Should I bother with a battery? 14 February 2022 at 4:35PM edited 14 February 2022 at 4:37PM in Green & ethical MoneySaving 35 replies 1.4K views

Cottage_Economy

I have a 6kw system, fitted in 2919, and am thinking about whether to add a battery to it given the rising cost of electric.
In 2021 I generated 3473 units from the panels and imported 4090kw from the grid. My FITS payments are 3.92p per unit generation and 5.57p for exports. 

I’m with Ecotricity for my electricity and am currently paying 32.8p per kWh. Going forward our usage will come down a bit, as the rising costs of electricity has made us reevaluate our usage and we’re currently experimenting with what we can realistically change but I’m estimating a conservative 15% savings in that respect (no tumble drier, 20 degree washes, changing all bulbs (not just some) to LED, etc.) Could be more but I’ve lost touch with what all these things cost so it will take some time for me to get reacquainted. I should add this is a small holding so we do have a bank of freezers in our barns of which the large one is running 24/7/365 and the other two medium and small for about 6-9 months. We also have a 4-foot and a 3-foot tropical aquarium running in the living room as well. 

In the past we have exported 50% of what we’ve generated in the height of summer, which drops to 34% by the end of December.
A brief conversation with a supplier of batteries has resulted in a ball park figure of £4,000 + 20% VAT. 

I’m reading all the threads on the subject of batteries but it seems to be going in one eye and out the other. Will having a battery really save us that much money, or are we better off looking at making the maximum savings that we can through our usage and just forget about it? For example, this could be the last year we keep animals so over the next two years our freezer usage could drop drastically as we deplete our stores. 

QrizB

Cottage_Economy said:

Will having a battery really save us that much money, or are we better off looking at making the maximum savings that we can through our usage and just forget about it?

That's the $64k question, isn't it!

At the moment, mains electricity is 21p/kWh but you're selling it back to the grid for 5.57p/kWh. Each kWh you currently export that you can store and use, rather than importing, will save you 15p (although, if you're on deemed export, you could argue that it's worth 21p).

Let's guess and say that, with that £5k (£4k plus VAT) battery, you could store and use 2000kWh/yr that you currently export. At 15p/kWh that's a saving of £300/yr. It will take you 17 years to pay for the battery.

OK, but say on deemed export each kWh is worth 21p, now those 2000kWh are worth £420. Payback falls to 12 years. Still not great.

Essentially, battery economics aren't great, even with electricity prices as high as they are now.

Petriix

You have to do your own sums. It will vary massively depending on how much you're able to shift your usage into off peak. If I can get my peak imports to a minimum, that would save me ~£400 per year before I even consider additional solar self consumption, which could save another £100 per year.

It also varies depending on future prices and, in particular, the longevity of the large differential between peak and off peak prices. 

Cottage_Economy

QRiz - the long payback time is something that bothers me. I keep looking at the sums thinking there must me some merit in doing something battery-related, and at 32.8p it might well be (assuming Ecotricity doesn’t put the price up even further in April).  For every unit I use I’d save 26.8p, based on what we exported last year (1736) that would be £465 a year. Even if I stored and used all of that it would still take over 10 years to get payback? Is that right?

BUT we may end up moving on before then. DH is 61 this year and we want to be gone before he’s 70. Keeping a large place going is hard work when you’re older. 

Petriix - there is no off-peak price. Ecotricity has a one standard green tariff. Should add they are not included in ofgem’s price cap. I’ve been with ecotricity for many years and want to remain with them as long as I can.

I should have previously mentioned we have an iBoost system so it heats our hot water during the day ready for the evening. That’s an added benefit.

michaels

I'm thinking payback times might be very low just via time-shifting if the off peak 'ev' type tariffs remain available regardless of solar.

We use 8000 unit a year - if we could purchase all of these at a night rate of say 10p rather than the cap rate of 30p then that would save us £1600 per year!  On another thread there is a 5kwh battery for £1300 so 20kwh (our daily usage) for £5200 - don't know how much inverter plus fitting plus control systems would be on top but suppose the total is 8k then that would give a 5 year payback period.

Does anyone know what current cheap off peak hours tariffs are available? Octopus GO (via a switch to a different Octopus tariff?) any other?

A night rate less than 8/9p would also mean cheaper to heat our hot water using electricity rather than gas for that period - with an EV as well suddenly our 100A main fuse is looking a bit puny....

Cottage_Economy

Well, after some thought we're probably not going to go down the battery route this year, and maybe not at all. 

We don't think we're going to get as much benefit as we hope and at the moment the payback period is too long. We're going to work on getting our consumption as low as possible, and optimise our use of the panels during the day better. 

Ecotricity may put up its prices again in April, which could change the payback period so we'll revisit the idea in the autumn/winter and do some calculations, but at the moment it doesn't look good value for money for us. 

QrizB

Cottage_Economy said:

Qriz - the long payback time is something that bothers me. I keep looking at the sums thinking there must me some merit in doing something battery-related, and at 32.8p it might well be (assuming Ecotricity doesn’t put the price up even further in April).  For every unit I use I’d save 26.8p, based on what we exported last year (1736) that would be £465 a year. Even if I stored and used all of that it would still take over 10 years to get payback? Is that right?

Yes, I get the same £465/yr maximum saving based on your numbers.

You can buy a battery for less than £5k - potentially as low as £3k for a 4.8kWh system - but then you'll reduce the amount of surplus electricity you can capture for use. On those sunny days where your 6kWp of PV can generate 35kWh you'll only be able to store 1/7th of it.

I've just gone back to your initial post:

Cottage_Economy said:

I have a 6kw system, fitted in 2019, and am thinking about whether to add a battery to it given the rising cost of electric.
In 2021 I generated 3473 units from the panels and imported 4090kw from the grid.

Are those numbers right? 6kWp of panel would normally generate 5-6000kWh/yr, if installed to face south-ish. Do you know why you're only getting 60% of the typical performance from your system?

Cottage_Economy

QrizB said:

I've just gone back to your initial post:

Cottage_Economy said:

I have a 6kw system, fitted in 2019, and am thinking about whether to add a battery to it given the rising cost of electric.
In 2021 I generated 3473 units from the panels and imported 4090kw from the grid.

Are those numbers right? 6kWp of panel would normally generate 5-6000kWh/yr, if installed to face south-ish. Do you know why you're only getting 60% of the typical performance from your system?

Good question. I hadn’t spotted that. 

From fitting in March 2019 to December 2019 they generated 5483units. Between December 2019 and December 2020 they generated 6244 units. Between December 2020 and December 2021 they generated 4207. 

No reason I can see for the drop.

Verdigris

If the panels are connected in three strings it suggests one string has failed, as it is about a 30% drop.

Cottage_Economy

The panels are connected in two lines across a shallow roof. 13 portrait orientation with 7 below in landscape orientation 

Verdigris

By "strings" I mean electrical connections, not physical placing. Panels are connected in series, so the voltage adds up, but there is a limit that each connection on the inverter can handle. Because the panels are in series, if one fails the whole string fails, although it could just be a wiring disconnect, of course. The inverter may show fault codes, which would confirm if a string was not working and, possibly, why.