Shelly EM reads more than expected: causes and solution

Is your Shelly EM showing much higher consumption than you expected, or not matching your electricity company's meter? It's one of the most common problems when installing this energy monitor, and in the vast majority of cases it's not a device fault, but a setup or installation detail that's fixed in five minutes.

In this guide we go through, from most to least frequent, why the Shelly EM can give higher-than-normal readings and how to fix it step by step.

How the Shelly EM measures (and why it matters)

Before diagnosing, it helps to understand how it works. The Shelly EM is a single-phase 2-channel meter that calculates active power by combining two measurements:

• The voltage (V), taken directly from its own power terminals (L/N).
• The current (A), measured with a current clamp (current transformer or "CT") that goes around the cable.

Power is calculated by relating both measurements, including the phase shift between voltage and current. This has a key consequence: if the clamp measures a current and the reference voltage doesn't correspond to that same phase, the calculation comes out wrong. That's the origin of many anomalous readings.

Cause 1: the clamp set in the app doesn't match the real one

It's by far the number-one cause of crazy readings.

The Shelly EM ships as standard with a 50 A clamp, but there's also a 120 A version. In the Shelly app you can select which current transformer you're using, and that value must match the physical clamp installed exactly.

If in the app you have a different clamp type selected from the one you've fitted, all readings are scaled by the wrong factor and consumption appears inflated (or reduced).

Solution:

1. Physically check which clamp you have installed (it's marked on the clamp itself: 50 A or 120 A).
2. In the Shelly app, go into the settings for the corresponding channel and verify that the selected current transformer matches.
3. Save, restart the device and see whether the reading corrects itself.

Cause 2: the clamp isn't properly closed or goes around the wrong cable

The current transformer needs to close completely around a single phase conductor. There are three typical scenarios:

• The clamp doesn't close fully (dirt, an incomplete click or an object in the core). The small air gap distorts the measurement and usually causes erratic readings.
• The clamp goes around phase and neutral at once: the magnetic fields cancel out and the reading drops to nearly zero.
• The clamp goes around the wrong cable (for example the main one instead of the circuit you wanted to measure): you'll measure far more than expected.

Solution: open the clamp, clean it, make sure it only surrounds the correct phase conductor and close it until you hear the click. Also check that the arrow printed on the clamp points towards the load: this affects the sign of the consumption. If it appears negative, you can turn the clamp around or enable the "Reverse CT measurement direction" option in the device's web interface, without touching the installation.

If, on opening it, you find the core is damaged or the clamp no longer closes properly, we have original 50 A replacement clamps and 120 A ones with shipping from Spain.

Cause 3: voltage reference from a different phase

This is the "invisible" fault that causes the most headaches in installations with more than one phase.

If you power the Shelly EM from one phase (for example L1) but the clamp measures the current of a different phase (L2 or L3), the phase shift between the reference voltage and the measured current is incorrect. The result is a completely wrong active power: it may appear too high, too low or even negative.

Solution: make sure the Shelly EM's supply voltage comes from the same phase whose current you're measuring with the clamp. In a normal single-phase home this is no problem; the fault almost always appears in three-phase panels.

Cause 4: electrical noise from extending the clamp cables

If you've extended the current clamp's cables to reach the panel, or they run near sources of interference, they can pick up electrical noise that translates into phantom readings, especially noticeable at low consumption or with the circuit nearly off.

Solution: keep the clamp wiring as short as possible, away from other power cables, and avoid poor-quality splices. If, when you disconnect the load, the EM keeps reading watts "out of nowhere", noise is usually the cause.

Cause 5: you're comparing active power with another quantity

The Shelly EM records active energy (kWh), which is what your company bills. If you compare its reading with a device that shows apparent power (VA) or with very reactive loads (motors, pumps, certain appliances), you'll see differences that aren't an error, but different quantities.

The EM itself also shows you the power factor: if it's low, it indicates a significant reactive load.

How much can it differ from my company's meter?

A small difference from the official meter is normal and to be expected. Shelly's original current clamps (both the 50 A and the 120 A) declare a 1% accuracy; with low-quality generic clamps the error is considerably greater.

On top of that, the Shelly EM and the meter measure at different points of the circuit. That's why differences of a small percentage are perfectly acceptable. Differences of double, half or absurd values always point to one of the causes above (usually 1 or 3).

Quick diagnostic checklist

If your Shelly EM reads too high, check in this order:

• The clamp type (50 A / 120 A) in the app matches the physical clamp.
• The clamp closes completely and goes around only the correct phase conductor.
• The clamp's arrow points towards the load.
• The EM's supply voltage is from the same phase the clamp measures.
• The clamp cables are short and away from interference.
• The device firmware is updated to the latest version.
• You've restarted the device after any physical correction.
• You're not comparing kWh (active) with VA (apparent).

Frequently asked questions about the Shelly EM

Why does the Shelly EM read consumption with everything off?

It's usually due to electrical noise picked up by the clamp cables or a poorly closed clamp. Shorten the wiring and check the current transformer closes properly.

Can I use the 120 A clamp instead of the 50 A one?

Yes, but you must select the correct clamp in the app so the measurement scaling is right. The 120 A clamp lets you measure larger loads.

Consumption appears negative, is it a fault?

Not necessarily. It indicates the clamp is fitted the wrong way round (the arrow must point towards the load) or that there's energy being fed in, for example with solar panels. Turn the clamp around or reverse the measurement direction from the web interface.

Is it normal for it not to match the company's meter 100%?

Yes. The clamp declares a 1% accuracy, so a difference of a small percentage is normal. You should only worry about large differences.

Still getting incorrect readings?

If after checking all the points above the Shelly EM keeps giving anomalous values, it may be a faulty clamp or a unit with problems. At ShellyMadrid we help you with technical support in English and we have replacements and units with shipping from Spain and official warranty.

Related products

• Shelly EM — Wi-Fi Energy Meter with 50 A Clamp: the complete unit, ready to install.
• Shelly EM with 120 A Clamp: for more demanding services and loads.
• Shelly 50 A current clamp: original replacement with 1% accuracy.
• Shelly 120 A current clamp: to measure higher-consumption circuits.
• DIN-rail bracket for Shelly EM: clean mounting in the electrical panel.

You can also check the full technical specifications of the Shelly EM or write to us with your specific case and we'll help you diagnose it.