"Savings" calculation for solar PV with storage batteries?

[Agile]

There are a lot of firms wanting to sell storage batteries/inverters in order to enable solar power generated during the day to be used after dark in the evenings.

A simple look at this makes me think that this will never be financially beneficial.

Lets assume you already have say 6 kW of solar PV panels fitted. If you have say 4 kWh of storage then if this is fully charged during the day AND you use all this charge every evening then you will save 30p per kWh totalling £1.20 per day. Or annually £438 ! In practice due to the difficulties of using the full capacity beneficially it might only be about £200 benefit every year.

But a typical cost seems to be about £1000 per kWh. so you spend £4000 to receive a benefit of just 5% or £200 !

Then there is the life of the batteries. Many only last for 500 to 1000 recharge cycles! So the batteries will need replacing every 2-3 years. That will reduce the benefit from 5% to about 2% to 3% !

So my conclusion is that no storage system is ever going to be financially beneficial.

Have I missed something or is this just another industry mis-information situation?

 

[winston1]

You are about right. Also I don’t think I would want 4kW battery storage in my house. Mobile phone batteries make an awful mess if they catch fire/explode. Think would a 4kW battery would do.

 

[Adam_151]

Other than the pricing being a bit off from what I can see (£4k is probably about right for inverter & battery kit... if you had solar without battery storage you'd still need the inverter. Additional batteries seem to run to about 2k when purchased with inverter. The inverter would need to be compatable with battery storage, so would probably be fair to say 2.5k over not having the storage.

AC coupled battery is likely to cost more though, its got to come with its own inveter, along with appropropiate mains charger etc, and due to the number of conversions, looes enery thorugh conversion losses multiple times, so you gain less, but its more universal.

Even at 2.5k cost over non storage, not likely to make sense in todays world.... in a couple of decades though where the pricing is per minute and the cost in the evening quite sharp, then who knows, but batteries installed now wont still be good then

 

[flameport]

Many only last for 500 to 1000 recharge cycles!

Total fiction, batteries purchased today are typically rated for 6000 cycles, or 15-20 years with daily use, after which the capacity has reduced to 80%, so they are still usable after that.

But a typical cost seems to be about £1000 per kWh.

Also untrue, typical example https://midsummerwholesale.co.uk/buy/pylontech/pylon-US3000C
which is about £1500 inc VAT for 3.5kWh, well under £500 per kWh.
or this larger battery https://midsummerwholesale.co.uk/buy/pylontech/pylon-us5000
which is under £400 / kWh.

 

[flameport]

For the smaller 3.5kWh battery and 30p/kWh, if it's used fully every day, savings of about £1 per day or £365 per year, so it's paid for itself after about 4 years.
Even if it only lasts for the length of the 10 year warranty, that's still 6 more years of electricity that doesn't have to be purchased from the grid, or well over £2000 even if the price per kWh stays the same.

 

[flameport]

Also I don’t think I would want 4kW battery storage in my house

Install them and the inverter in a cabinet outside.

 

[JohnW2]

Also untrue, typical example https://midsummerwholesale.co.uk/buy/pylontech/pylon-US3000C which is about £1500 inc VAT for 3.5kWh, well under £500 per kWh. ... or this larger battery https://midsummerwholesale.co.uk/buy/pylontech/pylon-us5000 ... which is under £400 / kWh.

For the smaller 3.5kWh battery and 30p/kWh, if it's used fully every day, savings of about £1 per day or £365 per year, so it's paid for itself after about 4 years. ... Even if it only lasts for the length of the 10 year warranty, that's still 6 more years of electricity that doesn't have to be purchased from the grid, or well over £2000 even if the price per kWh stays the same.

Unless I'm misunderstanding, the above seems to relate only to the batteries/inverter, rather than the whole picture including the capital cost of the PV installation.

The total capital cost of a PV installation plus the batteries/inverters is presumably going to be a lot more than £1,500, so it would take a lot more than 4 years to reach the 'break-even point', quite possibly a period in excess of the warranties, even if one did manage to use the full capacity of the batteries every single day (which seems improbable).

In any event, as I'm always saying, even if did take only, say, 6-8 years to 'break even' (in terms of the total capital outlay), whether or not it makes financial sense depends very much on 'personal circumstances - since 'breaking even in 6-8 years' is another way of saying that one will be financially worse off for the first 6-8 years (and substantially so for the first few of those years).

Kind Regards, John

 

[flameport]

Unless I'm misunderstanding, the above seems to relate only to the batteries/inverter, rather than the whole picture including the capital cost of the PV installation.

It does. However with PV, you can use some of the generated electricity during the day as well as charging the batteries for use in the evening/night, so the cost of the PV isn't thrown away.

Whether it's viable or an acceptable timespan depends on the individual circumstances, but for most installations that already have PV, adding batteries is a viable option if the individuals intend to stay at the property for more than a few years. The batteries must be correctly sized, so that it's possible to use 90%+ of the stored capacity every evening/night, and the PV is capable of fully charging the batteries each day for a majority of the year.

There can be other benefits such as being able to operate at least some of the house in the event of power failure.
Batteries can also be charged from the grid at times when electricity is cheap, to avoid importing electricity at times when it is far more expensive.

 

[Jackrae]

Batteries can also be charged from the grid at times when electricity is cheap,

??? When will electricity ever be cheap ??? I know what you mean, but the days of "cheap" electricity are long gone

 

[flameport]

the days of "cheap" electricity are long gone

They certainly are for those on a single fixed price or the now obsolete E7 style.

With variable pricing that changes every 30 minutes, the batteries charge when it's cheap and power the house when it's more expensive.
At times of excess generation and low demand, that price can be extremely low or even negative.

Another option is to sell what's stored in those batteries into the grid when it's profitable to do so.

