I have 14 panels, a 5kw inverter and a 9.5kwh battery, I’m thinking of purchasing a second battery to take some of the load, granted more this time of year
The ROI aside, one question I had was will a second battery in tandem produce twice the output at peak load times? Or does one battery deplete 1st then the load is moved to the second?
[unknown] they should empty and fill at roughly the same rate. Although the primary (best to make that the new one I think) will be hammered a bit more than the secondary as they are (usually) daisy chained.
The output will be limited based on the inverter's throughput rating which might be different for DC battery to AC Vs DC solar to AC.
Check the product data sheets for the exact details.
Midlands_Solar Depends what tariff you are on (Eon drive I think?) but the economics of a second battery pretty much always look awful.
The saving between night rate and day rate x number of days a year you need the extra capacity x 0.8 (20% conversion losses) gives a payback measured in 10-20 year range.
Which is annoying. I would like to add more capacity too but it just doesn’t add up.
TX200 comments on throughput are correct
geoffreycoan
Though as a (perhaps weak) amendment to that pithy and accurate analysis, if you charge/export/charge overnight then a larger battery can perhaps pay for itself slightly faster.
On my single AIO I can clear about £1 each night doing that. So about £400 pa. Which pays for new batteries in ten years, or reduces the payback period (on top of normal usage) to a reasonable level.
As long as it is possible to import at 7p and export at 15p ...
If (when?) I got an EV id probably end up on Go or Intelligent Go.
With a great overnight rate, I'd certainly be tempted to upgrade my existing battery (5.2kWh/4.8 usable).
I was on go faster when I first had a battery, it was great that the off peak started around 8.30pm. If the solar wasn't that great, I wouldn't have to wait long with an empty battery.
My battery is perfect for intelligent flux and is doing fine on agile too at present.
But if I went to Go/IOG I'd be worried about the peak rate usage. I think I'd end up running out a lot of the time. Plus the electric shower would be used during peak rate unless I got up really early. 🤣
I don't mind moving my dishwasher usage to overnight but the washing machine is a bit too noisy if I want to get a decent sleep!
TX200 I don't mind moving my dishwasher usage to overnight but the washing machine is a bit too noisy if I want to get a decent sleep!
I don’t mind the laundry being done overnight. But, as has been forcefully pointed out to me, I don’t do the bloody laundry
It won’t double your output but it will increase it slightly in some situations (see below).
Is that a hybrid 5.0 inverter?
Discharge Under normal home load max: 3.6kW.
“Timed Export” rate: 4.0kW (constant).
“Timed Charge”: 3.3kW (when “empty”) increasing to 3.5 kW up to 90% SOC. Then the usual decreasing stepped down charge from 90% up to “100%”.
And in answer to your next question: Yes, they will (just) charge from empty to full in the 6 hour IOG cheap rate.
The pay back economics very much depend on what tariff you can henceforth get on. (or not)
If you try to calculate savings based on the same tariff with the same or slightly reduced average import cost then it may take quite a while to pay back as @geoffreycoan says, even if it reduces your average import rate somewhat. Especially if you have a heat pump (and IOG may not be feasible). But that can lead to an unfair comparison when working out “payback”. As the aim of getting an extra battery should be to get on the cheapest important rate (IOG) which will drastically slash your import costs! (and hence make the payback time look great).
As @PianSom correctly points out;
If adding a second battery enables you to get onto IOG and bring your average import cost down to as close to 7p as you can get, then the payback time will be far far quicker.
Assuming adding an extra battery will allow you to switch to IOG;
You need to multiply a years worth of your import at your current average unit cost (say 12p for arguments sake, may be more may be less). And then take off a years worth of your import at 7p. That’s Part A) of your annual savings. (ie 42% cheaper pa overall !) or looking at it the other way around 71% more expensive to be paying 12p rather than 7p !
Also you need to add part B) - That when it’s not winter you will be filling it for 7p overnight (as usual) and exporting all that’s left for 15p later in the day. So, if nothing else that’s about say an extra 9kWh x 0.15 x 365 = £263. Or say it’s only for 9 months a year (3 months winter) £200 savings. Roughly, That’s part B) of your savings.
So say you use 10,000kWh a year. You’d save £700 plus £200 = £900 a year roughly. Then divide the cost of the extra battery installed by £900 to give the number of years for payback.
There are other intangible benefits as well such as;
1) Being able export more during the savings sessions without running out later. (cough, cough, what savings sessions?)
2) Free power up events yields more (you can import more to export more later)
3) Being able to do a cheeky discharge for a couple of hours in the morning in winter (after 05:30) then plugging the car in to finish by 11am and charging the batteries back up again (so you’ve got more to discharge later in the evening!). Not sure that’s in the spirit of things but who’s judging?
All these intangibles add up. Say £100pa conservatively.
