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Author Topic: Are Import/Export calculations real time or averaged on a FIT?  (Read 4382 times)
al_uk
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« on: April 15, 2010, 07:00:11 PM »

Can anyone tell me whether the measurements are real time, or averaged?

eg. If the PV panels are generating 1KW right now, and I switch the 2.2KW kettle on, am I then importing for 1.2KW x 5minutes?

Or is it averaged over a month, so if I generate 250KWh during a month, and consume 240KWh, is the calculation then 10KW export?

cheers
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guydewdney
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« Reply #1 on: April 15, 2010, 07:43:07 PM »

Instant - to the second at least.

The import meter still works as a normal meter - and any power IN is metered, and charged for. What you make (total generation) you get paid for. The Export meter tots up that power that you make when you are exporting.

pv gets paid 43p for generation and (I think) 3p export. Say you pay 15p / unit import / normal type then:-

1kw production and 500w house load = 500w export@3p + 1kw at 43p = total = you get paid 44.5p for that hour
1kw production and 2kw house load = 1kw import @ -15p (say) + 43p = total = 28p
1kw production zero house load = 1kw export = 1kw @ 43p + 1kw @ 3p export = 46p
1kw production and 4kw house load (oven on say) = 43p -(4x15p) = you pay them 17p

you can see from above that its hardly worth exporting - so why bother? use that to charge batteries / heat things etc etc - hence outtasights mad control system and my 'reverse power meter bodge' contraption that Alan, All Praise The God That He Is made for me.
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al_uk
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« Reply #2 on: April 15, 2010, 11:32:29 PM »

Great, thanks for that, answered my question!
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Tobi K
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« Reply #3 on: April 16, 2010, 07:17:58 AM »

... but then, chances are, if you're <30 kW, your export metering will be "deemed" at 50% anyway for the next few years.
So while your import payments (i.e. normal electricity bill) will directly be influenced by when you do or don't use your home-made electricity, the export tariff payments of 3 p/kWh will probably just be set to 50% of what you generated.

Note: in earlier version of this post, I wrote ">30 kW". As pointed out below, that's, of course, wrong. Thanks for pointing this out
« Last Edit: June 04, 2010, 12:19:47 AM by Tobi K » Logged
JohnS
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« Reply #4 on: April 22, 2010, 06:57:26 PM »

... but then, chances are, if you're >30 kW, your export metering will be "deemed" at 50% anyway for the next few years.

Tobi,

That is the wrong way round.  >30 kw, you must have an export meter.

Whilst 50% is generally talked of as the deemed export figure, there is nothing to say it has to be that.  If you have a 4kW system your export will be a lot higher than if you have a 2kW system.  Unless you are using a lot during the day, your export will be higher for a bigger system.  If the power companies only deem 50%, there is the option to fit an export meter.

Perhaps, when the power companies get smarter, they will reduce the deemed export percentage in winter.

In my case, I have 'volunteered' to be an early adopter for smart metering which will include an export meter.  I dont't know when it will be installed.

I know a lucky few who have old meters which can spin backwards. 

John
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Ted
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« Reply #5 on: April 22, 2010, 08:21:25 PM »

The 50% deemed export figure is written into the FITs legislation so would require an amendment to that before any electricity companies could vary it.
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Ivan
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« Reply #6 on: April 22, 2010, 11:42:07 PM »

If you have an old spinning disc meter like me, the changeover to 'import' isn't instant. As I discovered recently, the meter disc spins backwards for half a turn or so, until the resistance of the spring stops it going any further. I'm not sure what amount of power half a turn is equivalent to, but I'd guess probably about 100 watt-minutes. So for very short bursts of power demand, you take advantage of averaging.
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