Solar PV: How does the metering work?

[Mark42]

I’m new to PV – I first investigated the technology this morning. There’s one (probably obvious!) point I don’t understand. If all that happens is the inverted PV supply is fed into a spare way on the main DB, how is the entire load taken from the photovoltaics, rather than also partly from the mains supply, when the sun is shining but only a low load is called for.

Here’s an example.

Say a heavy load like an immersion water heater is timed to operate from midday in the summer. Assume there is 3kW of (free) PV electricity available to power it, and no other loads in the house.

Without this particular load being disconnected from the external mains by some complicated changeover relay system, surely some of the supply current would come (unnecessarily) via the external meter, and hence be charged for as used, and at the peak (day) rate? Conversely, the PV feed-in meter would not be running as fast as it could. A double-lose situation.

Before getting a PV quote today, I’d assumed there would be some kind of new, complicated metering system installed, with electronics which apportion the public supply in relation to the PV supply available. But this is not so. It appears both supplies are simply paralleled. Is this right? Or have I misunderstood something?

 

[Gary K]

The inverter produces a current at a slightly higher voltage than the mains supply so that the excess that is not used flows out of the system to the grid- You could liken it to a pressurised water system, if you fed more water into it at a higher pressure than the mains supply, it would be pushed back into the mains.
The generation meter is installed before the DB so it is not effected by current usage from it or the mains supply.

 

[liquidity]

Let me try to answer... The PV system has no knowledge of the level of consumption in the house nor does it care. If the inverter can take the DC generated by the panels and output AC at a higher voltage than the grid connection it does so and the generation meter installed as part of the PV system starts counting up merrily.

If consumption in the house is greater than the output of the PV system then electricity is imported and that is reflected on the standard meter. If consumption is less than the output of the PV system than electricity is exported to the grid. HTH.

 

[Mark42]

Gary & Liquidity – thank you for your excellent answers.

Shame the chap who priced the system today couldn’t explain this! It sounds an elegant solution, so long as the inverter gets it right. Is the over-voltage significant?

My installation may be complicated by the fact that the barn where the PV panels will be installed is 160m from the main house. The installation company will throw in the SWA required - so they probably won’t be too generous on cable gauge – so might voltage drop complicate things?

I have a UK ‘Farm supply’. The incomer usually meters around 240V on the two phases available (480V between phases).

Let’s say the voltage at the end of 160m of (say) 10mm at a given time is 234V.

If the inverter sees the dropped mains voltage at its end of the long cable, might the device take this voltage, add, say, 5V (or whatever it is), feed back the resultant 239V, which, using Gary’s excellent water pressure analogy, would only get close to the DB before dropping to below mains voltage *at the meter*, all of which end is connected up properly and over-specified (I did it myself) with 25mm everywhere.

Therefore all the solar would do is heat up a cable and attempt to cook my garden. This is not ideal.

Is this sort of thing taken care of in the inverter software or settings? Are the inverters that clever?

Even using a very heavy cable back to the board would not eliminate the problem, only reduce the voltage differential. Is this a known issue, or am I again barking up a wrong tree?

BTW, the final install will probably be two separate PV systems with two inverters, one for each phase. I assume the two phases act as separate circuits so this should not be a problem? (There are no legacy split-phase 480V machines).

Lastly, the voltage here goes up and down like the proverbial -----’s drawers, sometimes reaching 248V. Can the inverter top that? And if so might it damage equipment? (I have a lot of electronics, much of it rated at 230V).

 

[Mark42]

Oops – I just had an afterthought. If the generation meter is installed at the barn end, where the panels are, might this solve the problem? (If this is allowed.)

Then any wasted power would be metered at the generation rate.

And what stops someone connecting the PV supply to a constant (off-grid) heavy load when solar is not needed in the house (for example when on holiday), so as to ensure maximum metering of the FiT? And/or switching in additional panels in the winter to keep the generation capacity up to the 4kW allowed for a higher-tariff domestic install?

What does the 4kW really mean in the regs? The max possible in an August heatwave? Or average capacity? The installer had no idea on this one.

 

[Earthstore]

Where are you based in the UK?

