Wrong.
It's higher.
In Dec. '07's electric bill, I used 2,630 kWH of electric power, 82.2 kWH per day. In Dec. '08, fully 2,757 kWH, 86.2 kWH per day!
True, Dec. '07 had an average temperature 3° higher than the 37° F. of Dec. '08, here in Baltimore. But I would have thought the storm windows, plus not wasting heat in closets and bathrooms on the cold side of my town home, would have offset the difference.
Wrong.
After the shock of receiving the new bill wore off, I determined to figure out why keeping the house at a comfortable 72° is so costly.
Doing that is a tall order, because it depends in part on how much time my HVAC system is on during the course of a month. I have no way to measure that. But I can measure how much power it draws when it is on.
To do that, I look at the large, horizontally mounted spinning wheel in the meter. I time how many seconds it takes for the wheel to make one revolution. (If the wheel is spinning fast, I time ten revolutions and divide by ten.)
The I plug S, the number of seconds per revolution, into the formula:
Watts = ( ( 3.6 x 7.2 ) / S ) * 1,000
3.6 is a constant which makes the formula work. 7.2 is the kH figure from the face of the meter; your meter might have a different kH figure.
First, I turned off my HVAC system and took an S reading: 53 sec. Using the formula, that gave me a baseline wattage of 489 watts. All the things in my house which were on and drawing electricity were using 489 watts of power.
Next, I turned on the HVAC system and set the thermostat so that just my outside heat pump and the indoor air blower were on. The indoor backup resistance-heating coil was off. This was "stage 1" of my two-stage heating system. It gave me an S reading of 8 sec. The formula said that 3,240 watts (3.240 kW) were being used.
Subtracting the baseline 489 W, I determined my heat pump and blower were using 2,751 W (2.751 kW) of electric power. That's more than 27 100-W incandescent bulbs.
Next, I kicked up the thermostat a few degrees until the indoor heating coil came on to supplement the heat pump — "stage 2" of my two-stage system. Now, S was 2.8 sec., and my power usage was 9,257 W! In addition to the 2,751 W the heat pump/blower were using, the resistance heating added fully 6,506 W to my overall power consumption.
For my final experiment, I switched the thermostat over to "emergency heat," a setting which cuts out the heat pump entirely. If the heat pump is broken, you supposedly can use this setting to bypass the crippled "stage 1/stage 2" kick-in sequence.
When I did that, my S was just 1.5 seconds! The formula spit out a whopping 17,280 watts, or 16,791 W more than my baseline. 17,280 - 9,257 = 8,023, the number of watts the emergency heat setting used above and beyond stage 2's usage of the heating coil. Apparently, the heating system doesn't drive the resistance coil as hard for stage 2 use as for emergency heating.
And that came as a surprise to me. I have been told by service techs that I should switch over to emergency heat when the outside temperature is well below freezing. Rationale: running the heat pump is a waste of power when it's bitter cold out. It can't squeeze enough heat out of the air to justify spinning its mechanism.
Yet I now find that spinning the heat pump plus using stage-2 coil heating uses just 9,257 - 489 = 8,768 watts, while running in emergency mode uses 16,791 watts. That's nearly double!
True, the latter mode gives me more heat quicker than the former, since the doubled power usage presumably makes the coil twice as hot. So the system doesn't have to run as long each time it is activated. If it runs half as long each time, I'm using the same number of kilowatt-hours per day or month.
But if emergency-heat mode is less efficient overall than stage-2 mode when the mercury is subfreezing, I could be wasting power and money by switching to it (something I did more than once during a mid-December 2008 cold snap).
I don't really have enough information to judge the relative efficiencies of stage-1, stage-2, and emergency-heat modes, for different outside temperatures and wind factors, given the propensity of my home to leak heat through its antiquated windows.
I do know that new, energy-efficient windows are on their way in January, as is a programmable thermostat that will let me keep the house cooler at night than during the day.
I also know that turning on any of the three heat modes makes my meter spin like crazy, and the stage-2 mode and especially the emergency-heat mode are true electron guzzlers. When my pocketbook can afford it, I'll have to look into installing a really efficient HVAC system. How else can I continue to call myself green?
Anyway, for quick reference, those numbers again are:
baseline: 489 W
stage 1: 3,240 W (incl. baseline); 2,751 W (excl. baseline)
stage 2: 9,257 W (incl. baseline); 8,768 W (excl. baseline)
emergency heat: 17,280 W (incl. baseline); 16,791 (excl. baseline)
In round numbers, the baseline is 1/2 kW, stage 1-plus-baseline is 3 1/4 kW, stage 2-plus-baseline is 9 1/4 kW, and emergency heat-plus-baseline is 17 1/4 kW!
My baseline of 1/2 kW of power usage generates 1/2 kW x 24 hrs./day x 31 days/month = 372 kWH/month of energy usage. As a rough guess, my non-baseline, non-HVAC power usage (TVs, computers, electric range, washer/dryer, water heater, interior lighting, etc.) probably brings that up to 1,000 kWH per month. Now, if I am in stage-1 heat pump usage half the time, but the inside resistance heating coils never come on, that comes to about 1,200 kWH a month. The total of the two comes to 2,200 kWH. When my Dec. '08 bill shows nearly 2,800 kWH used, there must have been roughly 600 kWH of "excess" HVAC use going on.
There are three possibilities:
- If it came from using emergency-heat mode, which happened on two or three occasions, 36 hours (600 kWH ÷ 16.791 kW) of heating in that mode would have accounted for it. A day and a half, or roughly 1/20 of the month.
- If it came from going into stage-2 mode, which may have happened without my becoming aware of it, 68 hours (600 kWH ÷ 8.768 kW) of heating in that mode would have done it. Approximately 3 days, or roughly 1/10 of the month.
- Of course, some or most of it may have come from being in stage-1 heating mode more than half the time, as opposed to the HVAC system being quiescent.
It is impossible to be sure which possibility predominated. Also, these calculations are rough estimates. Still, they make it clear that heating my house electrically is currently far too costly, both environmentally and financially. I desperately need to get the new windows, the programmable thermostat, and perhaps a more efficient heat pump going for me.
I also would like to find a way to record how much time I'm spending in the various heating modes. I know of no way to do that, at present.