On-grid domestic battery storage

orrery

https://forums.moneysavingexpert.com/discussion/comment/82061133#Comment_82061133

The intent is to replace the FIT system like-with-like, so the generation meter stays the same in order to retain the FIT. The new additional panels and the battery will be outside of the generation meter. The issue is that any new hybrid inverter will still need to 'see' the export from the FIT system and divert it to charge the battery where necessary - presumably with a clip on the meter tails. Hence my question … one supplier has told me that a hybrid inverter will only charge batteries from its own panels and won't switch to AC charging to charge from surplus being exported by the FIT system. Is this, in general terms, true or false?

Martyn1981

Purely my experience with a Solaredge hybrid inverter, it does grab any excess generation from my other PV system. That's a pain for me, as it means the leccy goes through a bi-directional FiT meter, so the charging losses are a negative.

For what you are describing, it sounds like it would be ideal, noticing the 'export', and grabbing it to charge the DC side battery on the new system.

My Solaredge hybrid is 3.68kW, and when on force discharge, it will dial down to prevent house export exceeding 4kW. No idea why 4kW not 3.68kW!

So, let's say I tell it to discharge, then in the afternoon the west system is generating 1.5kW, then with low house demand, the hybrid inverter will dial down to around 3kW. But will go back upto 3.68kW if I pop the kettle on,or similar.

I believe the Solaredge inverter can be told not to charge from the A/C side, but I think that's an installer control, so I haven't bothered them, as I'd need to change it every spring and autumn, to stop it grabbing home excess, but needing winter charging. IYSWIM.

No idea if other systems do this, or not.

Martyn1981

BTW, you can add to a FiT system, if that's easier, and the generation will be apportioned. But sounds like having the new system separate, especially with DC batts, may be a great solution too. For me, I expanded the FiT system, behind the 3.68kW inverter, as the DNO wouldn't let me have any more export.

Qyburn

https://forums.moneysavingexpert.com/discussion/comment/82061214#Comment_82061214

On Solaredge the setting is AC Charge Policy which I have set to "Always Allow". That doesn't mean it charges from AC all the time, that depends on the Storage Command Mode, which would need to be set to "Charge from Solar Power and Grid".

Have you managed to set yours so it only charges from surplus AC? I haven't seen how to do that.

My Solaredge hybrid is 3.68kW, and when on force discharge, it will dial down to prevent house export exceeding 4kW. No idea why 4kW not 3.68kW!

That sounds like it has its Export Limit set to 4kW.

QrizB

https://forums.moneysavingexpert.com/discussion/comment/82061063#Comment_82061063

As I understand it, it's false for most/all of the branded kit on sale in the UK.

You can generally install a hybrid inverter and battery without installing any solar panels at all and still use it as an AC-coupled home battery system.

The inverter will need a CT or similar to measure house load, and if house load is negative (ie. solar PV is exporting) it should charge from AC to balance that out at zero.

I'd be interested to know what make/model of hybrid inverter your daughter's installer is proposing which doesn't work like this!

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/82061284#Comment_82061284

Hi. How / where can you set that AC Charge Policy, I may be being stupid, but I can't see it?

I haven't set it for excess AC, it just does it, which I'd actually prefer it didn't (purely for FiT reasons). It became obvious to me, because I have energy monitors in the living room, showing what the two systems are generating. But the 'big' system now really shows what the battery is discharging or charging.

So, if the battery isn't at 100%, then in the afternoon I might see the small system showing 1.5kW, and the big system monitor showing 1.2kW (as an example). That's when it dawned on me that it's showing the charge going in from the AC side. If for instance I turned the 1kW kettle on, the small system would still show 1.5kW, but the big one would drop to say 0.2kW ….. giving the game away.

Not the end of the world, but I'd rather export that excess, not run it two ways through my batts and bi-directional meter.

Regarding the export limit, yep. In reality I have a DNO 5.9kW export limit, but that's the sum of the two inverters, 4kW is fine, now that I have batts to buffer, and the systems don't all peak at the same time.

Qyburn

When I get a chance I'll check if that setting is available on the inverter web page. (I access everything via Modbus).

Could you confirm your topology? Earlier you said you expanded "behind the 3.68kW inverter", but elsewhere suggests you have two inverters.

Regarding chacharging from surplus AC, if I set mine to "charge from grid and solar" then it take as much AC charge as needed to reach the configure charge rate limit. For example if the limit is set to 2,000W and we're getting 1,000W of solar, then the other 1,000W comes from AC.

Qyburn

The bad news is that AC Charge Policy doesn't seem to be available as such on the inverter web page. There's a setting AC Charge Limit under Power Control / Energy Manager / Storage Control, so I expect you could set the limit to zero to disable AC charging.