Finding the Home Solar / Battery Energy System that is right for you
This short guide is designed to help you know what questions to think about when considering a home solar / battery system, based on the experiences of Sustainable Wantage members who have already had systems installed. We hope that this will give you more confidence in looking for the best installer, and for ensuring that the system best suits your requirements.
We’ve found that while finding a solar photovoltaic system that suits your needs is fairly straightforward, its the introduction of attractively priced home battery energy storage systems that means that there are now many more things to think about. Suppliers understandably have an interest in making the purchasing decision as simple as they reasonably can, and so they may not offer all the options unless you ask for them. Fortunately we haven’t come across anyone who regrets installing their system, but some people wish they had understood more about the options before committing to a system.
The most important thing is to talk to more than just one supplier. Not only have we found this a good way of learning more about systems, but we were surprised to see variations in price for substantially the same system of up to 30%. This backs up the Microgeneration Certification Scheme recommendation to always talk to at least three suppliers.
There is lots of experience within SW that we’re happy to share, and we’re always keen to hear the experiences of others on the same journey.
You may notice that we haven’t mentioned carbon savings in these notes. The headline is that having a home energy system will make a very big dent in your household carbon emissions, but looking at the very detailed ins and outs of this is beyond the scope of this document.
What do you want your home energy system to do?
You know that you want a solar system – but why should you fit batteries too?
The most common reason to add batteries to your solar system is so that you can store solar energy generated during the middle of the day for use later on, maximizing the financial value of your generated energy. But there are other reasons too:
- to provide back-up power to your home during power outages.
- to make money by buying and selling electricity at different times of the day (known as arbitrage) – with or without solar panels
If either of these reasons are of interest, then make this clear to your possible suppliers so that they can offer you a system best tailored for your requirements.
What functions should your home energy system have?
To get best value from your system, consider how it might operate with other appliances that you already have, or might install in the future:
Solar and Electric Vehicles (EVs). EV charging is a really good way of using any electricity that you can’t use in your house.
Solar and Hot water heating. Devices that automatically use any surplus solar energy to heat hot water have been popular. But if you have batteries fitted, then storing surplus solar generated electricity in your batteries for later use in the home is likely to give a better financial return.
Solar and Heat Pumps. It’s unlikely that your solar system in winter will produce enough for your non-heating needs, and so there just won’t be any left over for your heat pump. So while both solar systems and heat pumps are good things, solar panels won’t really help to power your heat pump.
Heat pumps for hot water heating. If you have a heat pump, then its worth using electricity to heat water using the heat pump rather than using an immersion element. This is because the heat pump will typically give 3-4 times more heating power than an immersion element consuming the same electrical power.
Batteries and Heat Pumps. We haven’t tried this, but we think that this could just be a game changer for heat pump economics. Very simply, if you can buy off peak electricity and store it for powering your heat pump later in the day, then the cost of running your heat pump will be much lower. The downside of this is that you will need a larger battery, but it’s worth doing the sums.
Maximum PV size
If you have a large roof, it is possible to export more than the 3.7kW maximum usually offered as standard, but this requires the installer to apply to the network operator to check that the local network can cope. This can be immediate and free, or it can take several months. There may be a non-refundable fee of typically £100-£350. In practice, much above 3.7kW the amount of this that you can use or store decreases, and so there are diminishing financial returns for having larger systems. On the other hand, because solar panels are now so cheap, the marginal cost of additional panels is not large, and so it does very much depend on your circumstances.
System Economics
Payback calculations
All suppliers will include detailed calculations that show the energy and cost savings that you can make with their proposed systems. Do look at these carefully, and make sure that you are comfortable that any assumptions made are a good match with your circumstances.
If you have the skills and patience, then it’s worth creating a spreadsheet to do the sums for yourself, which has the bonus of giving really good insights into how you want the system to operate. It’s also interesting to play around with different PV and battery sizes, and look at the impact of different tariffs.
We noticed that payback calculations factor in future energy price rises, but this doesn’t take account of future inflation in other living costs. So if the model shows that energy costs are going up by 5% pa, but you think that other costs are going up by say 3%, the annual increase of energy costs in real terms is only 2%.
