Here’s my take.

We have 30kwH of battery storage

if we run the ASHP on it then it runs out by 10am. However that 30kw costs us 8p/keg so still keeps costs lower

EBay we’ve been doing is running the ASHP on 17c room temp then using log burner to top up the room temp. Batteries then last most of the day. 

that means we’re paying between £5-10 per day for elec instead of £15-20. Our average price is about 12p/kEh (Octopus Go)

in the summer we should be able to go all of the day on solar, or off peak electric. 

@si-fillo the answer is, it depends. But if you can get the price right per kW you can save significantly in the shoulder months and take the sting out of the winter with time of use (ToU) tariffs.

It's pretty easy to work out the benefit and payback - given solar is free and you know your tariff rate. In the winter you use the difference in ToU and peak rate. Any solar in winter will never go into the battery anyway.

So three months of the year you basically won't have a bill. 6 months it's reduced significantly and over winter it's arbitrage. Of course as I said at the start it depends on how much storage you have and at what £/kW. Batteries right now are very expensive to buy off the shelf. Plus you'll need a battery charger and controller.

Posted by: @si-fillo

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I'm trying to understand if investing in battery storage will be worthwhile. We have a 3kW solar PV array. During 2022 it generated 3228kwh and we also pulled 7250kwh from the grid

We have an ASHP which can consume ~60kwh on the coldest winter days.

During the spring/summer, due to the reduced heating demand and increased solar generation it is a fair assumption that a battery would keep our pull from grid very low.

Between Oct - Mar, due to increased heating and reduced solar its fair to say a battery will have next to no impact on our pull from grid.

Then throw into the mix energy tariffs. If I can get a cheap overnight tariff to charge battery during the winter then it sweetens the deal. However, I believe the Octopus Go tariff where you can access even cheaper overnight rates only apply if you own an EV (I don't). Then I have to consider the size of a battery if I wanted it to take care of much of my demand in the winter. With the HP taking 50kwh+ on some cold days it seems certain I will be pulling from grid for much of the winter (and probably at higher daytime rates with Octopus).

What am I missing in my presumptions? Is an ASHP & battery storage a match made in hell?

@si-fillo I was scratching my head on this very issue before I purchased mine. I've 2 x 5kWh batteries with a 5.6kW array. In the end, after crunching various numbers, I opted for 10kWh storage as a single 5kWh unit seemed too small in relation to household usage (around 7-8kWh/day for DHW and general use) and the capacity of the array. The other thing to bear in mind is that usable battery capacity is less than the quoted value, mine are set to limit drawdown to 12% for example. 

I did read that having batteries will take your PV utilisation from (typically) 50% to 80% or so. In your case with a generation of around 3200kWh/yr, it's an extra 800-900kWh for you, saving 34p/kWh imports but 'losing' 15p/kWh export. Adding some clever TOU tariffs should help to make the numbers better. 

Thanks for the insights. 

I don't really want to buy an off the shelf battery as it would over inflate my £/kw return. A DIY battery bank is the most appealing but I don't yet have the understanding to be able to do this but looking around this forum, there is info in abundance so there is hope.

Im on the FiT scheme until ~2041 so will continue to receive export payments even if I expert nothing. I shall have to confirm if adding battery storage will in anyway void my FiT contract. 

I also wonder where I would locate the components of battery storage. The obvious place would be in the loft where the solar inverter is situated but with this option, safety would be a concern. Another possible option is the garage. However, I have further ideas of capping my gas supply and utilising HP (maybe also adding solar thermal) for DHW which would also mean I need room for HW tank. 

It becomes a job knowing where to even start 

@si-fillo brave man for the loft. Not for safety (lifepo4 are very safe). More for the lugging for 80+kg up there.

Posted by: @si-fillo

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I'm trying to understand if investing in battery storage will be worthwhile. We have a 3kW solar PV array. During 2022 it generated 3228kwh and we also pulled 7250kwh from the grid

We have an ASHP which can consume ~60kwh on the coldest winter days.

During the spring/summer, due to the reduced heating demand and increased solar generation it is a fair assumption that a battery would keep our pull from grid very low.

