With solar panels, the question that matters is not how much they produce. It is how much of it you keep, and how much goes to the grid at less than you later pay to buy it back. The inverter usually shows you production only, and the utility sends the figures days late.
The Shelly EM has two measurement channels, and that is enough to know in real time: one watches the main feed, the other watches production.
Two clamps, two channels
- Channel 1, on the main feed. It measures the exchange with the grid both ways: positive when you import, negative when you export.
- Channel 2, on the inverter’s AC output. It measures what the panels produce. Always positive.
And the condition that applies to any EM installation: power the device from the same phase the clamps are reading. If the reference voltage comes from another, the calculations are wrong — set out in detail in why the Shelly EM reads higher than expected.
Two measurements, four figures
Those two physical readings give you the complete energy picture of the house:
- House consumption = production + imported − exported
- Self-consumption = production − exported
The figure that actually judges an installation is the self-consumption percentage: self-consumption divided by production. Thirty per cent means you are giving away two thirds of what you generate.
The sign on channel 1 tells you everything
That is the heart of the setup. Positive, you are buying; negative, you are exporting. If the signs come out backwards on first start there is nothing to dismantle: turn the clamp round, or reverse that channel’s measurement direction from the device’s web interface.
Which clamp goes on which channel
A common buying mistake: the two channels do not carry the same current and do not want the same clamp.
The main feed carries the whole contracted supply, so the 120 A clamp belongs there. On the inverter output, a domestic 3 to 5 kW system does not exceed about 25 A, so the 50 A clamp has plenty in hand.
Select the clamp type separately on each channel. It is the mistake that produces the most nonsensical readings.
What a day looks like
The pattern is always the same: early morning and evening you import; in the middle of the day, once production passes your base load, you start exporting.
That middle band is the opportunity. Every watt you use then is free; exporting it is compensated below what you pay to import. The EM keeps 365 days of history, so you can compare seasons and see whether self-consumption is improving over the year.
Automating the surplus
This is where the EM stops being a meter. It has an output that can drive a contactor and start a load when it detects surplus: the immersion heater, the pool pump, the car charging during daylight.
A safety warning, and not a small one: that output is rated 2 A. It is there to drive a contactor coil, not to feed the load. A water heater or a pump needs an intermediate contactor sized for its power. Connecting them directly destroys the device.
Watching the surplus as it happens
Tuning the threshold at which the load kicks in is trial and error, and doing it in front of the board is awkward. In the console the telemetry refreshes by itself, so you can change the threshold and see the effect there and then, from a browser.
And when the installation is handed over, the client can have the device assigned to their user: they see their own production and consumption, and nothing from your other installations.
Common questions
Is one clamp enough? If you only want to know when there is surplus, yes: the one on the main feed already separates import from export, which is enough to automate. The second adds production and lets you calculate the self-consumption percentage.
Can I extend the lead to the inverter? You can, but the longer it is the more noise it picks up and the less steady the reading. If the inverter is far from the board, a second meter beside it usually works out better.
Does it work on three-phase? No. The EM is single-phase and both channels must be on the same phase. Three-phase needs a three-phase meter.
Is it accurate enough to do sums with? The original clamps are rated at 1%. Enough to size and automate with, but not a replacement for the official meter for billing.
Does it need internet? No. It works locally and stores the history internally. It integrates over MQTT without depending on any cloud.
If you are fitting several meters — production, main feed, a sub-meter — provisioning them in one go with the SIT-IoT app saves configuring them one at a time and having one end up with the wrong clamp declared. Compatible with Gen2, Gen3 and Gen4.