Let's price up some bits:

• Sofar ME3000SP battery storage inverter (3kW output): £780
• Pylontech US2000 2.4kWh battery: £728
• Total for inverter + 9.6kWh batteries: £3692

Prices from Midsummer Wholesale (who I think anyone can buy from).  Plus VAT, but then the Chancellor has just set VAT to zero if somebody is installing it for you (but not, I think, for DIY installs).

Unlike a solar panel install, the install is not very complicated: you need cabling and some switchgear for isolation and you need to do some DNO paperwork, but there's no scaffolding or anything like that.  Maximum a day for an electrician, unless there's anything time consuming like running underground cabling. If you want to power the house in a power cut the wiring is a bit more complicated as you need a changeover switch upstream of your consumer unit.

This is at the cheaper end of the spectrum - I can't comment for Solaredge's capabilities.

The price from Seplos in China for an integrated battery comes in at about $2640 including shipping (+VAT+customs fees) for a 6.9kWh battery which will talk to the Sofar inverter (this has gone up a fair bit of late).

Ballpark for the bare LiFePO4 cells from China: last time I checked $1300-1900 for 10kWh delivered (+VAT+fees).  Enclosure and BMS (eg Seplos) is your problem.

IMHO the Solar Edge doesn't sound like a very good deal, but I'm not familiar with the unit.

One question: what's the maximum power output, and what's the maximum taken by your ASHP.  eg my ASHP takes 3-4kW which means a 3kW battery inverter wouldn't fully cover the ASHP load and I'd still pay for some grid power when it ran, even with a full battery.

Hi @justinsb

I think that the first thing that you should do is get at least two further quotes.

If you have DIY skills you could talk to Batalto about installing your own system. Now that I am making much more use of our A2A heat pump, I am seriously considering installing battery storage myself. Since you don't live too far away, I may be able to help with the electrical side, if you don't have a friendly electrician in your area.

We had two 14kWh Tesla batteries fitted six years ago (£15K) by Stratford Energy to enable us to keep most of the energy generated by our existing 6kW solar edge panels, and at the time I though this would be simply a lifestyle choice (i.e. unlikely to get money back) however, an analysis I did last year on performance indicated that I was in fact going to recover the cost of the batteries within 12 years. This was helped by being able to also charge the batteries from the grid using economy 7 tariff - essential during the winter months. With the current massive jump in electricity prices this repayment appears to now be 8 to 9 years (For a new installation this could be 6 to 7 years).

The Tesla system comes with a great mobile app that enables you to check your battery status and manage your power usage / charging, download data on usage an performance from the time of installation, and even go Off Grid with a few clicks. I have already enjoyed three days this month totally off grid, and last year between May and September over 100 days were totally off grid.

For example, using the Tesla app, I know that this month (March) I generated 465 kWh solar electricity, 31% was consumed directly by the house, 66% was stored for later use in the Tesla Batteries and only 3% was exported. If I did not have the batteries 69% of my solar generated power would have been exported and I would of had to pay peak prices for most of the approx 300 kWh I was able to store, saving me this month around £75 (old prices, so probably around £120 after 1 April).

Batteries in an all electric house like ours (ignoring the log burner) are well worth the investment.

Cheers everyone. Today was Day 1 of gathering quotes. I've found out soooo much in the past couple of hours, but even before I had properly got stuck into the research a quote of seven & a half grand did ring all of my mental alarm bells. Thanks @batalto and @derek-m - I possibly do have enough DIY skills to take that on, so I will expand my research on that front. Also, I do have a friendly electrician, but I don't think that he knows anything about Battery or PV systems (yet). I may well come back to you on this one Derek.

@prunus that site looks awesome, https://midsummerwholesale.co.uk and your ballpark costings really made me sit up & take notice.

I have a 14Kw Ecodan ASHP, but I'm not sure what it usually peaks at in the bleak mid-winter.

@justinsb although peak is a worry, I wouldn't worry too much. Over spec'ing can be dangerous. As an FYI I have the Sofar ME3000SP and its maximum discharge is 3.2kw - the biggest issue you will have is charging is discharging. Some batteries have peak outputs (so check that) and inverters also do - so even if you have 6kw of PV for example, you'll still only be able to charge at 3.2kw (in my example).

There is a 5kw charger out there - Goodwe - GW5000S-BP - 5kW - its about £1,700. But as I said, you'll need the batteries which can match it, or its a waste.

I'll do loads more proper research before I begrudgingly force open my wallet...

Speaking of which, Midsummer Wholesale actually sell the 10Kw Solar Edge Energy Bank systems for £5655 & the advertising blurb says that the installation is basically just connecting up a couple of wires. That would place it at nearly 2 grand for the "quick" install on my first quote, plus they were uncertain if they had to add 20% VAT on top of their £7,500 quote, just to add insult to injury. It's always good to know what the worst case scenario is, & it's no wonder that they wanted me to sign up so quickly.

