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Which technologies do you have? Which tariff are you on? Multiple answers, please Poll is created on Dec 09, 2025

  
  
  
  
  
  
  
  
  
  
  
  

Poll for Time of Use, tariffs, technology

 
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Electricity, Tariffs, Monitoring & Providers
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 Old_Scientist
(@old_scientist)
Prominent Member Member
Joined: 1 year ago
Posts: 314
13/12/2025 10:05 pm  

Posted by: @sheriff-fatman
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Posted by: @jamespa
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Posted by: @sheriff-fatman
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Are you restricting your choice to specific high-cost batteries in your calculations, or is there some specific non-obvious constraining factor being reflected, as it's the only way that I can think that you'd conclude that the payback wouldn't work?

I dont know TBH.  Quotes for installed batteries seem very similar, a bit more than: 2.5K=5kWh, 5k=10kWh, 8-9K=20kWh.  I am including cost of capital.  I am assuming 4% interest but also accounting for tax on the interest (which is favour of the investment).

If I stretch things I can just about get payback = 8 years, mostly its 10 years.  This is based on EON Next Drive (2025 edition) which has a large differential between cheap rate and nighttime rate (6.7p vs 25.4p)  Roughly speaking I do 50% of my import at night rate and 50% at day rate.   

Does that answer your question.  Im trying to make it work, I'm not trying to find reasons not to do it!

So, assuming any technical issues could be overcome, if you factored in a 16kWh battery cost at £2,000 or a 32kWh battery at £3,400 and added on a cost to install (they've been described as 'plug and play', but this is generally referring to swapping out Sunsynk batteries for these, so that might be an oversimplification), does that price level make a big enough difference in your overall calculations?

I'm an accountant, so am very familiar with opportunity cost type calculations, and DCF analysis, which I assume yours are if you're figuring in interest lost and tax on interest saved, but the underlying price of the batteries appears to be the big constraint in your calculation.  The Fogstar ones seem to be generally acknowledged as being the best value cost/kWh of reliable storage options currently available so if the calculation doesn't work at those prices, then I guess it won't work at all.

Not necessarily. Some more expensive batteries (e.g, GivEnergy, Tesla) also give access to more lucrative tariffs (Intelligent Octopus Flux), so one could base a strategy around the purchase of the cheapest battery that would give access to those more lucrative tariffs (see my post above) which may change the economics (or not).

I think it also depends on how you treat at the purchase. Considering a purchase of a "solar + battery system" is very different from considering adding a battery to an existing solar system where the initial gains (low hanging fruit) of solar are already baked in and the expensive battery then offers a diminishing return.

 


Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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 Sheriff Fatman
(@sheriff-fatman)
Reputable Member Member
Joined: 8 months ago
Posts: 115
13/12/2025 11:21 pm  

@old_scientist Interesting point, but I suspect that's not a typical usage case for most people.

I'm using Intelligent Octopus Go.  I've looked previously at the Flux options and, on each occasion, have concluded that they're not as cost effective as the EV rate in my case, particularly when I factor in the car charging requirements for 2 vehicles.

Based on the difference in pure battery cost I doubt that tariff savings on the more lucrative rates would offset the capital cost difference over a reasonable payback period, unless the solar/export capability was on a much bigger scale than a typical system.  I don't think my 6.3kWp system with 5kW inverter would have sufficient capacity to 'trade' a saving on that sort of scale with our typical usage patterns.


130m2 4 bed detached house in West Yorkshire
10kW Mitsubishi Ecodan R290 Heat Pump - Installed June 2025, currently running via Havenwise.
6.3kWp PV, 5kW Sunsynk Inverter, 3 x 5.3kWh Sunsynk Batteries
MyEnergi Zappi Charger for 1 EV (Ioniq5) and 1 PHEV (Outlander)


   
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