@jamespetts I agree it's extremely hard to calculate ROI on batteries as it will also see changes in usage patterns as well as opening up tariff options.

Factors include

• EV? Usage? Charger?
• Solar panels? Size?
• Inverter charge/discharge rates?
• Batteries? Size? 
• ASHP? size? Usage?
• Automation options including HA, WonderWatt  Homely etc?

In my case

• EV? Yes Usage? Around 2000kWh/pa. Charger - zappi
• Solar panels yes Size 8.5kw
• Inverter charge/discharge rates? Only 2.6kWh
• Batteries? Size 24.6kWh
• ASHP yes size? 16kw Usage: 6000kWh/pa. Weather compensation controls
• Automation options including HA, WonderWatt  Homely etc? Yes. WonderWatt

• I crank up heating of house and hot water at night on cheap rates
• Solar probably produces around 6500kWh/pa (not optimal orientation etc). Endeavour to export majority

Usage: say 6k ashp, 2k ev, 2k all other usage Inc lighting, cooking. 

Import: say 3k kWh/pa at cheap rate overnight for later force export as well as filling batteries for own usage + charge ev + run ashp 

Expected TOTAL imports 13000kWh @ 8p Intelligent Go = £1040

Expected TOTAL exports 6500kWh @ 15p = £975

Expected Net cost £65 for 1 year usage in 2025

So I conclude the batteries are a no brainer for me. But I could not have assumed that in advance. Charge/discharge rate is a significant barrier to maximising but newer installations are likely to be better especially if part of a setup such as Tesla, GivEnergy All-in-one etc

@tim441 As you say Tim, very difficult to quantify any savings or ROI even, for a battery; I have often wondered about the savings I make from my solar PV - the figures might be very different with or without a battery. Without a battery, I might quantify my solar ‘gain’ at 25 pence per kW/h as I would not likely be using a TOU tariff but a fixed one. As I do have a battery (Tesla 27kW/h Powerwall), I opted for Cosy and pay ~ 13 pence per kW/h and let the battery take the strain. But wait! I am only showing a solar ‘gain’ of ~13 pence per kW/h now!?😉. In fact, my exported energy realises 15 pence per kW/h with Octopus Outgoing Fixed (at the moment anyway!) Oh, now my brain hurts! Regards, Toodles.

@toodles I guess it all comes down to import averages. Which is going to be impacted by ToU tariff, time-shifting usage etc.

In my case I'm importing vast majority at night at 7p/kWh... for own usage and filling batteries.

In summer using batteries to force export + maximise solar export. 

In winter using batteries to supply home ... and refill during day from solar (if any excess) + any bonus cheap slots.

I think I should keep the average for year sub 8p as a result. But with max exports it's a game changer.

When I was on Agile in 2024 the average was around 12 or 13p I think. But far less exports as the round trip was not worthwhile.

Tracker in 2023 was a decent saving vs fixed or standard contracts.

We are still on agile and it works ok'ish to charge the EV. But the way the market is working I am not sure we will stick with it for long. We have HA but without PV I suspect our consumption will benefit from the predictability of a lower overnight rate.