Is It Worth Investing In Solar Battery Storage To Power My Home?

Should I Invest In Batteries To Power My Home?

If you already have solar panels, that's great! But if you don't have batteries, you are loosing out.

To benefit fully from the energy generated by your panels, you would need to be at home all day using the power as it is being produced, but in most cases, that is not practical as we have to work and do other things away from the home during the day. It is safe to say we will be away from home at the time when the solar panels are producing most of the power. So where does that power go? All is not lost as the panels will be powering the refrigerator and other things that are plugged in such as Wi Fi routers and chargers as well as other devices left on standby. These will only draw a few hundred watts at most, where's the rest going?

Grid tied Inverter

If you've got solar panels, then you will almost certainly have a grid tied inverter. This converts the DC voltage produced by the panels to the AC voltage coming into your home from the grid. This also has to synchronise with the 50 Hz frequency of the mains in the UK (60 Hz in the US). If the solar panels are active, the inverter will inject a slightly higher voltage than the mains which will be absorbed by appliances in the home, any excess will go out through your meter onto the grid and power some of your neighbour's appliances. Think of it ! Your power from your panels powering other people's homes and the utility companies getting paid for it! You may be part of a scheme where your utility company buys your surplus power, however, this is at a reduced rate, perhaps 4p per KWh, some may be a little higher, but nowhere near the 25p or so per KWh that you have to buy from them.

Battery Storage Is The Answer

The answer is to store all of that energy that the solar panels have produced for You to use it when You need it. Technology has moved on and battery prices have come down enough for it to be within reach of a lot more people. So while you are out during the day and your home is using very little of what the panels are generating, rather than feeding the grid, your batteries are 'banking' that energy for when you come home and start using appliances as you normally do. Only this time, you are not drawing from the grid, but the free electricity stored in the solar batteries.

The Tesla Powerwall 3

I mention Tesla because it's a well known name associated with Electric Vehicles (and Elon Musk) but they have also got into the home battery market in a big way and this will be a name banded about if you search the subject of home battery storage on the internet. Here's the cost of having the Tesla Powerwall 3 in the UK - 

• The Tesla Powerwall 3 costs £7,995 + ~£2,800 for installation & hardware.

Almost 11 Grand! Ouch!   Granted, that's for a 13 kwh battery. Some may be happy with that as the quality is there and the backup is good.

What Size Battery?

The lion's share of the cost is going to be the batteries as these need to be big in order to handle the demands of the home, we are talking kilowatts. A kettle alone may use 3kw, then there's the washing machine, clothes iron, hairdryer, not to mention the tumble dryer - anything that uses electricity for heat draws a lot of current which will quickly run down the batteries.

You need to find out the average daily electricity consumption of your home in kwh. If you have a smart meter, you may be able to view the history over the past week or month. It might be best to look at the winter months usage as this will give a better idea of the maximum daily consumption your household will use. My peak is around 7 kwh so I would need a battery of at least that capacity to supply power to the home each day. Every home is different so your daily consumption may be more or less than mine.

It's not a good idea to fully charge the battery, then fully discharge it every day as this would greatly reduce the lifespan of the battery. That's why something like the Tesla with it's 13 kwh capacity is desirable as this leaves a good reserve margin and will prevent the batteries getting 'hammered' meaning they will last longer.

A/C Coupled Or Hybrid Inverter?

If you've got solar panels, chances are you have a grid tied inverter. If you plan to install batteries, you will need either a A/C coupled or hybrid inverter. If your system was installed under the Feed In Tariff scheme (no longer available) then you will not want to change the system in any way as you could lose out on the FIT payments. (The longer you have had the system, the more generous the payments are.) you may also be registered under the SEG export scheme.

The way around this is to install a A/C coupled inverter which will work alongside the solar PV system you already have, The batteries are connected to this inverter which managers charging and powering the home from the batteries. 

If you are not on any scheme or you are starting from scratch, then a hybrid inverter is the way to go.

