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  3. Energy Calculator

Energy Calculator

How long will it take to heat, and what heater do you need?

Enter your tank and temperatures. You'll see the heater size you need (or how long your heater will take), the cost of each heat-up, the electrical supply it needs, and heaters that fit.

What do you want to work out?
What are you heating?

The amount you heat, not the tank's full capacity.

Temperatures
°C
°C

Mains cold water in the UK is usually 10–15°C.

Some heat always escapes into the air and the tank walls. Choosing honestly here stops you buying a heater that's too small.

p per kWh

Check your latest bill. Used only to estimate the running cost.

Heaters for your setup

Heat-up times are worked out for your liquid, amount and temperatures.

    How the calculation works

    Every liquid takes a set amount of energy to warm up by one degree. Multiply that by how much liquid you have and how many degrees you're raising it, and you have the energy needed. Divide by the heater's power to get the time.

    Energy = amount × density × specific heat × temperature rise ÷ efficiency

    For example, 100 litres of water from 15°C to 60°C needs about 5.2 kWh of heat. In an uninsulated tank with a lid (80%), the heater has to supply about 6.5 kWh. A 3 kW heater does that in roughly 2 hours 10 minutes; a 6 kW heater in about 65 minutes.

    LiquidSpecific heat (kJ/kg°C)Density (kg/L)
    Water4.191.00
    Water–glycol (30%)3.751.04
    Cooking oil2.000.92
    Mineral or thermal oil2.100.87

    Common questions about heating air

    Should I choose a bigger heater than the calculator says?

    Usually by around 10–20%. Ducts lose heat through their walls, and incoming air is often colder than planned. A temperature controller stops a larger heater overshooting your set point.

    Can I use several finned strip heaters together?

    Yes. Add their power together and use "How warm the air will get" to check the result. Spread them evenly across the airflow so all the air is warmed, not just a strip of it.

    What happens if the fan stops?

    Finned heaters rely on moving air to carry their heat away, so without airflow they can overheat quickly. Wire the heater so it can only run while the fan is running, and fit an over-temperature cut-out.

    I don't know my airflow. How do I find it?

    Check the fan's label or data sheet first. Otherwise, measure the air speed in the duct with an anemometer and multiply by the duct's cross-section. A 0.2 m² duct with air moving at 3 m/s carries 0.6 m³ per second, or 2,160 m³ per hour.

    Does this cover drying or humid air?

    No. This works out the heat needed to warm the air. Drying a product or evaporating water takes extra energy on top. Call us and we'll help you size for that.