So if it’s £4k to install a second battery and that allows you to get on IOG and it saves you £1k pa off your bill you could be looking at a 4 year payback.
If it doesn’t allow you to get on IOG and your average import rate doesn’t come down much then payback could be terrible and quite frankly not worth it at all.
It’s VERY personal and very variable the payback time. Diminishing returns on the same tariff after a point but if it allows you to get on IOG (or EON next drive.) Happy days ! Even if you do still have to import a bit of peak on the coldest days in winter.
Tariffs change though, so everything could be out of the window in 5 years - who knows.
…so payback currently could be as good or as bad as 4-20 years and everything else inbetween depending on tariff and usage.
A) should be £500 savings not £700. Just assume it’s 12,000kWh usage a year instead and the maths works !
Apologies.
You get the idea though.
Pete UK great analysis, and if you can get onto IOG or a similar EV tariff then the extra battery capacity can change the dynamics quite considerably
but a slight counter point is reminder that you need a compatible EV or charger so if you haven’t already got one of those then the £500-1000 cost of the charger and the EV needs to be included in the equation
Personally an EV for me doesn’t stack up as I don’t do enough miles for the cost-per-mile to be a significant number so I just run an older and very inexpensive ICE car. With a heat pump as well I would need to add a lot more battery capacity to cover the daily consumption. Maybe if I started now I would have an AIO or the 8/10kW hybrid, but I’m not starting afresh
geoffreycoan No EV is required for Eon Next Drive and the current v4 rates are comparable to IOG
Avenir geoffreycoan No EV is required for Eon Next Drive and the current v4 rates are comparable to IOG
Yes I had heard that, I’d also heard that Eon was changing the policy and there was a replacement tariff for battery owners
I just ran the numbers for my own consumption with Eon Next Drive, and over the 3 months of winter Eon (£872) is remarkably similar to what I’ve paid on Octoput Agile (£879). I think if I had more battery storage (13.4kWh currently) so I could run more of the ASHP off the overnight rate, it’d be compelling.
Yep. There is that too. But if you have an EV and charger already or if you need to replace your car anyway or if your company car is going to be an EV you’ll get the cost of a charger install back pretty quickly; that’s only a couple of ICE services…. You’ll never really get the cost of the car back but then when do you ever with a car ?
It all just depends where you are on the journey.
One size rarely fits all (as in the economics of adding more battery storage). I’d really like a heat pump and I recon I’ve got probably enough battery storage already - but it’s not looking like it’s going to be anywhere near cost effective to install in my older solid wall property even with the grant it’s about £17k for two massive heat pumps and that doesn’t include changing all the radiators! So I’ll likely just continue paying about £190 a month for gas and water heating, just have to accept it’s not to be. I’d like one. I’d really like one and I’d probably save 50% off my gas bill at 7p/ kW for electricity but the numbers just don’t add up - in my case anyway.
I’m pleased with our electricity bill being zero (or a slither negative). And free EV fuel for about 14,000 miles a year though. That is a miracle in itself compared to before solar/batteries & EV. Had the car coming up to 3 years now. Zero services, zero road tax (till April 2025 anyway!), zero congestion charge, zero maintenance. I’ve only had to buy 1 new tyre (as it got a nail in it). Solar and batteries have been in 18 months now.
All this stuff is a very large initial outlay to get the maximum savings benefit. In some cases it just isn’t worth going the whole way. You getting an EV or me getting a Heat Pump. In other cases it makes a lot of financial sense. If you’ve got relatively low costs in one area already then little point spending a fortune to save a little bit more. Solar panels/ batteries, EV, Heat Pump. 2/3 will have to do me !
geoffreycoan I’d also heard that Eon was changing the policy and there was a replacement tariff for battery owners
I beleive the Next drive v5 T&C now are only for EV (V4 was for battery owners as well) - do you have any details of their tariff for battery owners? I couldn't find anything
CW They don't have one, well not unless you get the PV panels and battery fitted by Eon.
CW Did some digging on the Eon forums. According to Eon support, the solar + battery tariff is 'in trial', but until that launches they are accepting battery-only customers onto Next Drive v5.
Avenir Any details on what the Solar + Battery tariff looks like?
geoffreycoan not that I could see
Midlands_Solar in case you hadn't seen it, there was some good discussion (and some attempts at modelling the costs of) second batteries here: https://community.givenergy.cloud/d/5318-second-battery-economics
In my case I can't realistically justify it. Bidirectional EV charging, if/when that ever happens... well that's another story.
Thank you all! and sorry for the late reply, recent job change and it's been a heavy few week!
Midlands_Solar Did you come to a conclusion in the end?
Josephiah I opted to not get a 2nd battery, it seemed a very high cost outlay with a long ROI, for a storage system I will only really need three months of the year.