How Sensitive are electricity meters

I have been monitoring my solar generation and consumption using a setup I created myself which reads an Elster A100C generation meter and my L&G Import Meter together with a clamp meter for Total Consumption.


I  currently have a combi boiler and my summer gas consumption is mostly Hot Water. I am thinking of getting a 300L Hot water tank. I want to divert my excess solar energy to the water tank and have seen some sample projects to do this.

Many of them use a Triac circuit to turn the power as different time delays during the phase to regulate the power. I was wondering if it would be possible to use a zero cross SSR and just fire it for so many half cycles in each second. So If I have 1KW of spare energy and I have a 2KW immersion heater I could fire the SSR once every other half cycle and draw 1KW. If I build in a 10% safety margin and only fire it 45 of the 100 half cycles per second  this should use up most of my spare consumption. 

What I was wondering Is how sensitive are the meters I have a standard Ampy/L&G how many jewels have to flow forward before they register. According to the spec the start current is 0.4% of the BASE current which on my meter is 20A ie 80mA (Approximately 20W).

The meter binks once per W/H IE once for every 3600 jewels. If I have an excess of 1KW or power I have 1000 j/s going out through the meter which is 10 jewels every half phase.  My Immersion heater would draw 20 J per half phase (2000J/S/100th of a S) .

So does the meter count upto 3600 J before if registers a W/H unit? From what I gather they subtract outflow IE if you had a reactive load that say drew 3KW for half the phase and returned 500W for the other half it would measure 2.5kW on the meter.

If the minimum measured unit is 3600J (1WH) then it should be posbible to turn the heater on for a second then off for a second. As in the first second its on would have an inflow of 1000J (2000W  (Load) -1000W(Generation) ) the register would then be zeroed by the outflow of 1000J of generation.

So what I was wondering was does anyone know how many Joules there is the minimum measures amount for a standard electricity meter? 

Alternately I suppose the meter could just add up all the total outbound energies per half phase or phase if it were per phase I could rectify the supply to give an alternative of 1Kw or 2Kw dump load? 

JSHarris's picture

Re: How Sensitive are electricity meters

I've recently been looking at all of the core chips used in the electronic energy meters.  All of the chips are capable of resolving to energy levels of less than 1Wh, but the meter manufacturers seem to vary in their implementation.  Some stick to the same sort of energy resolution that was an inherent feature of the magnetic drag "anti-creep" feature built in to induction disc type meters.  This is typically about 1 Wh (3600 J).

The advent of electronic meters has allowed far better resolution, with lower thresholds, but many manufacturers stick to 1Wh as the threshold.  I know of one that has a 1250 J threshold, though, so it is possible that energy threshold figures will reduce as meter manufacturers decide that lower thresholds can still give reliable operation.

As a typical immersion is rated at 3000W, a single cycle will be 60 J, so there is plenty of scope for fine control of power to an immersion heater, even using zero crossing control.

Robert Wall's picture

Re: How Sensitive are electricity meters

I think you need to read the articles on Energy Meters and Diverting surplus PV Power

You cannot use a half wave rectifier to control the power into your load. A half-wave rectifier generates a large proportion of even harmonic current in the supply that is guaranteed to exceed the permissible limits here in the UK, and most probably elsewhere in the world too.

ffimon's picture

Re: How Sensitive are electricity meters

The tank I have my eye on has a 2KW immersion heater in it as I only ever have a maximum 2.6KW excess. They do have one with two 2KW heaters I did contemplate this one as I could use SSRs to switch one one or both in series this   would give me the option of 1K or 2KW. If I can find one with 2 * 1KW heaters I could swich two on 2kw 1 on 1 kw or both in series 500w that would probably be enough variation for me.  

JSHarris's picture

Re: How Sensitive are electricity meters

It's straightforward enough to use a single SSR to vary power to the immersion in single cycle increments, but to get acceptable flicker performance (caused by small voltage drops) it's necessary to do some jiggery pokery to either psuedo-randomise the "on" pulses over several cycles, or accumulate them and concatenate all the "ons" into a single, sub-detection threshold, pulse.

There's no real need for lower power immersion heater elements, as power can be controlled pretty much irrespective of the element rating (subject to the max being the rating).

calypso_rae's picture

Re: How Sensitive are electricity meters

I think you'll find that all the basic problems have been already sorted out.  If you can provide a load that's sufficient to mop up all your surplus power, then you can feel confident that a Mk2 PV Router will be able to control it in a way that is effective and reliable. 

All Mk2 variants will heat water in a similar manner, but you may prefer one version than another depending on what else you want to do.  The code for the original version was rather bulky.  It includes a "debug" mode which was useful for me at the time but has generally not found much favour.  A trimmed down version of the original code, still with floating-point maths, is available as Mk2_PV_Router_mini_3.

Mk2a versions uses integer maths and are very much faster.   Mk2i versions are interrupt-based, and are faster again.  The Mk2i_rev5a version can support multiple loads, one of which can be controlled via an RF link.  All of these versions, along with various tools and videos etc. can be found here.

Equivalent solutions have also been posted by other designers.  As far as I'm aware, the only difference is the way that the flow of energy at the supply point is determined.  The "energy bucket" stage, and the means by which its level is regulated by varying the power to the load, seems to be pretty much standard.

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