Smart vs manual thermostat: what the difference actually costs

People compare heating mats. They are nearly identical. The component that decides what underfloor heating costs to run is the thermostat, and it is usually the part nobody asks about.

What a thermostat is actually doing

Electric underfloor heating does not run continuously. It heats the floor to a set temperature, then stops, then tops it up as the floor cools. How much of the time it draws power — the duty cycle — is what appears on your bill.

A programmable thermostat manages that cycle against a target temperature and a schedule. In a typical Tenerife apartment it means the system draws power roughly 30–40% of the hours it is on.

A manual switch does none of this. It is a light switch attached to a heating mat. It runs at full power until a person walks over and turns it off. If that person forgets, or is on a plane, it runs all night.

The difference over one winter

Take a 12 m² living room in an older Canarian building — around 9.6 m² of heated floor at 150 W/m², so roughly 1.4 kW installed. Five hours of use a day, five months, at €0.20/kWh:

This is why underfloor heating has a reputation for being expensive to run. It generally isn't. What is expensive is heating a floor with no way to stop it.

You can run these numbers for your own rooms in the running cost calculator, which shows the arithmetic rather than asking you to take a figure on trust.

What app control adds on top

A programmable thermostat solves the bill. Connecting it to your phone solves a different problem: being somewhere else.

• Heating a property you are not in. Holiday lets and second homes — switch on before arrival, off after departure, from anywhere. A manual switch cannot serve this at all.
• Changing the schedule without being in the room. Weather here moves; a fixed dial set in November is wrong by February.
• Seeing what it is doing. Knowing the heating is off is worth something when you are three thousand kilometres away.
• Home automation. A modern thermostat works with Alexa, Google Home and Home Assistant, so it joins whatever you already run rather than sitting outside it in an app of its own.