[sylvesterjay]

Can someone with a degree in electrical engineering please explain to me:

When I run my air con 24/7 in the summer, with the big ol' compressor chugging away, why then is my electrical bill for the last month 50% higher for running my tiny little floor heater for half a day with just a fan and some heating coils?

[PerryCo]

Power = Current (squared) x Resistance

Heaters are current hungry. Compressors are less current hungry.

Welcome to Hong Kong where power meters are estimated, and not read by humans.

[sylvesterjay]

one would think a big compressor would use more power though, errrr current.

[jgl]

Well, heaters typically use something in the vicinity of 1500-2500 watts, and a wall mounted AC unit uses 400-1500. So just looking at the specs would explain why there is a difference in your electricity bill.

But here's another way to look at it that just occurred to me. It's all around how much heat each device generates.

Your heater converts electrical energy into heat (duh). There is a bit of waste in that some of the electicity is converted into kinetic (air blowing around and soundwaves), but most of it goes into heat.

An AC also mainly converts electrical energy into heat. This sounds counter-intuitive, but all the cold air being pumped into your room is offset by a load of hot air being radiated out the other side of your AC unit, hence all the radiator fins set in the back. Again, some energy gets turned into moving air and noise, but most of it is converted to heat waste.

If you've ever stood on the outside of a running AC, you can feel that it pumps out hot air. But if you compare it to a small fan-forced or oil heater, the amounts of heat generated is relatively small. The heater generates much more heat, and in the process uses a lot more electricity.

[sylvesterjay]

mmmkay thanks. I feel a wee bit better now about paying my electrical bill, still annoyed tho.

[Skyhook]

Qouted from JGL's post

"Well, heaters typically use something in the vicinity of 1500-2500 watts, and a wall mounted AC unit uses 400-1500. So just looking at the specs would explain why there is a difference in your electricity bill."

JGL is part of the way right.
Also, factor in that comparing a dry bulb thermostat equipped split/box type air conditioner, you need to understand co-efficiencies of performance or whats known as C.O.P. The least efficient of which is the electric radiant heater, fan aided heaters aren't much better.

A heat pump reverse cycle air-conditioner will work on 6 - 8 air ( room volume ) changes per hour, so, providing the air conditioner is mathematically the right capacity output for said room ( meeting design condition ) the compressor will engage/disengage approx 4 times over an hour as it maintains the temp of the room set on the control at a plus or minus 2 degree variation.

Example:
Temp is set for 21 degree's
Ambient Room temp = 18 degree's
Air-conditioner will run the compressor until the thermostat realizes 23 degree's at return air position. Fan ( blower ) will continually run ( low current device ), once the temp drops to 19 degree's, the compressor will engage again, so the average temp will be a maintained 21 degree's ( set point ).

So, if a 6kw output 2.5hp reverse cycle air-conditioner has a COP of 4, it would have a maximum current draw of 1.5kw per hour of operation, if no thermostat was installed inline.

Because there is a thermostat that will cycle the compressor to meet demand ( design condition ), you will use more likely 50% less than the rated current spec on a kw/hour basis, we know the max hour draw of the appliance = 1500watts, so we can achieve a consumption of 750watts used per hour, not to mention being able to heat a room area 2.5 times larger than the electric element heater based on COP alone.

Electric element heaters have a COP figure of 1 to 1 lol, very inefficient, hence why most people get bill shock, who actually use heaters during a Hong Kong winter.

Our last electricity bill was $500 for 2 people.

Heat pump A/C costs @ $5* per kw hour = $2.50 per hour of operation.

An eqv 6kw electric heater to be able to heat the same given area = $30 per hour of operation.

As most people under size their heating and use a tiny fan heater of say 2KW it would = $10 per hour of operation. or 3 to 4 times more than their air conditioner.

*= figure just used for this case .