yes
Pylontech US2000B (2.4Kw Stackable) 2 1.92 5 No
Wattstor 6 5 6 5 yes


Please note

·         Warranty cycles taken into account for some products

·         This is just a basic summary and you should always check current up to date data sheets when comparing products.

·          You can add the price quoted by your installer and divide it by the usable storage capacity to work out the best value per usable unit of storage capacity.

To summarise, there are a lot of options on the battery market and this can prove to be a bit of a minefield when making a decision! There are a few clear market leaders when it comes to home battery systems and our advice would always be to choose the best available today and not the cheapest!

Take your time making the right decision and grill your installer to check that they are offering you the best solution and not just what they stock!


 Image credit: John Englart



About the author:

Ollie Gibbs is a PV & battery system designer for SunGift Energy.

He estimates that he’s designed around 3000 systems during his time!

Ollie himself is an avid fan of solar technology, running his own house from his home PV system. He is a strong believer in the value of renewable technology and sustainable non-polluting fuel sources. 

He has an honest and up-front approach to designing and advising customers on the types of system that will best suit their needs. If a technology isn’t for you, he’ll let you know and he’ll explain why! 

Ollie also likes to get involved with installing the systems he’s designed and can sometimes be found up on a roof with a solar panel or two.

Outside of work Ollie enjoys all sports and likes to spend his weekends kite-surfing along the coast in Exmouth. Ollie’s travelled a lot of the world but, luckily for SunGift, is now happily settled in the Westcountry with his family, finding he can’t be away from the coast for too long!

If you have a question about anything in the above blog, please ask it in the comments section below.

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9 comments - read them below or add one


ChrisSullivanComment left on: 8 January 2019 at 8:05 pm

EDF are offering 8kWh batteries at £5,000 for PV and economy7 use when connected to their smart grid. It comes with a new inverter which I suppose is worth £1-2,000 if my existing 8 year old one were to break down. Estimating £200/yr of savings it would have a pay back of 10-15 yrs. (15 × 200 = 3000 where £3000 is the net cost excluding the inverter). Is it now time to commit to a battery? Is this a good deal?

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GaryComment left on: 6 July 2018 at 5:22 pm

Thought I would re-awaken this thread.

In answer to the question about changing appliances the answer is no.  Power from the battery goes through an inverter to turn it into normal AC mains voltage in the same way the invert converts DC from the solar panels to AC.  

Do we have any updates on the artical itself or people's experience with batteries?  For me the maths doesn't quite work out yet.  Even if I covered all current import (unlikely) I would only save about £200 per year, so realistically, with battery replacements and install costs I would seem to be looking at an ROI of about 30 to 40 years, by which time even the daisies will have stopped being pushed up.

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LongtaleComment left on: 28 February 2018 at 1:05 pm

Will I have to change my elictrical appliances and lighting to 12V to make most of battery power?

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SOLiC 200

SOLiC 200Comment left on: 10 November 2017 at 9:30 am

As the maker of the SOLiC 200, you could say I am biased, but often at under £400 fitted and able to save you up to £200 a year. It is a no-brainer compared to batteries. 

The SOLiC 200 uses your existing hot water tank to divert the energy you would have exported to the grid to offset your normal heating method for the water, so saving carbon and money. 

With its class-leading 10-year warranty and zero export threshold with thousands of UK installs you really should check it out as a considerably cheaper halfway house alternative. 

Chris Rudge reviewed an early version of it on  here

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sashtonComment left on: 25 August 2017 at 12:16 pm


The Powerwall II is only availble in AC connect (as in it having an integrated inverter) but what you say about the FIT loss in a DC coupled battery isn't strictly correct.

Yes, you do not gain FIT as the battery is charged but you just get it later as battery discharges. You will lose a small percentage through battery losses but this is probably less than you would lose through the DC > AC > DC conversion on an AC coupled device.


I'm not sure of your maths. My, like most older small PV systems, has its export deemed so displaced energy purchased would be at full price - in my case about 15p/kW.

I do agree that you need to be able to work you battery to its limits in order to justify the expense. Ideally to be able to fully charge the battery every day and fully discharge at night but there are additional shart charges that help (slightly).

e.g. on a intermittently cloudy day there could be times when my base load is greater than the yield and 10 minutes later the opposite is true. The battery will cover this.

...but the main advantage is that I don't have to worry so much about scheduling the loads (Washing etc) according to the weather. ...which may even save my marriage! 

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sashtonComment left on: 23 August 2017 at 9:20 pm

As an update to my earlier post:-

I have found it is possible to monitor the Tesla battery without extra clamps. Assuming your setup connects home via WiFi or wired ethernet ( not via 3G) then there is a URL "your gateway address"/api/meters/aggregates which returns a JSON file with all the current and cumulative metrics. It does not actually give you a State of charge but I calculate it roughly as follows -

(power in since last fully discharged * turnaround losses (specs say this is about 11%) - power out since last fully discharged) 100 / battery capacity (they state around 13 kWh but mine seems to take about 14.5).

So I can now regulate the car charging with consideration of the SoC of the PW II. ... after a bit of fiddling.


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Andy in Hawick

Andy in HawickComment left on: 31 July 2017 at 9:32 pm

Batteries are expensive to run. Even the longer-lasting Li-ion cells are costly (and very few people will use anything like the number of cycles predicted). It is worth crunching the numbers to see how much a stored kWh is costing you. If the installation is costing more than ten times the annual saving, it is unlikely to be able to pay for itself. Divide the complete install price by the rated kWh capacity and then divide that by the rated number of cycles. If this is more than about 9p, it will be cheaper to sell (export) to the grid and buy back at standard rate when you need. It would need to be less than half this price to really make sense. (If the rated cycles are stated as more than 5000, think how many years this is equivalent to and use 2500—about ten years' heavy cycling—instead.)

There are cheaper ways of storing energy and even heating water or charging storage heaters is better than electrochemical batteries. Charging an electric vehicle does make sense because the marginal cost is minimal.

Once battery prices halve and then halve again, I will reconsider the situation. In the meantime, the only storage battery that I will be installing in my house will be to run the lighting circuits directly (without using an inverter) and power USB sockets.

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richmcComment left on: 31 July 2017 at 6:43 pm

I have a Wattstor system, one reason is it's an "AC side" system Tesla is DC side, so charges the batteries before the FIT meter so you loose out. Wattstor have given me first class support, as one of the early take up customers they upgraded my batteries when they found my installation under performing and upgraded the operating system hardware and software free of charge. The monitoring system is via Vectrons portal website (they use vectron inverters). The system is truly flexible and although the batteries are lead gel as opposed to Lion it is still saving me shed loads of money. During this summer I had weeks when the import meter didn't budge and I only exported when the batteries were full and water heated.

All this in my mind makes Wattstoe a viable contender as far as batteries are concerned.

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sashtonComment left on: 25 July 2017 at 9:34 pm

As the owner of a Tesla Powerwall II I would like to warn potential purchasers about an issue that affects me and should be weighed when chosing a battery system.

The only way Tesla permit you to interact with the battery is via their app.

While this will not affect users who simply want to shift their solar or other generated energy to leaner times it does mean that you will not be able to integrate the system with any other smart home devices.

e.g. stopping the car from charging when the SOC (state of charge) drop below a define threshold.

It pretty much restricts the device to being a dumb battery.

I have contacted their support but no alternatives to using their app has been suggested. I will update this post if they come up with an http/JSON solution.

NB the Fronius hybrid/Datamanger solution does offer open interaction.

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