Top tip 03 · Metering
Metering is two jobs sharing one word
A power meter tells the system what is happening right now so it can decide what to do. An energy meter counts kilowatt-hours so somebody can be paid. Conflating them is why meters get left off the design — and why systems get commissioned that cannot see their own grid connection.
From our founderTim Gauntlett TMIET
Power now, or energy over time
“Metering” covers two things that have almost nothing in common beyond the fact that both involve a box on a DIN rail. One measures power; the other measures energy. Ask which of the two a meter is for and most of the confusion on site goes away.
Power meter
What is happening right now
Measures instantaneous flow, in kW, and in a direction. The system reads it to decide what to do this second — charge, discharge, hold, or throttle the export. It is a control input.
Energy meter
What has happened over time
Accumulates kWh. Nothing reads it to make a decision — it exists so somebody can be paid, or audited. It is a record.
Get these confused and you end up in one of two familiar places: a system that cannot control itself because the only meter on site is a billing meter, or an expensive MID meter fitted to do a job a simple power meter would have done.
Topology decides what you measure
How the battery is coupled to the system decides what has to be measured. This is about power meters — the control inputs — not energy meters.
DC-coupled: meter the grid only.The panels and the battery are behind one inverter, which already knows everything happening on its own DC side. The only thing it cannot see for itself is the building's exchange with the grid.
AC-coupled: meter the grid and the PV. Here the battery system is a separate machine from the PV inverter, and it has no way of knowing what the array is producing — that generation arrives as AC on a circuit it does not control. It needs telling. The same goes for any other generation on site.
That single difference is the most common cause of an AC-coupled battery that “works” but never charges from surplus solar: it is metering the grid, it has no idea the PV exists, and it is behaving perfectly sensibly given what it can see.
The utility meter
After the main fuse, the utility meter counts the energy in and out, in kilowatt-hours. It belongs to the supplier, it is the basis of the bill, and it is not yours to design around.
It is worth naming it explicitly at the start of a job, because it anchors everything else: it establishes where the boundary of the installation is, and every other meter you fit is measuring something on your side of it.
The generation meter
Where owners were paid for generation rather than export, a MID meter was required to keep an accurate record of what the system produced.
That is the Feed-in Tariff, and it explains most of what you will find in an older meter cupboard. The payment was for every kilowatt-hour generated regardless of where it went, so the generation had to be counted at the source, accurately, by a meter somebody was prepared to be paid against.
FIT is closed to new applicants, but the meters are still out there and still being read. When you are adding storage to an existing FIT installation, that meter is not decoration — interfering with what it counts can affect what the customer is paid. Establish what it is doing before you change anything upstream of it.
The net meter
Following on from the FIT payment system, installers felt compelled to install bi-directional energy meters — and MCS often agreed.
The habit outlived the reason for it. MID certification is a billing requirement— it applies where the meter's readings are the basis of a payment. An endemic lack of understanding about which requirement applies to which situation means it is easier to go with the flow and fit the expensive thing.
Two things follow, and they pull in opposite directions. An Eastron SDM230 will do the energy-billing job where one is genuinely required. But most of the time it is not billing anything: it is there for accurate control — measuring power, so the system knows what is happening right now — and that job needs no MID certification at all.
For the control job a power meter with its serial number visible from the front will suffice. The serial number being readable in situ is not a detail; it is what makes the installation auditable without pulling the board apart.
So: know which of the two jobs the meter is doing before you price it. That is the question the whole page has been circling — power now, or energy over time.
Where the meters actually go
Three layouts, three answers. The letters mark measurement points, and the note under each says what that point is for.
The pattern across all three: the meter is the cheapest part of the specification and the one most often left off. Leave it out and the system still runs — it just cannot tell you, or the network operator, anything about what it did.
Field notes, and a starting point for a conversation — not a design authority. The current edition of BS 7671, your DNO's own requirements and the equipment manufacturer's instructions all outrank anything on this page.