On-grid domestic battery storage

Martyn1981

You don't need to fear the export meter, it's only if the TGM is being impacted that raises concerns.

Is your battery on the PV or household side of the TGM?

Reed_Richards

I have two meters next to my consumer unit; I don't know for certain which you would call the TGM but I presume it is the first (which is the one I referred to earlier).

One is in the same box as the isolator switch for the solar PV system. This is wired into the taps that were installed on the wires that connect my electricity meter to my consumer unit. It is preset to measure the net power from the solar panels but doesn't know (on account of its position) where this power goes, to the house or to the mains.

The other meter is preset to report the instantaneous power output from the solar panels. It has one wire that goes to something that surrounds one of the two wires from the electricity meter to the consumer unit so presumably can sense the direction of current flow and report this back to the inverter (which "knows" roughly how much power is being used by the house and/or exported to the grid).

The battery is on the PV side of the of all the meters, in fact it's on the PV side of the inverter so there is no means of recording the actual Total Generation, just the generation minus any losses that come from charging and discharging the battery.

P.S. I seem to have "won" the "debate" I was having when the other person understood that I have a Net Meter; I guess they don't all have that capability.

Martyn1981

The TGM is the total generation meter which records generation from the PV, and is the one you use to report to your FiT provider.

If your battery is DC side, then you are absolutely right, import from the mains to the battery, will be recorded as PV gen when it goes back out from batts to the house.

I don't think it's a clever inverter you need, but some sort of FiT approved TGM that deducts import. You'd then lose more FiTs due to battery losses, but at least it might be doable. [Sorry - yes you'll need an inverter that can also change AC to DC for batt charging.]

I have no idea if such a meter has FiT approval, I'd suggest contacting your FiT provider and asking them, also, you'll need to do this anyway, since the FiT provider will have the reference number of your TGM, and will need to change it for the 2yr check that takes place.

1961Nick

I think from a your description that you have a DC battery system - just one inverter that handles solar & batteries. If so, your generation meter should record the net of import & export. Usually these meters automatically scroll through the various registers - or there may be a button to do that.

If your set up is as above, have a look at the functions on the front panel of your inverter & see if you can find a program for grid charging. Have you got the instruction manual - if not, see if you can find it online. If it’s a popular model there may even be YouTube videos about setting it up for grid charging.

Reed_Richards

As I hoped I had explained, my meter does deduct import. I hadn't realised hitherto that the don't all do that.

My battery is charged DC to DC.

What's this about a 2 yr check? I have not heard anything about this.

Reed_Richards

Thanks @1961Nick, I am sure you have correctly interpreted my description. At present I don't have an Economy 7 meter so charging from the mains is looking to some hypothetical future; I just wanted to know how feasible it would be.

Martyn1981

Reed_Richards wrote: »

As I hoped I had explained, my meter does deduct import. I hadn't realised hitherto that the don't all do that.

My battery is charged DC to DC.

What's this about a 2 yr check? I have not heard anything about this.

Cool, if the TGM deducts import then you should be OK. Basic installs just get a bog standard meter just like the (non-smart) domestic import meters. But obviously your FiT will be reduced by battery losses, so you might want to factor that in too when considering the true cost - price of import + battery cost (per kWh) + FiT losses. Of course the pro's column has a big tick under fun too.

The FiT providers have to check your TGM every two years, so a meter reader will pop by to see/read it.


PS - I still can't find any small wind or hydro turbines that output in AC. Any chance we weren't talking about the same thing, or something got lost in translation?

Reed_Richards

Martyn1981 wrote: »

PS - I still can't find any small wind or hydro turbines that output in AC. Any chance we weren't talking about the same thing, or something got lost in translation?

No I am sure I am correct. But the AC that is the immediate output from a wind or hydro turbine I have seen described as "wild AC", not the domesticated type that you use in your home. So it must be immediately converted to DC, possibly just using something simple like a rectifier. But I cannot tell you why the AC to DC conversion is done at the turbine end of the cable that carries electricity to the end user rather than at the user end.

Reed_Richards

Martyn, have a read of this: https://www.howacarworks.com/basics/how-the-charging-system-works which explains how a car charges it battery (I mean any car). Rotary motion is converted to AC and then rectified to give you the DC you need to charge your battery. Turbines work the same way.

joefizz

Reed_Richards wrote: »

Martyn, have a read of this: https://www.howacarworks.com/basics/how-the-charging-system-works which explains how a car charges it battery (I mean any car). Rotary motion is converted to AC and then rectified to give you the DC you need to charge your battery. Turbines work the same way.

Late to this discussion but cars used to have dynamos (direct DC) and now alternators (its in the name).
As mentioned the AC is just alternating current in that it alternates, its not 240v 50hz so converted at source to a DC value which is measurable (12v, 48v etc etc).



When I was at uni we had certain projects to do and one of my tutorial mates' dad had built a small hydro system from scratch pretty much on their farm. It was a DC system to power lighting etc in the livestock barns but worked pretty well (was in Donegal, lots of rain, good head on the system) but first real problem they came across was small fish getting mashed up in it. Still needed an output voltage regulator and was direct to batteries IIRC.


I wonder do electrical engineering students get to play with magnetos, dynamos and alternators today. Probably against health and safety.



As others have mentioned its the head of water and the regularity/flow. When I was in Iceland I went to a few derelict local hydro plants (single car garage sized things), the ones I saw were at small drops in rock (i.e. waterfalls of about 3-5m in height) with a separate pipe feed from further up the river.

All AC and powered local lighting etc.



Have often thought of putting a small dc motor in my downpipe and seeing if I can charge a couple of AAs from the rain ;-)


Id certainly be looking at it if I lived up the street where the river runs down the hill ;-)
Even a constant say 300w output fed straight into the batteries after losses would be all that was needed really in winter to make a big difference.