@webcmg That crystal ball I mentioned…, well hindsight is wonderful, but who has foresight? I had solar and battery installed in August 2022 - look what has happened to energy prices since then! Rather than ‘payback’ I prefer to think in terms buying my energy as cheaply as I can ATM but, having the security of supply as of great importance too. @old-scientist is enjoying his fling with IOF just now and that looks like a good prospect to improve ‘payback’ too. Regards, Toodles.

Posted by: @webcmg

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I do need to track what the peak usage is outside of Cosy slots in order to determine what the smallest battery I could get away with is... that would reduce the investment cost and minimise the breakeven period. I think I will need to contact OVO to get an annual hourly usage breakdown...

I estimated our usage on the following basis.

The most challenging 6h window for us is 4-10pm as it coincides with cooking/meal time (electric oven, air fryer, microwave, kettle all potentially in use) and no solar generation to help out.

I estimate our heat pump runs at around 1.2-1.5kWh input, so heating for the 6h may require somewhere between 7-9kWh. Background use is another 2-300w (~1.5kWh). Then we may want the electric oven on to cook using another 2kWh?? (I know it draws 2.7kW don't really have a firm number on average usage per hour as it kicks in and out). So our 13.5kWh Tesla Powerwall seems ideally sized for us to meet our peak demand in that 4-10pm window. We could reduce the demand slightly by overheating the house by 1C in the run up to 4pm, and then knocking the flow temp back slightly if we know we are going to be cooking a large roast dinner and battery capacity is going to be tight. The house should not cool noticeably with the heating still on low and the additional heat output from cooking.

Laws of diminishing returns. It may be the case that a battery large enough to cover your usage on 80-90% of days is good enough, and doubling it's size for those last 10% of days makes less economic sense. If batteries were only available in 5kWh modules, we'd need 15kWh, but 10kWh would do most of the time and that's what I would go for. The Tesla PW3 at 13.5 kWh just so happened to hit our sweet spot.

We’re high energy users other than the ashp so the OVO tariff wouldn’t work for us, so cosy was the next best. We averaged 17p per kWh using a 9.5kWh battery to get our 7kW ashp through peak hours. I did calculate the financial payback for a bigger/additional battery but it was just not viable. Too small a saving.

Resurrecting this thread because OVO have discontinued the Heat Pump Plus Addon. I looked at the worst days usage over the past 12 months and these are the reaults for the periods that Cosy rates do not apply:

00.00-4.00 = 10 kwh
7. 00-1.00 = 17 kwh
16.00-22.00 = 12 kwh 

Average winter day:

00.00-4.00 = 9 kwh
7. 00-1.00 = 13.5 kwh
16.00-22.00 = 13.2 kwh 

Can anyone recommend a battery that would cover that type of usage? I looked at the fogstar but the 80% DOD may be problematic. Good price though... 

Posted by: @webcmg

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Resurrecting this thread because OVO have discontinued the Heat Pump Plus Addon. I looked at the worst days usage over the past 12 months and these are the reaults for the periods that Cosy rates do not apply:

00.00-4.00 = 10 kwh
7. 00-1.00 = 17 kwh
16.00-22.00 = 12 kwh 

Average winter day:

00.00-4.00 = 9 kwh
7. 00-1.00 = 13.5 kwh
16.00-22.00 = 13.2 kwh 

Can anyone recommend a battery that would cover that type of usage? I looked at the fogstar but the 80% DOD may be problematic. Good price though... 

Looking at your figures, I'm guessing you'd ideally need around 13.5-15kW of usable charge as an absolute minimum for a tariff like Cosy. That higher usage in the 7am-1pm slot often receives a little help from solar, assuming you have a decent south facing unshaded array.

Recharging is also a consideration. If you had a 15kW battery with a charge rate of 5kW, you could fully recharge in the 3h cheap slots, and recharge 10kWh in the 10pm-midnight slot ready of the 10kWh usage that follows. So you ideally want something with a charge rate of 5kW

The other main consideration is the rate of discharge. What is your maximum power draw? If you want to use only cheap rate electricity then the battery (plus inverter) will need to be able to fully meet the house load during non-cheap rate slots. This is where I find the Tesla PW3's 11kW maximum discharge to be very flexible - we are not running round turning things off to balance a small 5kW inverter (having the ability to empty that 13.5kW battery in just over an hour is very impressive!)

I have a Tesla PW3 and it works very well for us with Octopus Cosy in Winter keeping us running on Cosy cheap rate electricity only, and gives us access to the highly lucrative IOF tariff in summer. We haven't really tested our 13.5kW capacity yet as the weather has been so mild, but I'm confident a single PW3 is sufficient for us. If you went this route, I'd almost certainly add an expansion pack to double the capacity given your higher usage. Other batteries are available. The advantage of any modular system is that you can start off conservative, and add another module later if required (although don't skimp on the inverter rating)

I found because I have very low heat loss I can set back the temp by only 1C during the higher rate periods because the Daikin overshoots 1-1.5C regardless. The HP rarely starts again until the next "cheap" rate period. During very cold weather between 4PM and 10PM is the mostly likely time the HP might start again between 8:30 - 9:30PM then ramps to 1-1.5C over the set point temp or cuts off again at midnight.

I don't believe my HP as ever run between the 4pm - 7pm period.

I tried a 24/7 schedule with a 4pm - 7pm 1C setback but the above method had a lower temp variance so I'm sticking with it. The HP spends less time off losing system water temp so the starts required less energy to get back to the set point.

Posted by: @webcmg

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I was considering a Fogstar 32kwh battery + inverter. I think this coupled with Cosy would likely reduce all our electricity use to 12.29p, ..

I am wondering if anyone with an EV could find the 7p/kwh overnight rate of Octopus INTELLI-FIX-12M-25-08-29 worth it? 

With some extra batteries, of course. And there may be better options than Fogstar, a few examples here.