If you have solar panels, the question that really matters isn't how much they produce, but how much of that production you're actually using and how much is being given away to the grid. Most inverters show you production only, and your utility sends the data days late.
The Shelly EM solves this with its two measurement channels: one watches the grid and the other the solar production. With those two real-time figures you can work out your true self-consumption and, above all, automate loads so you don't export a single free watt.
The setup: two clamps, two channels
In a single-phase home with self-consumption it's simple:
- Channel 1 — on the grid tail. Measures the grid exchange both ways: positive when you import, negative when you export.
- Channel 2 — on the inverter's AC output. Measures the panels' production. Always positive.
The critical point is the same as in any EM install: power the device from the same phase the clamps are measuring. If the reference voltage comes from another phase, the power maths comes out wrong. We cover it in detail in the guide on why the Shelly EM reads higher than expected.
The four figures you get
With just two physical measurements you have the whole energy picture of your home:
The two formulas you need are these:
- House consumption = Production + Imported − Exported
- Self-consumption = Production − Exported
The self-consumption percentage (self-consumption divided by production) is the metric that really tells you whether your array is well sized. 30% means you're giving away two thirds of what you generate.
How to read the sign of channel 1
This is the key to the whole system. The grid channel measures both ways, and the sign tells you instantly what's happening:
If the signs come out reversed at first, no need to dismantle anything: rotate the clamp or enable «Reverse CT measurement direction» in the device web UI for that channel.
Which clamp on each channel
This is the most common buying mistake. The two channels don't handle the same current, so they don't need the same clamp:
All the home's contracted power flows through the grid tail, so a 120 A clamp is the right call there. On the inverter output, a 3-5 kW home unit doesn't exceed about 25 A, so the 50 A clamp that comes as standard is plenty.
Remember to select the clamp type for each channel separately in the app. If the EM has a different clamp configured from the one fitted, every reading is scaled by the wrong factor.
What a full day looks like
Once installed, the daily pattern is very clear in the history:
Early morning and at night you import from the grid. In the middle hours, if production exceeds your base consumption, you start exporting. That midday band is your opportunity: every watt you use then is free, while exporting it is paid back at a lower price than you pay to import.
The Shelly EM keeps 365 days of history, so you can compare seasons and see how your self-consumption evolves over the year.
Use the surplus automatically
This is where the EM stops being a simple meter. It has an output to drive a contactor, so it can switch a load on when it detects surplus:
Typical uses are heating the electric water tank, running the pool pump, or charging the EV during sunny hours.
An important safety warning: the Shelly EM's output is only 2 A. It's for driving a contactor coil, not for powering the load directly. An electric tank or a pool pump needs an intermediate contactor sized for its power. Wiring them straight to the EM output would damage it.
FAQ
Is one clamp enough?
Yes, if you only want to know when there's surplus. With the clamp on the grid tail you already tell import from export, which is enough to automate loads. The second clamp on the inverter adds the production figure and lets you compute the self-consumption percentage.
Can I extend the clamp cable to the inverter?
It's possible, but the longer the cable the more electrical noise it picks up and the less stable the reading. If the inverter is far from the panel, a second meter next to the inverter usually works out better.
Does it work in three-phase installations?
The Shelly EM is single-phase: its two channels must be on the same phase. For three-phase homes or businesses you need a three-phase model like the Shelly Pro 3EM.
Is the data accurate enough to do the maths?
Shelly's original clamps declare 1% accuracy. That's more than enough to size and automate, though it doesn't replace your utility's official meter for billing.
Do I need internet for it to work?
No. The Shelly EM works locally and stores the history internally. You can integrate it with Home Assistant via MQTT without depending on the cloud or subscriptions.
What you need to set it up
- Shelly EM with 50 A clamp: the base unit, with one clamp included.
- 120 A clamp: the second clamp for the grid tail.
- DIN-rail bracket: for a clean mount in the panel.
Not sure about your particular install? Send us your panel and inverter details and we'll help you pick the right setup. Technical support and shipping available for your market.
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