It is not just about the financial return
It’s important to be aware that fitting a home energy system will not make you lots of money, with a payback of perhaps 8-12 years being typical. There may be financially better ways to invest your money, but if reducing your carbon emissions or having some energy security is also important, then it makes lots of sense.
Everyone who has fitted a system also feels good that they’re “doing their bit”, and the return is still probably better than you would get from a building society. If energy costs do continue to rise above inflation, then your system will help to hedge against this.
Tariffs
New installations can no longer get the generous Feed In Tariff for each kWh generated, but there are now lots of different tariffs aimed at encouraging the export of power at peak times, and the import of power at off-peak times. Choosing the right tariff can really change the economics of your system. Have a look around available tariffs, and do think about what would happen if any of the currently attractive tariffs were withdrawn.
You don’t have to keep to the same tariff, with some people changing tariffs during the year to maximise solar generation income during the Summer, and then moving to a tariff with a lower electricity purchase price during the Winter.
Although rarely a problem, do check that the system you are being offered is compatible with the tariff that you are considering.
Tariffs will not only determine the economics, but also the design of your system. For example, if you have a tariff with two low cost charging times a day, then this might impact the size of battery that you install.
But however accurate the calculations, if tariffs change, then so will your financial return – and there’s no way of knowing what will happen in the future.
On the roof
Shading
In most installations, if just one of the panels has its output reduced due to shading, then all the other panels that are connected to the same “string” will have their output reduced by the same amount.
Most systems will be designed to completely avoid shading, and so it isn’t something to worry about. But if you think that it might be an issue, it’s worth understanding what impact it might have on the energy produced.
All suppliers have to use an MCS accredited method to calculate the reduction in energy produced over the year. Because detailed analysis takes time, suppliers are allowed by the MCS to use on-line software based on on-line 3D mapping. This isn’t quite as accurate as traditional field measurements, but it does greatly reduce the cost of analysis, and we understand that any errors in the shading calculations are likely to be in your favour. But if you have shading from nearby objects such as trees or chimneys, it’s worth confirming with the supplier that the model is taking sufficient account of these.
In practice, if parts of your roof will be shaded at different times (such as if you have an East West roof). then a two channel inverter will be offered that treats them as two separate systems. While the total yield isn’t as big as with a south facing roof with the same total rating of solar panels, you will get more power at the start and end of the day, which for many users may be a better match to their home energy use pattern.
Some solar panels have built in electronics that individually optimises each panel. These can work well, but the low price of solar panels and inverters means that the cost premium of these alternative panels might be too high.
North facing solar panels
Although the sun never shines from the north, north facing panels will still generate some electricity, indicatively up to half that of a south facing panel.
Equipment location
Suppliers will want to fit the control equipment in a place that is easiest for them, and so if you want it somewhere else, then ask them!
Bird-proof netting
Although rarely a problem in practice, the cost of retrofitting netting will be much higher than fitting it during the original installation. Mesh type guards allow for much better air circulation.
Adding batteries to your home energy system
Back-up power power socket
Several inverters have a single power socket in to which you can plug essential loads during a power outage. For many homes this will be sufficient for very occasional power outages, and there’s a good choice of inverters available that will do this.
(Wired in) Whole-house back up
This type of system allows you to power your whole house from the batteries during a power outage, but there are several important things to consider before doing this:
In order to limit the peak load on your battery system, you will need to split the circuits in your consumer unit into critical (maintained), and non-essential (non-maintained) circuits that you don’t mind losing during a power outage. This will require some re-wiring of your consumer unit.
Or you can leave the consumer unit as it is, enabling the battery to back-up to power all your home circuits. The problem with this is that any heavy loads (e.g. an electric oven, tumble drier, electric shower, heat pump) will run the batteries down quickly. A pragmatic approach might be to leave the consumer unit as it is, manually turning off heavy loads when there is a power cut.
If you already have a solar scheme, then your battery system will be connected to the mains separately (sometimes referred to as “ac connected”). But note that this will mean that during a power cut, your solar panels will not be able to charge your batteries.
If you are fortunate enough to have a solar system installed before 2019 where you receive Feed In Tariffs (FITS), then it’s important not to make any changes that might mean that you lose your FIT.
