Powerwall 3 or Myenergi or something else?

AdamK · 2025-08-20T20:18:02Z

Bit of background, 4 bed house 7.35kw heat loss, gas disconnected this year, Vaillant 7kw arotherm and 250L HW tank, induction hob, 4kwp of solar feeding a 3.68kw inverter installed in 2014 on FIT. On Octopus tracker for electric with Octopus export at 15p kWh. previous house usage per day during the worst 2 months in the winter was about 10kwh of electric per day, that was obviously before we ditched the boiler and gas hob. so need to get some storage for the winter months to make the HP cheaper to run than the Gas boiler and maybe in the summer become 0 import. Nice to have would be power cut cover, though we only have a few per year and they don’t last very long maybe an hour at most. had one company want to install a Sigenergy, though haven’t had price yet. The other does Tesla but seems to think I’d be better off with a Myenergi Lbbi stackable system. I’m pretty open to suggestions really. I’d like to be able to run in the winter on cheap rate juice, I like the EV tariffs but don’t have an EV so that’s not possible unless I fib. I can’t see why house storage can’t be charged this way, it’s like being discriminated for not having enough money for an EV.

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Majordennisbloodnok

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22/09/2025 12:17 pm

Posted by: @old_scientist

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It is true that you can buy a Fogstar 16kWh battery for around £1750 whilst a comparably sized Tesla Powerwall expansion pack will cost in the region of £4000-£5000 (Octopus are offering them at £3773 installed)

However, the Fogstar is not IP rated for outdoor use and has no active thermal management so will find it's usable capacity and charge rate significantly diminished in winter, just when you need it most if relying on it to power a heat pump. Not everyone has the capacity to locate a battery indoors or wants a battery inside the house (I'm looking at my wife), at which point you are going to pay significantly more for a product with IP67/IP55 rating and that has active thermal management to keep the cells in an optimum temperature range when it's -5C outside.

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Let's also bear in mind that Fogstar supply more than one solution. They certainly do have an IP rated and heated battery solution that, half populated for 20kWh is £4,000 and fully populated to 40kWh is £7,000.

Fogstar Energy 51.2V 40.96kWh IP56 Outdoor Battery System and Cabinet

I'm certainly not providing any recommendations but it does allow for once again comparing like with like.

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"

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