Some of these things are available now, others will be in the future, but battery installations are there for a decade or three.

 

[Jackrae]

I recall a comment from one of my school teachers, in 1960 when he said the price of electricity generated by nuclear power would be so cheap that it wouldn't be worth the "'Board's'" effort to charge for it. We live in hope

 

[EFLImpudence]

I remember that; not just teachers - the Government.

Don't forget the new ice age that was coming.

 

[winston1]

I recall a comment from one of my school teachers, in 1960 when he said the price of electricity generated by nuclear power would be so cheap that it wouldn't be worth the "'Board's'" effort to charge for it. We live in hope

Something similar was said about the abundant North Sea gas.

 

[JohnW2]

It does. However with PV, you can use some of the generated electricity during the day as well as charging the batteries for use in the evening/night, so the cost of the PV isn't thrown away.

Again, it depends crucially upon circumstances, since I presume that I am not unique in saying that, if I had a PV installation I could achieve that aim without suffering the cost of batteries/inverter. I could very easily shift anything up to 10 kWh/day or so of usage from the hours of darkness (currently at that time because those are the E7 "not quote so expensive" hours) to the hours of daylight, such that I would presumably never need to 'throw away' (export) energy that I had managed to generate with any probable domestic PV installation.

Whether it's viable or an acceptable timespan depends on the individual circumstances, but for most installations that already have PV, adding batteries is a viable option if the individuals intend to stay at the property for more than a few years.

Maybe for some but, as we've both said, it depends crucially on 'individual circumstances' - and, as I've illustrated above (and unless I'm missing something) there would certainly seem to be no (financial) sense in some 'circumstances'.

However, as presumably was apparent, I was thinking more of someone starting from scratch, and contemplating installing both PV and batteries/inverter - which, as I said, might need a decade or so to repay the initial capital outlay before one 'got into profit. If so then, at the least, to be worthwhile (or even 'sensible') one probably needs to think one is likely to still be around, and in the same property, in a a lot more than a decade's (ideally a couple of decade's) time, and be financially able to cope with the first few years during which one would be 'thousands' out of pocket.

The batteries must be correctly sized, so that it's possible to use 90%+ of the stored capacity every evening/night, and the PV is capable of fully charging the batteries each day for a majority of the year.

There's a fair few implied "IF"s in that. In fact, if the PV system is capable of fully charging batteries during Winter, the batteries would presumably not be able to store anything like the spare amount of energy generated during the 'peak-generation' periods of the year?

There can be other benefits such as being able to operate at least some of the house in the event of power failure.

True, and I'm not totally sure about the technicities of that (in the absence of a grid supply), but I am able to satisfy that requirement with the generator I bought from Lidl for about £140 several years ago.

Batteries can also be charged from the grid at times when electricity is cheap, to avoid importing electricity at times when it is far more expensive.

Also true, so long as there are times when electricity is "cheap", but the whole plan/policy seems to be to try to minimise the circadian variation in demand, which presumably means that the days of meaningfully "cheap periods" are probably numbered. Even today, the difference between my "peak" and "cheap" E7 rates is dramatically less than it was when I first started using E7 some 35 years ago.

Kind Regards, John

 

[ericmark]

What ever you spend money on to save money, you have to include loss of interest on that money if simply invested, or cost of borrowing the money.

To work out interest rates 10 years in advance needs a crystal ball. But unlikely to be less than 10% per annum, so

a typical cost seems to be about £1000 per kWh. so you spend £4000 to receive a benefit of just 5% or £200 !

That would mean minus £200, but be it interest rate or how much power you get, or maintenance cost, it is guess work. My daughter has a property in Turkey with a flat roof, with internal access to the roof, and the solar panels are easy to maintain, there is no real reason why the panels need to be stationary, the could easy follow the sun, and adjust azimuth, main point it a flat roof.

Putting panels on a sloping roof presents problems with maintenance, even a simple task like brushing off the snow, also the actual weight of the panels, and any damage to the roof due to panels.

I see no problem with a home designed from concept to have solar panels and other energy saving items built in, this includes the provision of large water tanks to store the energy so it can be used to heat the home, or batteries which can both heat and cool the home, however we have seen it all before.

I remember in the late 60's the MANWEB building in Chester it was claimed to be super Eco friendly, ahead of its time, the post war prefab homes designed to just bridge the gap, and not intended to last are still in use today, but MANWEB building gone by 2000 I think 1995 but not sure of the date. So this building only lasted around 30 - 35 years, we want homes to last far longer than that.

I was born 1951 and I have seen so many what seemed good ideas, now no longer used, the idea of a material which changes state at around 70ºC in the 70's seemed the answer, off peak power turned it to a liquid, then it turned back into a solid over the day liberating heat. Be it water or a chemical or as heat or change of state, there are many ways to store energy.

I see smoke coming out of chimneys today, in the UK we heat the home for up to 9 out of 12 months, and even in the summer we still want domestic hot water, so storing energy as heat is a good idea for most of the year, so when looking at storage batteries you need to compare to other methods or storing energy.

My brother-in-law had a house with two large storage tanks which would store energy from the wood burner, or solar panels, and use it to heat the home when required. It worked well, and likely would continue to work for some 40 years, the solar was not enough to keep home warm on its own, but he travelled to Germany on a regular basis, and while away the house kept the temperature just warm enough to stop freezing. So the geofencing would turn up the temperature on return, and it would be 3 days before he needed to light a fire.

So we need to compare battery with other storage methods, is it the best method, both in financial terms, and in planet saving terms, the latter we look at the cost of extracting the minerals required to make the battery, and once you consider that, it is unlikely batteries are best option in the UK to store energy. And remember in UK, not interested in if proved in Germany, it is what works in UK.