What is the difference between the Shelly 1 and Shelly 1PM

4 Jul 2021

Measuring just 41mm x 36mm x 17mm, these tiny Smart Relay’s are designed to sit in line within an electrical circuit and provide a smart switch capability incorporating any existing traditional switches you may have in place already. They are ideal to retrofit into existing lighting or power circuits and can also be a good fit for rewiring or new build applications.

Both the Shelly 1 and Shelly 1PM can handle resistive loads up to 16amps and connect to your existing WiFi network using the 2.4 Ghz signal and are managed via the Shelly Cloud App or directly linked up to services such as Home Assistant.

The Shelly 1 and Shelly 1PM both come with a programming/debug header which can be used to flash alternative firmware on the devices such as the Mongoose OS firmware for HomeKit or Tasmota. It has an ESP8266 inside, with a 2MB flash chip. A USB-to-UART adapter is needed as well as a reliable 3.3V with at least 350 mA drive capability. With the addition of the Shelly 1/1PM Temperature Add On, you are able to connect low voltage sensors such as the DS18B20 but note, you should never connect anything directly to any of the GPIOs of these devices when they are connected to AC Power without the addition of a Digital Voltage Isolator or Shelly 1/1PM Temperature Add On.

Whilst both of these devices appear very similar, there are a few important differences between the Shelly 1 and Shelly 1PM. 

Power Monitoring

The most obvious difference between the Shelly 1 and Shelly 1PM is in the name. The Shelly 1PM (Power Measurement) allows you to monitor the amount of power being consumed by the devices connected to it. Via the Shelly Cloud App you can view the status and history, as well as the current and past electrical power consumption for the day, week, month or year.

DC Power Supply

Both the Shelly 1 and Shelly 1PM work with a 24-60v DC power supply, whilst the Shelly 1 also supports a 12v power supply meaning it can be used in more low voltage applications.

Dry Contacts

The Shelly 1 has what is known as Dry Contacts, which can have a power source going through them that is independent of the control circuit. Also known as a volt free contact or potential free contact is where power/voltage is not directly provided from the switch but is instead always being supplied by another source. Dry contacts are known as passive contacts, as no energy is applied to the contacts.

Device Temperature Protection

The Shelly 1PM has overheating protection from an internal temperature sensor.

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