 

[Gary K]

It is normal for the inverter to be fitted as close as safely possible to the panels to minimise cable losson the dc side of things and as you say idealy the gen meter would be close to the inverter.
I understand that maximum voltage for an inverter is about 260v before it will cut out but I am sure one of the regular sparks on here will comfirm.
4kw is the max size system for the highest fit payment ( whatever that is now ) either measured by the max output of the panels or the maximum peak output of the inverter if money was no object.

 

[screwdriver]

It is normal for the inverter to be fitted as close as safely possible to the panels to minimise cable losson the dc side of things and as you say idealy the gen meter would be close to the inverter.
I understand that maximum voltage for an inverter is about 260v before it will cut out but I am sure one of the regular sparks on here will comfirm.
4kw is the max size system for the highest fit payment ( whatever that is now ) either measured by the max output of the panels or the maximum peak output of the inverter if money was no object.

err I think you may find that the AC volt drop is more of an issue than DC volt drop, A long cable run like that would need a huge AC cable to reduce volt drop to a point where the inverter wouldn't trip on over voltage, I think you need to make sure that your installation company understands the job in hand before you accept them fitting an undersized cable. If you can relocate the inverter then I would possibly consider a longer DC run as that may allow you to install a smaller cable and therefore reduce installation costs.

 

[sharpener]

Gary & Liquidity – thank you for your excellent answers.

My installation may be complicated by the fact that the barn where the PV panels will be installed is 160m from the main house. The installation company will throw in the SWA required - so they probably won’t be too generous on cable gauge – so might voltage drop complicate things?

<snip>

Lastly, the voltage here goes up and down like the proverbial -----’s drawers, sometimes reaching 248V. Can the inverter top that? And if so might it damage equipment? (I have a lot of electronics, much of it rated at 230V).

To be more specific, the inverter will generate at whatever output voltage is required to push the export current down the cable. BUT the UK G83 spec for inverters up to 16A/phase requires them to trip on overvoltage at 264.5V (230V + 15%) so you if you want to generate when the supply is 248V at the main house your supplier will have a big problem sizing the cable for such a low voltage drop over 160m. I don't have the SWA table handy but 10mm^2 T&E would drop 11V over that length at 16A, and this would be 4.7x the recommended 1% voltage drop (you are inside the 5% max allowed for power ccts in BS7671 but most people would consider that level of power loss uneconomic).

As others have said, probably the way round this is to run the DC all the way back to the house. It is often recommended that the DC runs should be minimized, but if you have say two strings with 8 x 240W panels in each string the string voltage will be about 240V so the losses will be the same either way. If however you can arrange the array as one string of 16 panels then the voltage will be doubled and for the same cable cross-section the losses will be a quarter (assuming of course you do not need more than one MPP tracker for reasons of aspect or shading, and you fit an inverter which can cope with the higher voltage).

HTH

 

[moggy1968]

I have a similar problem. I have an install to do (8kW) which involves a 300m run from the barn where the panels will be back to the house.
running this through sunny design to have the inverter in the barn would require a 120mm CSA cable to run the AC, ouch!! if I run the DC all the way to the house I just need a 16mm cable

 

[sherbert]

you should consider the mechanical protection of the dc cable as any damage to the cable would not trip out the ac supply and could cause fire or injury to livestock. Maybe consider dc trip but I'm not sure if that would protect a short on the dc.

 

[Mark42]

Again, thanks everyone for your input.

The idea of running higher voltage DC back to the house is very interesting. I like it!

It appears 2-core 10mm SWA would be well over-specified. At £2.34/m that’s within budget.

I’ve just looked up inverters and it appears the conversion efficiency is higher as DC input voltage is increased, another plus point.

The installers I’ve asked before said no to long DC, because it’s considered non-standard.

Maybe add a panel or two to the ‘standard’ 16 (possibly at a later date) to increase the DC voltage even further, to allow for any voltage drop or other inefficiencies.

I have another contractor coming to price the job next week: he’s not a salesman like before, but a real electrical engineer who might even understand this stuff, and so be open to planning a non-standard system. (I’m near Diss in Norfolk) Normally I’d research the technology and do it myself (I do everything else) but obviously can’t connect without being certified, so will let a contractor have this one.

 

[moggy1968]

If only I knew! I was up there begining of the week surveying my 8kW job with the 300m run in Shottesham. next time your driving towards harleston go through Scole and look left and you'll see my parents flat roof system I put on in nov!