With this type, the solar panels, the batteries and the mains are all connected to this inverter. It knows when to charge the batteries from the panels, when to charge the batteries from the mains - perhaps at a time when it's at a cheaper rate and also export power to the grid if the batteries are charged and there is surplus power.

Hybrid Inverter using solar to power the home and charge the batteries. Notice no power going to or from the grid thanks to the CT clamp and meter.

CT Clamp

For either of the above inverters to do their job properly they need to know how much electricity is coming in, or going out to the grid. The CT or Current Transformer clamps around the live mains lead immediately after the meter. The inverter can then monitor in real time the amount of electricity and the direction it is flowing. Some inverters can have the clamp wired directly, others need a separate energy meter in which the clamp connects to and then sends the data to the inverter. These are more accurate, especially if there is a bit of a distance between the inverter and the meter. Without the CT clamp, all of your battery power would go out into the grid because the inverter wouldn't know when to stop. 

Energy meter with CT Clamp. Notice the power is showing zero Watts. The hybrid inverter is doing an excellent job making sure no power is being drawn from the grid or going out to the grid. Solar is powering the home, the surplus is charging the batteries. 

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How Long Is The Return Of Investment?

What we need to work out is the actual savings gained by the solar battery installation Vs the overall cost of the system. I worked out that my system should save around £500 - 600 per year, it will probably be more, again, depending on how much electricity you use, could be more or less. Now if I was to go with the Tesla Powerwall 3 mentioned above, it would take 22 years for payback. That's not on! We should be looking at 3 years average, 5 at the most. If you look around on the internet including auction sites  (you know the one) you can pick up batteries with a decent capacity for a fraction of the cost of the Tesla. You can get 15 kwh batteries for £1600, or 9kwh for £1000, you can pick up a decent hybrid inverter for around £500. £1500 for a 9kwh setup - Payback time in less than 3 years ! That's the way to go. Some suppliers offer 5 or 10 year guarantees, you may have to pay extra for this. You need to ask- will those companies still be there in 5 or ten years time? If the batteries fail after 7 years, will you be bothered to chase them for the 3 years? Prices would probably have come down further by then and capacity increased so it may be time to replace them anyway, and hopefully, they would have well paid for themselves by then.

Is It Possible To Go Totally Off Grid?

That would be a nice prospect, just think ! No more hefty standing charges every day, but here in the UK and most other places, it is probably not doable due to the fact that the sun doesn't shine all year round. We can write off 3 months - December, January and February, we say 100 days of no sunshine, therefore, no or very little solar power. Yes, during these months you may get a few sunny periods, but nothing you could rely on. For a few weeks between December 2024 and January 2025, the weather was stuck in a very gloomy, dull, stagnant spell. There was hardly even any wind so no renewable energy during that time. If you were off grid then, it wouldn't be long before your batteries were completely drained. The idea then is to save as much as we can in the months when the sun is shining, but we need to be realistic when it comes to the winter months. keep in mind that the solar panels will still be producing some power during daylight hours in the winter and may even charge the batteries depending on house load and panel capacity.

Here's something to think about - Let's say the average electricity standing charge is around 55p per day 365 days per year, that's roughly £200 before you use any electricity - £1000 after 5 years.

Suppose you bought a generator with the money you would have spent on the standing charge and used this to charge your batteries? It is estimated that 1Ltr of diesel produces the equivalent of 10 kwh of electricity.  We'll say a litre of diesel is £1.50 at today's prices,  divide by 10  =  15p/ kwh. Don't forget there will be loses in the generator, but would it still be cheaper to use a generator rather than pay around 25p/ kwh from the grid with the added bonus of no standing charge? If you were to go down this route and be disconnected from the grid, you have to think about what would happen if the generator failed, or oil prices started to rise and you didn't have the grid to fall back on.

There have been suggestions made recently regarding changes to how electricity is charged per kwh. One idea is to do away with the standing charge completely and put it on the unit rate. This would suit the generator model above as it would be far cheaper for the diesel kwh than grid kwh and you wouldn't need to disconnect from the grid in order to avoid the standing charge. Decisions decisions!