What is the real effect of cable losses?

[timbo]

I am in the final stages of planning my PV and Wind Turbine grid connected system.

There will be 10kW of PV with the inverter/TGMeter about 100m away from the house, cable losses with 16mm^2 cable are about 4%
There will be a 6kW Proven turbine with inverter/TGMeter about 200m away from the house, cable losses again about 4%.
No export meter.

I keep getting lost trying to work out the real effect of the cable losses. Are all of the following statements correct or am I missing something or double counting?:

It is best to have the TGMeters at the array/turbine to maximise FIT's payments.

The Inverters will operate at 4% higher output voltage because of the cable losses, other than possibly reaching the upper voltage limit on the inverters there is no effect from this.

If my electricity usage in the house was ALWAYS MORE than I generate then I will import 4% more power from the grid (than if there were no cable losses) but not lose the 3p/unit export as it calculated from the TGMeter readings. (at 21,000kWh/year 4% at 11p is £92.40/year or "a £2300 decision over 25 years")

If my usage is ALWAYS LESS than I generate then the cable losses would have no money effect for me.

If my usage is sometimes more, sometimes less than I generate (the normal situation) then I will import power MORE of the time but it's not possible to calculate by how much, or is it?

Head is spinning....

Tim.

[EccentricAnomaly]

With 16 kW of (potential) generation, will they really go for the "deemed" 50% export or will they either deem something else or insist on an export meter?

[timbo]

I spoke to Loco2 today and they seem happy with the deemed output. I guess that since a typical house uses 5500 units then 50% favours them, I should probably ask for 75% assumed export.

Tim.

[BruceB]

If you have already chosen cable sizes then the most significant factor you can affect is the position of your total generation meter.  The FIT rate dominates in any calculations so put the meter by the output of the inverter.  You can do the calculation in your last question, but you would need to generate some sort of spreadsheet model that say for each hour of the day had inputs of power generated and likely load, perhaps also including an adjustment for seasons.

Regards
Bruce

[Ted]

Export meters are only mandatory above 30kW.

MCS guidance are that meters should be fitted next to the consumer unit.

PV and wind with no export meter can only be deemed at 50% - as this is written into the FiTs legislation by the government. The supplier has no discretion.

[timbo]

Thanks Bruce. I haven't chosen the cable size yet, I was assuming 16mm^2.

25mm^2 is possible but there doesn't seem to be a good economic reason for it, particularly as I will be almost always using less than I generate.

I would guess that going down to 10mm^2 would make the inverter voltage too high but otherwise make economic sense...I don't like it, I want it to make sense to get the best efficiency... :-)

Tim.

I see Ted's post now.

Thanks thats starting to make sense now. My MCS installer seems quite happy to fit the meter at the source though.

I think I should probably opt for export meter(s), it gives an incentive for efficiency and just feels like the right thing to do.

Tim.

[Justme]

What is stopping you from having a second & third CU near the PV & Turbine with a light & socket circuits in them for local use?

You would then have an RCD at both ends of the cable to protect it & you.

[BruceB]

You are quite right Ted, I had forgotten that about the meter position.

4. Position of meter
The meter shall be fitted to a vertical surface and be placed in a position so that the
register can easily be read by the customer without requiring the use of any
equipment such as tools, ladders or a torch. Ideally the meter should be positioned
adjacent to the consumer unit.

It is only a 'should' though......

[timbo]

4. Position of meter
The meter shall be fitted to a vertical surface and be placed in a position so that the
register can easily be read by the customer without requiring the use of any
equipment such as tools, ladders or a torch. Ideally the meter should be positioned
adjacent to the consumer unit.

It is only a 'should' though......

This sounds as though the intent of this paragraph was to make it easy to read, rather than being about not tweaking the most out of the FIT scheme. So my installers aren't breaking the spirit of the rules by putting the meter at the source.

I will have access to the data from the inverters in the house so wont need to see the front of the box.

Tim.

Sorry, I dont understand the point about 2nd/3rd CU.

[Justme]

If the regs said it HAD to go next to the CU there seems to be nothing saying which CU or that you cant have more than one (as it is normal in a house or industrial install to have secondary CU's for external areas like garages & sheds)

[timbo]

OK thanks I see what you mean now about the CU.

Tim.