Solar PV plus battery and standby generator

[AJH]

One for Adam maybe; what has to happen in the event of a power cut when
a 13kVA genset automatically starts up and triggers a changeover switch
between the mains meter and the consumer unit when a 4kW grid
interconnected array and 6.5kWh battery are on the consumer side?

Does the PV still work by sensing the AC from the genset or does it have
to be isolated with its battery during the time the genset is running?

[Andy Burns]

If you consider the solar+battery+inverter as a glirified inverter, I've
known several [decent] UPSes that aren't very happy when running from a
genny.

[Andy Burns]

Andy Burns wrote:

> If you consider the solar+battery+inverter as a glirified inverter

Or even as a glorified UPS

[Theo]

You can't feed power into the generator, so the grid output of the solar
would need to be isolated.

In normal times the solar is used to top up supply or demand - say if the
battery is offline, the solar outputs 2kW and the house demand is 10kW, the
remaining 8kW will be supplied from the grid. In that case things are OK.
But if you turned off the 9kW electric shower then the demand would fall to
1kW - normally this is fine as the solar pushes the remaining 1kW into the
grid, but the generator can't take that.

Unless the solar inverter has a mode by which it will disable grid upload
but still provide to local loads, which would need to be activated any time
the generator is connected. But as mentioned the generator might not like
even that if the waveforms don't match.

Theo

[AJH]

On 09/08/2021 14:28, Theo wrote:
> AJH <ne...@loampitsfarm.co.uk> wrote:
>> One for Adam maybe; what has to happen in the event of a power cut when
>> a 13kVA genset automatically starts up and triggers a changeover switch
>> between the mains meter and the consumer unit when a 4kW grid
>> interconnected array and 6.5kWh battery are on the consumer side?
>>
>> Does the PV still work by sensing the AC from the genset or does it have
>> to be isolated with its battery during the time the genset is running?
>
> You can't feed power into the generator, so the grid output of the solar
> would need to be isolated.

That was my guess but the generator would still charge the batterry as
long as its charger is AC and not DC straight from the panels. Easily
done with a tandem changeover switch I think.

In fact as long as the solar inverter accepted the near sinewave from
the generator was clean enough, thanks Andy Burns, why should the PV
inverter not backfeed the generator, surely it would just take load off
the generator? It's moot though as I agree the PV should not be
connected when the generator is on.

It's just that a PV installer I spoke to said it just could not be done
and in this situation the prospective PV buyers will not proceed.

>
> In normal times the solar is used to top up supply or demand - say if the
> battery is offline, the solar outputs 2kW and the house demand is 10kW, the
> remaining 8kW will be supplied from the grid. In that case things are OK.
> But if you turned off the 9kW electric shower then the demand would fall to
> 1kW - normally this is fine as the solar pushes the remaining 1kW into the
> grid, but the generator can't take that.

No but the battery charger could.

>
> Unless the solar inverter has a mode by which it will disable grid upload
> but still provide to local loads, which would need to be activated any time
> the generator is connected. But as mentioned the generator might not like
> even that if the waveforms don't match.

Yes as I said I think the changeover switch will have to isolate the PV
but I'll await further input.

[Andy Burns]

AJH wrote:

> the generator would still charge the batterry as long as its charger is
> AC and not DC straight from the panels.

I'd tend to think the battery *is* charged by the DC output of the
panels (MPPT/MPPC?)

Surely you expect the generator to run so infrequently, that costs of
genny fuel vs maximising battery are the least of your worries? Wire
the solar before the changeover switch so it "anti-islands" when you're
not on mains?

[Theo]

AJH <ne...@loampitsfarm.co.uk> wrote:
> It's just that a PV installer I spoke to said it just could not be done
> and in this situation the prospective PV buyers will not proceed.

It sounds like what you need is a control system with the following inputs:

Grid
Solar inverter
Battery inverter
Generator

and the ability to both disconnect and crossfeed between the power sources.

I'm sure that must exist, but it's not necessarily a feature of a particular
product that the installer might be familiar with. So 'can't be done' is
only because they only know brand X and that doesn't offer the feature.

I'd guess it can be done with a solar inverter that supports a generator
input, which is where most of the logic goes. And ideally one that can
have a battery hung off.

Theo

[Chris J Dixon]

Theo wrote:

>I'd guess it can be done with a solar inverter that supports a generator
>input, which is where most of the logic goes. And ideally one that can
>have a battery hung off.

I fear that the problem is that such an inverter is not permitted
to be connected to the grid.

Chris
--
Chris J Dixon Nottingham UK
ch...@cdixon.me.uk @ChrisJDixon1

Plant amazing Acers.

[Fredxx]

It would be fool hardy to connect your solar panel inverter to your
generator. Keep it connected to mains side.

If you want a combination of a solar, battery and generator then it's
not trivial. I would suggest you look at boat or RV power installations.

If you have access to your solar panels you will need to switch over to
your own MAPPT converter to charge your batteries, either 12V if you can
keep the LV cables very short but would recommend 24V or 48V batteries.
Then use an inverter to supply the mains power you need. Again 12V and
24V inverters are common. I recommend pure sine wave and avoid anything less

If you do want to use a generator then it's probably best to use this to
charge your batteries, as you won't be able to connect the output of
your inverter to any generator.

This is vastly more complex than I suspect you can imagine. 6.5kW
batteries is respectable 12V @ 550Ah or 24V @ 270Ah. But assuming these
are Lead Acid then at high loads not all capacity will be available.

The question you need to ask yourself, is what sort of load and for what
duration. A good 1kW panel will produce 800kWhr per year, Your 4kW
panels will produce very different amounts according to season, as this
indicates:

https://keyassets.timeincuk.net/inspirewp/live/wp-content/uploads/sites/19/2015/10/CardiffAnnual.jpeg

Taken from:

https://www.pbo.co.uk/expert-advice/solar-panels-everything-you-need-to-know-24455