Considering adding another battery - inverter options

ed110220

Hi all,

At the moment I've got 4.8 kW of solar, a Growatt SPH5000 hybrid inverter and 15 kWh Seplos battery. I'm on Octopus Go so almost all our imports are at the off peak rate (average cost of electricity imports are under 10p/kWh).

I'd really like to add another Seplos battery kit to have more storage and set us up for ASHP as at <10p/kWh even a pretty poor SCOP is going to offer a saving on gas.

But what about the inverter? The Growatt only charges up to 3 kW so the existing 15 kWh battery I have is as much as it can charge in the 5 hour overnight off peak window. 

So would I have the inverter replaced with a more powerful model - seems simpler but at the cost of lower PV efficiency because of over sizing. Or add a new inverter for the new battery?

QrizB

Adding more inverter capacity will mean going to your DNO for permission. That's not a huge hurdle but it's one with an unknown outcome.

Could you add a 48v battery charger without confusing the Growatt logic?

Or you could switch to EON Next Drive and get 7 cheap hours a night (at 7p/kWh) which would let you charge 21kWh rather than 15?

Martyn1981

Is there a HP tariff that offers several cheap periods during the day, so multiple chances to charge the battery? Or am I making that up?

QrizB

Martyn1981 said:

Is there a HP tariff that offers several cheap periods during the day, so multiple chances to charge the battery? Or am I making that up?

Cosy Octopus?

https://octopus.energy/smart/cosy-octopus/

Not as cheap as Go, though.

ed110220

QrizB said:

Adding more inverter capacity will mean going to your DNO for permission. That's not a huge hurdle but it's one with an unknown outcome.

Could you add a 48v battery charger without confusing the Growatt logic?

Or you could switch to EON Next Drive and get 7 cheap hours a night (at 7p/kWh) which would let you charge 21kWh rather than 15?

Thanks, the EON tariff looks good, especially as their website says it's available if you have an EV or battery. I don't have an EV, I just chanced it with Octopus Go by asking to switch to it and they never asked if I had an EV.

It would be pretty straightforward to charge a battery with a 48 V charger but I think you'd need an inverter to be able to charge and discharge it?

QrizB

ed110220 said:

QrizB said:

Adding more inverter capacity will mean going to your DNO for permission. That's not a huge hurdle but it's one with an unknown outcome.

Could you add a 48v battery charger without confusing the Growatt logic?

Or you could switch to EON Next Drive and get 7 cheap hours a night (at 7p/kWh) which would let you charge 21kWh rather than 15?

Thanks, the EON tariff looks good, especially as their website says it's available if you have an EV or battery. I don't have an EV, I just chanced it with Octopus Go by asking to switch to it and they never asked if I had an EV.

It would be pretty straightforward to charge a battery with a 48 V charger but I think you'd need an inverter to be able to charge and discharge it?

You only need an inverter for discharging, "inverting" the DC from the battery to create grid-synchronised AC.

But "use a battery charger" probably isn't a sensible suggestion anyway, unless you want to go poking around deep in the settings of your inverter (and even then you might find that it doesn't work). You'll be better served by either a hybrid inverter with more capacity, or a second inverter.

Do Growatt let you daisychain inverters, do you know? Some manufacturers do, some don't.

ed110220

QrizB said:

ed110220 said:

QrizB said:

Adding more inverter capacity will mean going to your DNO for permission. That's not a huge hurdle but it's one with an unknown outcome.

Could you add a 48v battery charger without confusing the Growatt logic?

Or you could switch to EON Next Drive and get 7 cheap hours a night (at 7p/kWh) which would let you charge 21kWh rather than 15?

Thanks, the EON tariff looks good, especially as their website says it's available if you have an EV or battery. I don't have an EV, I just chanced it with Octopus Go by asking to switch to it and they never asked if I had an EV.

It would be pretty straightforward to charge a battery with a 48 V charger but I think you'd need an inverter to be able to charge and discharge it?

You only need an inverter for discharging, "inverting" the DC from the battery to create grid-synchronised AC.

But "use a battery charger" probably isn't a sensible suggestion anyway, unless you want to go poking around deep in the settings of your inverter (and even then you might find that it doesn't work). You'll be better served by either a hybrid inverter with more capacity, or a second inverter.

Do Growatt let you daisychain inverters, do you know? Some manufacturers do, some don't.

Yeah I wasn't thinking of using a charger, only discussing it as it was raised above. The inverter's discharge limit of 3 kW is almost as much of a limitation as its charge limit so even if it was possible to get it to work with a separate charger it wouldn't be that much of an advantage (basically I'd be able to put more electricity into the batteries than I'd be able to easily get out). Daisy chaining is an idea, though I suspect a larger inverter and selling the existing might be easier and cheaper.

QrizB

ed110220 said:

The inverter's discharge limit of 3 kW is almost as much of a limitation as its charge limit so even if it was possible to get it to work with a separate charger it wouldn't be that much of an advantage (basically I'd be able to put more electricity into the batteries than I'd be able to easily get out).

Not really; you've got a max of 7 hrs to charge and 17 to discharge, so you only "need" half as much discharge capacity.

ed110220 said:

)Daisy chaining is an idea, though I suspect a larger inverter and selling the existing might be easier and cheaper.

Looking at the datasheet, all the Growatts are limited to 3kW DC. Unless you can add a second one, you might need to change brand.

https://cdn.shopify.com/s/files/1/0796/3698/8238/files/Growatt_SPH3000-6000_Datasheet-10.13.pdf

ed110220

QrizB said:

ed110220 said:

The inverter's discharge limit of 3 kW is almost as much of a limitation as its charge limit so even if it was possible to get it to work with a separate charger it wouldn't be that much of an advantage (basically I'd be able to put more electricity into the batteries than I'd be able to easily get out).

Not really; you've got a max of 7 hrs to charge and 17 to discharge, so you only "need" half as much discharge capacity.

ed110220 said:

)Daisy chaining is an idea, though I suspect a larger inverter and selling the existing might be easier and cheaper.

Looking at the datasheet, all the Growatts are limited to 3kW DC. Unless you can add a second one, you might need to change brand.

https://cdn.shopify.com/s/files/1/0796/3698/8238/files/Growatt_SPH3000-6000_Datasheet-10.13.pdf

True, though it takes some effort to keep our draw < 3 kW outside the off peak period to minimise more expensive imports. If we got ASHP I expect that a considerable part of our consumption would be day rate imports rather than from the batts.

The newer Growatt SPH series go up to 4 kW charge/discharge which would be usefully more, but not a big enough upgrade: https://en.growatt.com/upload/file/SPH_3000-6000TL_BL-UP_Datasheet_202303.pdf

There are various other Growatt single phase inverters with higher charge/discharge powers eg https://en.growatt.com/upload/file/SPM_8000-10000TL-HU_Datasheet_EN_202505.pdf but they don't seem to be available in the UK.

I'm not particularly attached to Growatt