Integrated battery: solar equipment
If you don’t already have a solar scheme, then you will probably be offered an integrated system that allows your solar panels to charge the batteries during a power cut. These are sometimes called “DC coupled” systems.
Your installer should check that your equipment can handle the rated current of your consumer unit.
Battery sizing
There are different ways to look at this, depending on what you want your system to do:
- Sufficient to hold any excess generated power for later use, or sale to the grid at a later time when you can get a higher export price. 10kWh is a typical size for this purpose, much less than this seems just too small to be worthwhile. If you have more capacity than you can fully charge on a regular basis, then there will be diminishing returns.
- Back-up energy for use during a power cut, in which case the amount of energy you want to have available will determine the battery capacity that you need.
Most batteries will have a minimum discharge setting of 10-20%, meaning that you don’t have quite as much capacity as you may think. If you are wanting your battery to be able to supply your home during a power outage, you can set this discharge setting to a higher value (say 40%) to make sure there is always sufficient remaining for when there is a power cut. But in this case you would only have 60% of the capacity of the batteries for regular use.
- Arbitrage (making money from buying and selling electricity to the grid). This is a fast developing new business for commercial scale systems, but in practice the numbers for domestic systems may not be sufficiently attractive.
Why your batteries don’t have as much capacity as they say
There are several reasons why your battery won’t give you quite as much capacity as you may think. Ask your supplier the actual capacity your batteries will have, and how long they are expected to last.
Round trip efficiency. Each time a battery is charged and discharged, there will be losses in the battery itself and in the electronics. This is likely to be the most noticeable factor in new installations.
Self discharge. Over time, all batteries will lose a little charge through internal leakage, but for batteries that are used regularly this will barely be noticeable.
Battery age. Fortunately batteries don’t now age as quickly as was previously thought, but their capacity will slowly reduce over time, driven primarily by the number of cycles they are used for.
Discharge rate. If you draw very high currents from the batteries, they will get hotter and have a reduced capacity.
Temperature. Batteries don’t like to be too hot, and really don’t like being too cold. If they get too cold then their capacity and available discharge current will decrease dramatically. Some batteries have built-in heaters to overcome this.
Which components should I use?
Most people just trust their supplier to pick the right components (electronics, batteries and solar panels), and this seems to work just fine. But even if you’re not technical, it’s worth asking a few questions so that you have full confidence in the system that you’re buying.
Solar panels: The rated power of the standard panel sizes keeps improving, so make sure that you are buying the very latest model. Your supplier can advise you on which are the premium quality brands, but we get the impression that even the cheaper panels may perform adequately over their life. The price of solar panels is now very low, so while the scaffolding is up, do consider fitting additional ones. Some people even reckon that they’re cheaper than fence panels, but you may struggle to find an installer happy to do this for you!
When comparing prices, work on the basis of kW, not the number or size of panels.
Electronics (inverters and battery chargers).
There is a lot of innovation in electronics for home energy, and lots of products to choose from. Some of us were happy to go with competitively priced products from newer market entrants, others would rather pay the extra for more mature products. From looking at on-line installer forums, we’ve seen that some products seem to be more robust and better constructed than others, but this isn’t something that is easy to find out from suppliers. It seems that generally the more you pay, the better the quality of products.
Although it may at first seem daunting, have a look at the key features of popular inverters to get a feel for what is out there; the common features and differences will soon become apparent. The inverter is the heart of your system, and so this is where it’s worth spending your time. You don’t have to go with the first inverter a supplier offers you, and while most suppliers stick to one or two inverters that they are experienced with, they should also be able to source any inverter that you ask for. But in this case, you need to be clear in the contract that while you may have suggested a particular product, they assume responsibility for checking that it will work in your application.
If you’re really keen, then visiting one of the regular-ish home energy shows at the NEC, Excel or similar exhibition halls is a great way to talk to different manufacturers and see and play with their products.
Batteries. These are similarly becoming much cheaper. The chemistry of batteries is evolving, so make sure that you are buying a well established type. Batteries are increasingly integrated with the inverter / battery charger, which makes system design and installation easier, but failure of one part could mean that the whole unit needs to be replaced. Batteries should be compared in terms of £/kwh.
