Help with watts and lecky things

Noel · 10 Dec 2006

I recall reading sometime ago about how one can calculate the cost of running an electrical appliance. In this instance an oil filled rad. say a 750w or a 1000w or WHY. How could I work out how much it would cost per hour (assuming it was on the high setting)?

TIA

Noel

CHJ · 10 Dec 2006

Noel, electricity is priced by the unit.
The price per unit will be on your bill and varies on quantity used.

1 (one) Unit is 1000 watts for 1 (one) Hour (One Kilowatt hour)

Therefore your 1000 watt heater uses 1 unit in one hour. (something like 7 p ?)

The 750 watt unit uses 3/4 of a unit an hour.
A 1500 watt unit uses 1-1/2 units an hour.

The above assumes the heaters are constantly ON, if thermostatically controlled they will probably consume less.

davy_owen_88 · 10 Dec 2006

CHJ is correct, but as he mentions, thermostatically controlled heaters are tricky to measure. This is simply because they don't come on for a fixed amount of time and the only way to work out the power consumption is to sit there with a stop-watch timing it.

Thats where this little gizmo comes in handy: Plug-In Mains Power & Energy Monitor

One assumption made when working out the power consumption using the rated wattage of a device x time is that it has a purely resistive load - I.e it draws a constant current. This is the case with immersion heaters, showers and electric heaters but isn't the case with motors and certain lighting types. Motors draw current depending on loading, on startup they require more current to get moving then when not being used only draw enough to keep turning. When you start cutting/planing whatever the load increases and up goes the power consumption.

Davy

Anonymous · 10 Dec 2006

Agree with above

I tested such a heater a while back and the duty cycle (time on as a ratio to time off) was about 50% - so the thing was drawing power from the mains for half the time it was switched on.

measured this over a period of 24 hours with an automated data logger into a laptop computer that took readings every 30 seconds.

So, it will likely cost around 3.5p per hour that it is switched on (this will depend on the environment in which it is used)

I got rid of mine and then piped radiaters into the garage asap