£5655

Posted by: @prunus

One question: what's the maximum power output, and what's the maximum taken by your ASHP.  eg my ASHP takes 3-4kW which means a 3kW battery inverter wouldn't fully cover the ASHP load and I'd still pay for some grid power when it ran, even with a full battery.

Heat pumps take a lot of current when starting. I checked the spec of a mid-range 9kW GSHP a couple of weeks ago and it draws 29A when starting (using a 'soft-start' design!). This drops to somewhere between 11-21A when running.

There's no problem drawing this amount of instantaneous energy from whatever chemistry of Lithium cells you favour. They will happily give you 20kA and weld together the contacts of your safety-isolation trip if you allow them to!

The limiting factor is the inverter. If it's grid-connected, then you're permitted 3.68kW (16A) per phase under the G98/G99 regulations.

You might imagine that the 29A start-up current will therefore be provided by taking 16A from the storage inverter and 13A from the grid. But in practice that isn't the case. The internal resistance of the inverter is higher than the grid and the electronics takes a finite amount of time to turn on the MOSfet transistors whilst maintaining synchronisation with the 50Hz of the grid. So almost all the start-up surge comes from the grid.

That switch-on surge introduces harmonics back onto the grid. The DNOs monitor this because harmonics are major cause of 'technical losses' at the transformers. This manifests as voltage variations between phases.

If your ASHP or GSHP is single-phase, then you also introduce phase-imbalance at the local substation transformer. This reveals itself as current variations. I can post graphs here if you want to see this.

The new RIIO-ED2 agreement between Ofgem and each DNO requires them to tackle these technical losses. If they don't, then their revenue is constrained, which hurts the shareholders.

Since HP installations require consent from DNOs using the downloadable LCT Application Form, I would anticipate the specs being tightened in the near future.

It is frustrating that the fitting of grid-connected storage doesn't do a better job in reducing these losses on the Distribution Grid.

Posted by: @batalto

@justinsb although peak is a worry, I wouldn't worry too much. Over spec'ing can be dangerous. As an FYI I have the Sofar ME3000SP and its maximum discharge is 3.2kw - the biggest issue you will have is charging is discharging. Some batteries have peak outputs (so check that) and inverters also do - so even if you have 6kw of PV for example, you'll still only be able to charge at 3.2kw (in my example).

There is a 5kw charger out there - Goodwe - GW5000S-BP - 5kW - its about £1,700. But as I said, you'll need the batteries which can match it, or its a waste.

One solution to that is multiple batteries and multiple inverters: dual ME3000SP each feeding its own stack of batteries. Then they can operate in parallel.

One trickiness is that 3kW inverters are designed that way because the limit under G98 rules is 16A per phase.  Above that you need G99 approval and your DNO could say no, depending on the state of the local grid. I think in that case you can work round it if the system is configured (internally or externally) never to export more than 16A - then the current is purely consumed locally and your DNO can allow it under G98.

@prunus yes but I'm talking about battery chargers, which can happily charge at 5kw - no grid involved

Posted by: @prunus

One trickiness is that 3kW inverters are designed that way because the limit under G98 rules is 16A per phase.  Above that you need G99 approval and your DNO could say no, depending on the state of the local grid. I think in that case you can work round it if the system is configured (internally or externally) never to export more than 16A

I've discussed this situation with the Western Power Distribution engineers who assess the grid-connection applications made by us consumers.

They don't accept the complete range of export-limitation technologies which are available to be installed in the UK. They cross-reference to a list of 'approved' inverter limitation technologies which is held by the Electrical Networks Association (ENA). That list is not available to the public.

When I needed a PV inverter installed alongside a storage battery as part of a funded trial, I was only permitted a 3.6kW PV string inverter (Growatt) because the export constraint system inbuilt to the battery was not approved/trusted by the ENA. The Growatt could otherwise have been rated 6kW with the overall system export limited to 3.68kW.

A good starting point is to assume that the inverter and the storage battery must be designed as a matched pair by the manufacturer. Even then there's no guarantee that the ENA have checked it out yet.

@justinsb I’m looking at a solaredge system would be curious what you paid and your thoughts / recommendations? 

Key thing with batteries, what’s important to you?

solaredge - DC coupled, integrates with your existing monitoring equipment but the warranty isn’t great I think it is 10 years to 70%. It does offer a 5kw continuous output which is pretty high compared to most systems.

If you use any equipment that is designed to utilise excess solar like an eddi or a zappi it would work with a DC coupled system, however solaredge do offer their own equipment for these needs. 

The route I will personally be taking is a Victron multiplus 2 5000va with plyontech batteries USC5000. This will be an ac coupled solution only with a 4kw continuous output but it’s ability to ramp up and down is much quicker than most ac coupled solutions like givenergy. It is often used in off grid situation so great support and modular build options around DC connected solar via mppt for future expansion. 

@mattengineer I didn't know pylontech now did 5kwh batteries and the price is actually very reasonable - £1,392 (£278/kWh). 

https://www.itstechnologies.shop/products/pylontech-us5000-4-8kwh-95-d-o-d-battery-storage