Adding batteries to existing solar system

Solarchaser

Reed_Richards said:

Solarchaser said:

2ndly as you will be on a very good FIT rate, you wouldn't really want a hybrid inverter as you will lose some of your FIT with every battery cycle, and then wouldn't be able to use a time of use tarrif to help make the batteries make sense, you would be far better with a stand alone battery inverter in your circumstances. 

I don't understand this bit because it does not apply to me; I only have the one inverter, my battery has no impact on my FIT.  But my battery is on the DC side of the inverter, along with the solar panels.  Is that not the norm?  

As a dc system the efficiency is I think about 80% round trip, so if you have 1000kw going through your batteries (not the whole system as it will be 100% efficient covering normal house loads) your fit will only report 800kw from that, so you really would be better with the batteries on the ac side, away from the fit meter.

Reed_Richards

Solarchaser said:
As a dc system the efficiency is I think about 80% round trip, so if you have 1000kw going through your batteries (not the whole system as it will be 100% efficient covering normal house loads) your fit will only report 800kw from that, so you really would be better with the batteries on the ac side, away from the fit meter.

80% sound very pessimistic; I hope it is.  I find it hard to believe that converting the DC output from the panels to AC then back to DC to charge a battery is somehow better than charging a (necessarily DC) battery directly from the DC output of the panels.  Is the DC to AC to DC conversion really lossless?

EricMears said:
... if you're charging batteries from DC that portion of the energy won't reach the generation meter so you won't get a FIT payment for it.

It reaches the generation meter when I discharge (i.e. use) the battery.

[Deleted User]

No two homes are the same. I have a 7kWp array and a PW2, and my annual consumption is c.4000kWhs a year. PW2 round trip loss is 10%. I need about 800kWhs/year top up from the Grid from late September through until mid March.

Solarchaser

Reed_Richards said:

Solarchaser said:
As a dc system the efficiency is I think about 80% round trip, so if you have 1000kw going through your batteries (not the whole system as it will be 100% efficient covering normal house loads) your fit will only report 800kw from that, so you really would be better with the batteries on the ac side, away from the fit meter.

80% sound very pessimistic; I hope it is.  I find it hard to believe that converting the DC output from the panels to AC then back to DC to charge a battery is somehow better than charging a (necessarily DC) battery directly from the DC output of the panels.  Is the DC to AC to DC conversion really lossless?

EricMears said:
... if you're charging batteries from DC that portion of the energy won't reach the generation meter so you won't get a FIT payment for it.

It reaches the generation meter when I discharge (i.e. use) the battery.

Is it lossless, no, but crucially for you, its after the fit meter, so the losses don't lose you FIT.
The conversion one way is about 90% usually, worse if you have the growatt I'd think, so 80% round trip seems about right 

pinnks

[Deleted User] said:

No two homes are the same. I have a 7kWp array and a PW2, and my annual consumption is c.4000kWhs a year. PW2 round trip loss is 10%. I need about 800kWhs/year top up from the Grid from late September through until mid March.

This is interesting.  My system is a little smaller at 5.25kWp, as is my annual usage at about 3,500kWh but my import averages about 1,700kWh per year, based on the last 7 years.  So, if I could get somewhere near your figures I could halve my import, say, reducing it to your 800kWh figure.  At 15p that equates to £135 saving per year from having a battery, ooo, err...
So even if I could get a battery of suitable size for £2,000 to £2,500 we are talking 15 to 19 years to break even on the investment...  I might well be pushing up the daisies by then!

EVandPV

Solarchaser said:

Reed_Richards said:

Solarchaser said:

2ndly as you will be on a very good FIT rate, you wouldn't really want a hybrid inverter as you will lose some of your FIT with every battery cycle, and then wouldn't be able to use a time of use tarrif to help make the batteries make sense, you would be far better with a stand alone battery inverter in your circumstances. 

I don't understand this bit because it does not apply to me; I only have the one inverter, my battery has no impact on my FIT.  But my battery is on the DC side of the inverter, along with the solar panels.  Is that not the norm?  

As a dc system the efficiency is I think about 80% round trip, so if you have 1000kw going through your batteries (not the whole system as it will be 100% efficient covering normal house loads) your fit will only report 800kw from that, so you really would be better with the batteries on the ac side, away from the fit meter.

Another benefit to ac coupled batteries is the ability to charge from the grid taking advantage of time of use tariffs during the months when pv is minimal, which helps with the financial calculations.
On Octopus Go for instance, you can charge the batteries at 5p a kw during the night for use during the day/evening.

Reed_Richards

EVandPV said:

Another benefit to ac coupled batteries is the ability to charge from the grid taking advantage of time of use tariffs during the months when pv is minimal, which helps with the financial calculations.

On Octopus Go for instance, you can charge the batteries at 5p a kw during the night for use during the day/evening.

Charging from the grid has nothing whatsoever to do with whether the battery is AC or DC coupled.  I could charge my DC-coupled battery from the grid but since I have a dumb electricity meter operating on a single tariff there is no point for financial reasons.  However if the state of charge of the battery gets too much lower than the specified minimum the inverter will take it upon itself to add a little charge from the grid.  

Solarchaser

Reed_Richards said:

EVandPV said:

Another benefit to ac coupled batteries is the ability to charge from the grid taking advantage of time of use tariffs during the months when pv is minimal, which helps with the financial calculations.

On Octopus Go for instance, you can charge the batteries at 5p a kw during the night for use during the day/evening.

Charging from the grid has nothing whatsoever to do with whether the battery is AC or DC coupled.  I could charge my DC-coupled battery from the grid but since I have a dumb electricity meter operating on a single tariff there is no point for financial reasons.  However if the state of charge of the battery gets too much lower than the specified minimum the inverter will take it upon itself to add a little charge from the grid.  

If you charge through your dc coupled system, when it discharges, the discharge that you have bought from the grid would go through your solar generation meter and so you would be paid FIT for it.... so you can't.

Reed_Richards

@Solarchaser, I'm sure I have been through this before.  My "generation meter" measures what goes in, what comes out and the default reading is the difference.  I report the difference to claim my FIT; no problem whatsoever!

Solarchaser

You must have a fancy generation meter then, good for you.

Most folk have a one way generation meter, which is checked every couple of years.