Is it a combined inverter limit for solar and battery in your setup?
Yeah
I have a decision to make on Monday after the Hypervolt wall charger is installed. 
Whether to remain on the Octopus Cosy tariff or change to Intelligent (or standard) Go.
You would think itās a no brainer because of the 6 hours overnight @ 8p/kWh on Go, but itās not as simple as that. Cosy has 8 hours @ 12p, but crucially, this is divided into three periods 4-7am, 1-4pm and 10pm-midnight. Cosy also has two further rates - 25p at all the other times, except peak 4-7pm when itās 38p.
Yes Goās cheapest 8p rate is a bit less than Cosyās 12p, but the difference in the daytime rate is bigger - Go @ 32p versus Cosy @ 25p.
In winter our heat pump is very juicy and the amount of lecky it consumes on cold days can easily empty the batteries, but the three charging periods on Cosy mitigate this by topping up the batteries at the cheapest rate throughout the day, particularly the 1-4pm slot which ensures that the batteries are topped up just before the peak rate kicks in at 4pm. With Go, yes the batteries will be fully charged by 5.30am, but on a cold cloudy winterās day, they could easily be depleted well before 4pm.
Perhaps the answer is to stay on Cosy over the winter, then swap over onto Intelligent Go for the summer months. More battery capacity (currently 15kw) would obvs also help.
Edit: just looked at our bill for January '26.
1002 kw at the cheap rate (13.9p back then)
184 kw at the daytime rate (28.4p)
4 kw at the peak rate (42.6p)
More battery?
Once you have an EV charger you can get an EV tariff, and that gives you some great deals. I have EDF and itās 7 hours at 7p/kWh.
Given that you have an inverter already, why not just add batteries?
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You didnāt actually read my post did you?
Yes Iām going to look at the options ā¦
It was very long and only one sentence made any sense. Get more batteries, no need to agonise over suboptimal tariffs then.
The question is whether you and your house can live with a lot of the heating input being shifted overnight in the Go period. We can most of the time, so it has to be sub zero for us to run out the battery on the heating during the day.
It may also depend how much of your grid use will be car as opposed to heating. Our car usage is the largest element so we would always go for a tariff accordingly.
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Thats Numberwang.
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Let me know how your quotes go.
Iāve just called a local company about heat pumps and they have an āindicativeā price of Ā£16,500, less the grant so Ā£9k of my cash. Sounds high to me - this is for a Vaillant Arotherm Plus, which costs under Ā£5k retail.
I think that my install is fairly easy, as the heat pump is close to where the boiler is and also close to the water cylinder. But I doubt that their indicative quote will come down much!
I also need to look at overall usage for the battery sizing. My peak gas use can hit 130kWh per day, although I wouldnāt size for the worst case. It would probably be best to add another 16kWh battery as well.
Then I also need to get a new cooker to get rid of gas completely, and get radiator thermostats to heat the rooms I need. Indeed it would probably make sense to heat the downstairs overnight to use the cheap electricity without draining the batteries. Sigh.
Ultimately Iām really struggling with the value proposition of a heat pump. I spend about a grand a year on gas, why would I spend Ā£9k on a heat pump that would cost not much less to run in fuel, and any possible savings would be lost in service costs?
I donāt mind a slow payback time, and I would like to move away from fossil fuels, but this seems hard to justify.
What are you guysā financial positions of heat pumps?
Servicing really isnāt expensive, I think less than our boiler. The running costs are a lot less given that we do run the main heating overnight and losing the standing charge for gas was a good thing.
Right now with solar and a piddly little old Powerwall our entire energy costs for a year are less than Ā£800 and I suspect almost half of that is car charging. The biggest monthly bill we have ever had (including the charging) was Ā£140 for a cold January.
Edit: and if a part of the bill is replacement rads then that is something you will have to do at some point anyway at the next system renewal, given that fitters are supposed to spec gas systems for a 50Āŗ flow temp.
I use 15200kWh/annum gas. If I use a figure of 9p/kWh thatās Ā£1368 per annum
Converting that to electricity for heating would be around 4000kWh at 7p, or Ā£280.
So letās conservatively say it saves me Ā£1000 per annum. RoI easy to calculate based on the install cost.
For me, itās Heat Geeks or nothing as they will calculate the heat loss, spec the system properly and provide a guaranteed COP.
House is old, no cavity wall, needs an innovative hw cylinder location and at least some rads replacing so the final number could just end up being too expensive.
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I would recommend looking at daily usage in detail. My EDF app shows this nicely.
My gas use is really peaky. Sizing the battery for this would mean half the battery is never used for over 9 months a year. Feasible, but price it in! If much of the heat pump usage is at 34p rather than 7p, the savings disappear very quickly.
Of course, keeping the house at 20 degrees 24/7 would work, but again increase overall heating use.
Also, if the 7p car tariffs end, overnight could revert to 15p or so. Not a problem for a battery with a 3-4 year payback, but if the payback period is 8 years plus, itās quite a risk.
Your premise of not using the battery for 9 months of the year isnāt quite right.Today I do quite a lot of energy arbitrage even with a small battery. With a favourable G99 and appropriate inverter size I would be working it very hard.
I also donāt buy into the car tariff argument. There will always be some energy shifting delta in pricing because itās the only way to not have the grid itself. It may not be 7p forever (or in my case 5.9p), but the cost of peak electricity and gas will be correspondingly higher so the RoI remains consistent
Yeah I havenāt got into the arbitrage yet. Axle havenāt used me yet, and I donāt have an export tariff with my current (ahem) provider.
You may well be right on the 6/7p tariffs, but Iām not certain. Economy 7 tariffs are 15p; to get the cheapest ones you need an EV charger. This strikes me as a promotional thing, with a risk of reverting to the 15p once enough people are taking advantage of it. If that would kill the finances of the heat pump, I would be cautious. If the payback period is almost ten years even with the best assumptions, and there is reasonable risk, itās not a good investment.
Of course, getting away from fossil fuels is also laudable, thereās that.
For context, we did the calcs on the battery just on storage rather than export (zero marginal income on old FIT) and with an 8p differential on Go the payback was a bit over 10 years. I would be very surprised if the day/night rates converged back to that extent.
Part of me does wonder if wider rollout of solar will take out some of the incentive to charge and discharge for part of the year, but then I am still in the mode of wanting to be as self sufficient as possible.
Another way to look at it is that you are bringing your old unsuitable housing stock kicking and screaming into the modern ageā
More and more estate agents are including EPC, details of heating etc.
More and more the next generation will be prioritising better EPCs and lower emission heating, and those properties will have better values.
Perhaps you will get back more than the invested when you come to sell..?
Itās no different to modernising a kitchen or bathrooms.
Plus the feel good factor and nice continuous warm home.
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