Thread by @ChameleonMitch

Wondering why the cost isn't right on your smart meter display? Well, let me tell you far more than you need to know about smart meters by way of explanation... This assumes that your display IS showing the correct usage, but the cost is wrong.
#ihd #smartmeters #gb
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First, let's quickly talk about how smart meters talk to the rest of the world. Your smart electricity meter has a little box on top of it called a communications hub, which is sort of like a router for your smart meters. (2/33)

It allows the meters to talk to each other and your IHD over a short distance radio network called ZigBee Smart Energy (lots of other things use ZigBee, but Smart Energy is a special version of it). (3/33)

The communications hub also allows approved companies to talk to your meters either over long range radio (in the North of GB) or mobile phone networks (in Central and Southern regions of GB). (4/33)

North, Central, and Southern regions are defined by the government, and not by how you pronounce "scone" in this case. (5/33)

So your gas and electricity suppliers can talk to your meter this way through an organizational called the Data Communications Company (@smartdcc), which might sound like a front for GCHQ, but is actually just Capita. (6/33)

These networks, particularly in the North, aren't the fastest, though they're getting better all the time. (7/33)

Next, let's talk about how meters figure out how much energy you're using. This is easier with electricity than gas. How new meters measure stuff accurately is governed by requirements set under the Measuring Instruments Directive/Regulations. gov.uk/guidance/mid-a… (8/33)

Smart electricity meters measure things the way they have done for years: by measuring the electrical current flowing through them at high frequency, as well as the voltage. (9/33)

If you remember your lessons from school, you multiply those two to get power in Watts, and by the time period to get Watt-hours, the units you're actually billed in. This isn't the full truth and the real formula involves extra pieces of information like power factor. (10/33)

Gas meters measure the volume of gas that flows through them. The energy contained in that volume depends on its pressure, temperature, and "calorific value" (how much burnable energy is in each mole of gas). Calorific value varies day by day and area by area. (11/33)

@nationalgriduk publish data on calorific values every day. nationalgrid.com/gas-transmissi… (12/33)

What do smart meters do with the measurements they take? Well, they're not constantly sending them to your supplier, the suppliers have to request the values when they want them, so the meters have to store a lot of data for when it's needed. (13/33)

Smart meters are required (by the Smart Metering Equipment Technical Specification or SMETS) to store energy consumption data in four main ways (there are extra variations on this, but these are the main ones we care about):
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1. The total energy consumed since meter installation (your meter reading);
2. Total energy used for each half-hour for the last 13 months;
3. Total energy used for each day in the last two years; and...
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4. Total energy used for each day in the last 8 days, each week in the last 5 weeks, and each month in the last 13 months.

Suppliers fetch this data from DCC to calculate your bill. The values they use to calculate your bill varies, usually it's the meter reading though. (16/33)

OK, that's all about consumption, but what about cost? Well the meters do also calculate cost. And here we get to the tricky part. Energy suppliers can send information about your tariff to your meters so they can work out cost of the consumption they've measured. (17/33)

Energy suppliers are obligated (under the Electricity and Gas Conditions of Supply set out by @Ofgem) to take "reasonable measures" to ensure your meters are up to date with information such as calorific value and tariff. (18/33)

The tariff information can be really complicated, allowing for tariffs with different prices every half hour, different prices for different days of the week and all sorts. (19/33)

The meters use that tariff information with the consumption information they've measured to calculate the cost of the energy you've used and store it with the consumption from point four above (total for each day in the last 8 days, etc) but not for every measure. (20/33)

Your IHD doesn't calculate cost of consumption. Chameleon and ivie IHDs show three types of price/cost information:
1. The current price per kWh and standing charge for gas and electricity;
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2. The cost of consumption from the last day/week/month as fetched from the meter; and
3. The cost of current electricity power usage if you kept it up for the next hour.
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These are all fetched from the meters apart from the last one which is based on the current power draw and multiplied by the active price.
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The IHD might cache some data, but if it's still wrong after you've turned the IHD off and on again, it's just showing the latest information it's fetched from the meters.
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So back to my original question... why could it be wrong? Who should you blame?
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1. The tariff set on the meters might not be right. Maybe the supplier hasn't been able to work with your meters because they are an older model, this should be fixed by the end of 2022. Maybe the radio signal to the meters is poor.
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2. The tariff set on the meters might not have been set at the right time. Future tariffs can be set in advance, but if it's set after it's meant to take effect the meters don't have to recalculate the cost of energy already used.
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3. The tariff might not be supported by smart meter technology. As flexible as the tariff data structures and messages are, the underlying ZigBee Smart Energy standard doesn't support negative prices like those (once upon a time) seen on Octopus Agile for example.
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4. The meter software might have small rounding errors or quirks when calculating the cost. This is the least likely, but as much as you might try to nail something down in technical specifications and technical testing, some issues will occasionally sneak through.
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At Chameleon we're constantly working on new approaches to testing and improving our compatibility of our products with different meters, but...
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your supplier is responsible for the version of software running on the meters at your property and the IHD they gave you, and that software can only be changed with their approval.
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So in summary:
• There are a few things that can result in bad price or cost data showing on your IHD. Usually it's something your supplier should be able to resolve quickly. Sometimes they'll be stuck until broader technical changes are made or approved.
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• The IHD itself isn't calculating cost at all.
• The cost shown on your IHD is not necessarily the same cost that you'll be billed for by your supplier, but that's also up to them to decide.
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