Internet connectivity
We were surprised that most systems just have very basic controls and lights, with connection to a phone or PC essential for full control and information. Suppliers of other internet-connected products are now much more likely to state how long the software upgrades will be made available. But unfortunately we couldn’t see any times for guaranteed software support for any of the electronic inverters that we were offered. If the ability to provide power during power cuts is important to you, then you may also want to check how well the system will perform during an internet outage.
Which supplier should I use?
The starting point is to only use companies that are registered with the Microgeneration Certification Scheme for the type of system that you are looking for – you can check this at MCS Certified | Giving you confidence in home-grown energy. But this is just a minimum qualifying criteria to allow you to connect your system, and in itself it is no guarantee of quality. Finding the right company can be daunting, especially when you’re new to the subject, and so there are several things that you can do to check out prospective suppliers:
- We reckon that personal recommendations are a good starting point.
- Look for comments on the company on social media, but remember these comments should be treated with some care.
Look on Companies House. All but micro-companies are required to submit annual accounts that will tell you something about the company. Even if you’re not familiar with looking at accounts, you can see who the Directors are, how long it has been established, how many employees it has, and an idea of the financial stability, but remember that accounts are always historical and so can not predict future performance.
- Membership of a trade body to promote quality suppliers can be useful, but remember that these schemes are themselves a business reliant on fees from traders. It’s worth checking that you are happy with the qualifying criteria to get on to the scheme, and for the policies for removing traders who have complaints made against them.
You could buy the components on-line and install it yourself, but unless you are legally competent to do this then it could be unsafe, and unless you are MCS registered you will not be able to connect the scheme up to the network.
If you’re not convinced by the need to go with a good supplier, internet forums have plenty of stories of cowboy installers who give you a system that might actually look fine to begin with, but that when it goes wrong turns out that it wasn’t installed properly. It can be costly to get the system retrospectively brought up to standard, and few companies are willing to do such work. A wider problem is that low cost installations undercut the suppliers who want to do a good job.
Quotations
We were surprised that many installers will give you an initial quotation without visiting your site, only visiting in person if you are happy in principle with their offer. We found that although this did slow down the process of getting a definitive quote, it did mean that when they did come to visit, buyers were much better informed about their offerings and able to get more out of the meeting. You should expect suppliers to want access to your loft and to inspect your consumer unit. It’s also important to have started to think where you would like the inverter electronics and batteries fitted.
We have heard of installers refusing to fit equipment in the location that the salesperson stated, so it’s worth checking how the company is structured. There are some companies who just use sub-contractors for all the installation work, in which case these sorts of problems are more likely to occur.
Contractual
Read the contract very carefully before you sign, thinking through your position for different problems that might arise. If there’s anything you’re unhappy with, then most should be happy to make reasonable changes or additions in order to secure an order.
If you asked for your system to have particular features, then attach this to the contract so that you have a clear right of complaint if the system they have offered you doesn’t do what you asked.
We strongly advise corresponding by email as much as you can, as this way everything is in writing and so there is less room for miscommunication within the company.
Warranty claims.
As your system will hopefully work for 20 years plus, it’s worth spending some time to understand exactly what the warranty is on the system and its components, and that you understand how to make a claim. Your first port of call in the event of a problem should be your installer, but if they should cease trading, will you still be able to claim on the warranty directly from the supplier?
Do you know where the support service for the equipment is from – we were surprised that one equipment supplier required you to speak to a helpdesk in China in order to get a software update.
The biggest risk is that your supplier may simply cease trading – so find one who is likely to be around for the foreseeable future, and have a plan in case they’re not. If you do need to find an alternative supplier to do any maintenance work, it will be much easier and no doubt cheaper if your inverter and batteries are a well known brand that other companies are more likely to be familiar with.
Some equipment manufacturers appoint premium installers with whom they enjoy a special relationship – it’s not clear exactly what benefit this gives, but if there are any equipment problems you may find that you get a better service. The possible downside of using a premium installer is that they may be keen to push that make of equipment in preference to others that might be more suitable.
On the whole we found that suppliers would start by recommending a cost competitive product, but if asked were happy to source alternatives. So if you do have any doubts about the products being suggested, do investigate alternatives.
While suppliers should give you a guarantee that the system will work, we didn’t find any company who will guarantee the power or energy generated by a solar system.