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 only show you the production, and your electricity company sends you 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 figures in real time you can calculate your real self-consumption and, above all, automate loads so you don't give away a single free watt.
The setup: two clamps, two channels
Installation in a single-phase home with self-consumption is simple:
- Channel 1 — on the main service. It measures the exchange with the grid in both directions: positive when you import, negative when you export.
- Channel 2 — on the inverter's AC output. It measures the panels' production. Always positive.
The critical point is the same as in any EM installation: power the device from the same phase the clamps are measuring. If the reference voltage comes from another phase, the power calculations come out wrong. We explain it in detail in the guide on why the Shelly EM reads more 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:
- Home consumption = Production + Imported − Exported
- Self-consumption = Production − Exported
The self-consumption percentage (self-consumption divided by production) is the metric that really measures whether your installation is well sized. 30% means you're giving away two-thirds of what you generate.
How to read the sign of channel 1
Here's the key to the whole system. The main-service channel measures in both directions, and the sign tells you instantly what's happening:
If, on starting it up, the signs come out reversed, there's no need to dismantle anything: turn the clamp around or enable the "Reverse CT measurement direction" option in the device's web interface for that channel.
Which clamp to use 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 passes through the main service, so a 120 A clamp is best there. On the inverter output, a domestic 3 to 5 kW unit doesn't exceed about 25 A, so the 50 A clamp that comes as standard has plenty of headroom.
Remember to select the clamp type for each channel separately in the app. If the EM has a clamp configured that's different from the one installed, all readings will come out scaled by the wrong factor.
How it behaves over a full day
Once installed, the daily pattern is very clear in the history:
Early in the morning and at night you import from the grid. In the middle hours, if production exceeds your base consumption, you start exporting. That midday window is your opportunity: every watt you consume there is free, while exporting it is compensated at a price lower than what you pay to import.
The Shelly EM stores 365 days of history, so you can compare seasons and see how your self-consumption evolves over the year.
Using the surplus automatically
This is where the EM stops being a mere meter. It has an output to drive a contactor, so it can activate a load when it detects surplus:
Typical uses are heating the electric water heater, starting the pool pump or activating the electric car's charging during sunny hours.
An important safety warning: the Shelly EM's output is only 2 A. It's for driving the coil of a contactor, not for powering the load directly. An electric water heater or a pool pump needs an intermediate contactor rated for its power. Connecting them directly to the EM's output would damage it.
Frequently asked questions
Is one clamp enough for me?
Yes, if you only want to know when there's surplus. With the clamp on the main service you already distinguish import from export, which is enough to automate loads. The second clamp on the inverter adds the production figure and lets you calculate 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 more unstable the reading. If the inverter is far from the panel, it's usually better to install a second meter next to the inverter.
Does it work for three-phase installations?
The Shelly EM is single-phase: its two channels must be on the same phase. For homes or businesses with a three-phase supply you need a three-phase model such as the Shelly Pro 3EM.
Is the data accurate enough to do the math?
Shelly's original clamps declare a 1% accuracy. It's more than enough to size and automate, although it doesn't replace your company's official meter for billing purposes.
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 install it
- Shelly EM with 50 A clamp: the base unit, with one clamp included.
- Shelly EM with 120 A clamp: if you're going to measure the main service from day one.
- Shelly EM double-meter pack: designed for energy management and photovoltaics.
- 120 A current clamp: the second clamp for the main service.
- DIN-rail bracket: for clean mounting in the panel.
Questions about your specific installation? Write to us with your panel and inverter details and we'll help you choose the right setup. Technical support in English and shipping from Spain.
Leave a comment