Metering in two directions

I just read this in some other place
“Even the excuse for the expensive bidirectional meter has a simple solution – use two conventional ones back to back. The one measures incoming energy and the other measures outgoing energy.”

This doesn’t sound right. Surely somebody would have thought of this by now. And it would be two meters in series both seeing the same incoming and outgoing current.

I was going to ask if the meters have a CT in them, but we know they don’t. If they did then then we wouldn’t have problems with people feeding back and the meter seeing the feed-in as draw from the grid.

If one is referring to the older generation one direction only spinning disc meters, yes, of course, it will work. Both meters will increment depending on the current direction.
This is OK if you are the person reading the meter knowing which one is which and can you basic arithmetic.
However, can you imagine the unbelievable lecky bills people would get if this daunting task is left to a meter reader, a computer and the collective intelligence of a municipality?

OK… I’ve heard that PV can make those meters run backwards. So it seems to me that one would do that, and the other would run forward by exactly the same amount and the two readings would cancel each other out.

The trouble is that the “conventional ones” simply don’t work like that. At least not the electronic ones. They are designed to prevent tampering (because connecting a meter backwards is one of the oldest tricks in the book).

The disk meters come in two variants. The one we are mostly used to, simply runs backwards if the power is going backwards, and two meters will indeed cancel out. But you do get some versions with “reverse lockup” essentially a ratchet mechanism, which prevents it from spinning backwards. If you had two of those back to back, it will work. For some value of work…

The lockup mechanism only operates at one point on the disk, to use the ratchet analogy again, the ratchet mechanism has only a single tooth. So you can spin the meter backwards, up to the point where the lock engages, which means small amounts of power can go forwards and backwards, just not so much as to make a full rotation.

When you put two meters back to back, there will be accuracy issues for this reason. Some energy is not going to be properly ignored in both directions, and the lockup points will be at different locations meaning some energy will be double billed or not accounted at all as well. Small amounts, but probably enough to rule it out as a certifiable solution.

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I have heard tell of such a mythical meter
This is a unicorn that should never be discussed out loud.
Legend has it that you must never export more than you use ( between meter readings).
Become one with the meter reading schedule and switch off exporting a week on either side of any possible manual meter reading.

At the end of this rainbow is a pot of gold worth the equivalent of a lossless infinite battery at no cost.

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I was told about old meters spinning backwards by the electrician who split my DB during the installation.

I said that doesn’t sound good. He was enthusiastic about it. But I said, OK… let’s say the meters have 6 digits to the left of the decimal place, your opening reading is 123456 and you closing reading is 123440, now what is the computer involved going to conclude? And do you want a bill for that amount?

The very least that is going to happen (assuming somebody is awake and spots the out of range number) is that the municipality is going to come and ask you some questions.

So yes, you’d have to manage it.

Even then the municipality is going to notice the drop off on your meter and - see above - come and ask some questions and have a look at what’s happening.

This happened to me when I got my system installed, and this is scenario has played out repeatedly in my neck of the woods as more and more folks have put in solar. Your consumption drops off substantially. It stays down for another month. Some rule on the City’s system is triggered and they send a technician around to do an inspection and ask you some questions.

The reality is that when you install solar your meter readings quickly become a big red flag for the city.

Even if you’re on prepaid, because you’re going to start buying less and less often. Again their systems spot the change and the red flag has been waved.

You’re not just going to pay less and smile all the way to the bank, not without somebody wanting to have a look at your meter and the connections to it.

If your consumption goes negative then I would expect the billing system to hit you with a large number. You, of course, will protest this. But all roads lead not to the end of the rainbow but to somebody coming to see what the heck is going on at your property,

There is no escaping it, that I can see.

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When you have an alternative power source, it is a reasonable expectation that you’re lecky bill will decrease substantially.
Supplying that answer on enquiry is perfectly legal and reasonable.
Red flag or not.
But, if you think red flags are raised by human intervention you’d be wrong, they are raised by a computer algorithm in reaction to a step-change. a gradual change would go undetected.
This is the first line of defence… just like a bank robber shouldn’t buy a Lamborghini, a solar user shouldn’t rip the arse out of things on day one.
Keep in mind:
It is illegal to achieve a bill reduction through exported power being subtracted from imported power. It is only legal to achieve a lesser bill by reducing the power that is imported.

It will give the game away if a reader sees your meter running backwards or if the exporter does not maintain a net import will give the game away though. Supplying back your entire usage and more takes some size system though and it is not easily done by accident.

But how to guard against that random inspection…
Companies like Xiaomi make inconspicuous sensors that sense things like light levels, IR and doors opening.
These use a Zigbee radio network. Through an app, they can switch other Zigbee power sockets on for certain times etc. It is easy to set up.
If you can achieve a 5kW net export, set up a 5kW element in a 50 l drum, or you could just bring existing loads on line.
Recall that famous Tom Cruise Mission Impossible scene where he hangs spread-eagled from the ceiling just inches above the floor?
I have it on good authority that that scene is based on a meter reader trying to get to a solar exporter’s meter without turning on the additional load that would force the meter to import before the meter box could be opened fully.

Unless a reader meter is 1) wide awake, 2) very sharp to catch small detail, and/or 3) specifically looking for it, he is unlikely to immediately notice that the meter is running backwards. He wants to get the reading, note it down, and get on with his job. He has a hundred other meters to read today.

But ending up with a reading lower than the previous month? That gets you noticed real quick.

I’ve had a look at my bill tonight. Several months in a row now, I bought exactly 600.2kWh of electricity :slight_smile: The 0.2kWh was because I rounded up to the next whole number, too lazy to type the cents.

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seems like officially the best one will be able to do is half a reader meter:upside_down_face:


EDIT: @plonkster, if not familiar with it, grab a copy of Oliver Sacks’s “The man who mistook his wife for a hat” (Takealot paperback seems cheaper than any of the eBook versions). NOT for diagnostic/self-pathologizing purposes (we do too much of that) but suspect you will enjoy the content. Putting this here to not derail things more…

Truth be told, I am amazed at the mistakes I make some days. Sometimes I use entirely the wrong word, but instantly see it on proofreading. And sometimes I don’t. Almost like a kind of non-permanent temporary Aphasia. So yes, this writer was quite the opposite of “wide awake” when he wrote “reader meter”.

Edit: OK, rabbit hole stuff. So temporary aphasia is a thing that sometimes occurs with migraines. I don’t get migraines, but I do get Scintillating Scotomas at times, which is a migraine in the visual cortex. Takes about 15 minutes or so to pass, really irritating and even dangerous if it happens while driving, although there is usually some warning that it is coming as the shimmering blob starts in the corner of the visual field and doesn’t immediately obscure everything. Happens more when I am stressed, sleep deprived… or both :slight_smile:

You didn’t need to admit it. When I first read it, I actually thought it was intentional and quite clever.