Between Oct - Mar, due to increased heating and reduced solar its fair to say a battery will have next to no impact on our pull from grid.

Then throw into the mix energy tariffs. If I can get a cheap overnight tariff to charge battery during the winter then it sweetens the deal. However, I believe the Octopus Go tariff where you can access even cheaper overnight rates only apply if you own an EV (I don't). Then I have to consider the size of a battery if I wanted it to take care of much of my demand in the winter. With the HP taking 50kwh+ on some cold days it seems certain I will be pulling from grid for much of the winter (and probably at higher daytime rates with Octopus).

What am I missing in my presumptions? Is an ASHP & battery storage a match made in hell?

Just before getting my solar PV installed last year I saw a battery as something to get later, however due to energy prices increasing I decided to go for it and I'm so glad that I did. It balances the solar on intermittent days, I've just switched to Octopus Cosy a few weeks ago and it really helps with that.

The only time its not really been used is in mid to late Nov, Dec and early to mid Jan as any solar got used up by the house, but I was on a Flex tariff then awaiting the switch to Cosy, so had I been on Cosy it would have made a big difference at that time.

Beforehand I was sceptical about the benefits, now having had it through the summer, autumn and winter I would say that ASHP/Solar/Battery is a match made in heaven. Down the line I'll probably add more capacity when funds allow.

I am certainly sold on the principle but I hesitate on the costs and more specifically the break even point. I currently have solar PV and solar thermal in balance with the HP. If I add say 4.4kW extra PV and a 8 kWh battery when I crunch the numbers no amount of clever tariffs get me to a sensible break even point given my age, approaching the 3 score and ten. It is also complicated by not getting access to any Smart Meter Arqiva WAN signal where I am, so dumb meter only.

so...

for my situation (3kw solar array FiT scheme), what are the pros & cons of having a battery on the AC side or the DC side of the inverter?

if I understand correctly...

1) having the battery on the DC side of the inverter, I will not lose energy from the conversions between AC/DC. 
2) having battery on DC side, would I lose kwh going through generation meter (because of energy going to battery) and therefore lower FiT payments?
3) does FiT scheme even allow for a battery to be added on DC side?.
4) if I wanted to charge battery from grid, is this possible if battery is on DC side?

Is my understanding correct?

Posted by: @si-fillo

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4) if I wanted to charge battery from grid, is this possible if battery is on DC side?

Your metering under these circumstances must satisfy FiT rules on page 30 onwards >>here<<. Key is single meter permitted only, no netting off different meters.

I think you may lose out on generation.  Solar->battery->meter->household means losses in battery are registered as losses in generation.

Grid->generation meter (negative) -> battery -> generation meter (positive) -> household: battery losses during grid-supplied charge/discharge are again registered as losses in generation.

Posted by: @si-fillo

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I shall have to confirm if adding battery storage will in anyway void my FiT contract. 

Hard to imagine how a hybrid inverter would be ok here.  Imagine you got an Octopus Go tariff, charged up the batteries at cheap rate.  Then discharged to the grid sometime..maybe not even when the grid needs it as the FIT meter won't notice ....  So now you're getting FIT payments from Octopus go cheap rate electricity? 

We have solar on FIT, and are planning on a battery - I plan to add it as an additional hybrid inverter, get some more solar somewhere too.

I’ve had a PowerWall for 3 weeks now, solar panels on the FIT schemes (and no, it doesn’t invalidate your FIT payments) and an EV. I’ve been signed up to Go since the battery was installed. Octopus did not check that I had an EV. Our PV output is around 2500 pa and we typically use around 2000 kWh per year excluding the EV which I think will add a further 1700 kWH charging at the Go rate, as is the battery. So far so good. The payback on the battery as it stands looks to be around 8 years.

This is where I share your concerns. I’m having a HP fitted in April. Throughout the summer my bills will be low so I’ll stay on Go. The battery will clearly not cope with the demand in the winter months so I anticipate I’ll need to switch to Cozy from Oct to end March and use the battery to tide me over the daily high peak rate. I can’t find a way to sensibly forecast my likely bills as I have no idea what the actual HP usage will be. I’m with @batalto on this.