Even in temperate North Carolina we have to turn on the furnace when winter comes around. A lot of folks here have heat pumps since they both heat and cool – and we need both. Electricity powers them, made plentiful by the many coal fired plants in the area. Since I now have my nifty power monitor in place I was curious to see how it impacted the home’s consumption.
It only reached the high 50’s today, and I set the thermostat at 67’ just to keep the edge off. The atrium collects some heat when its sunny and it was a little today. But the heat was definitely kicking on occasionally. Around 4pm in the afternoon I was getting cold so I turned the thermostat up to 72’. This evening I took a look at the monitor to see what had been happening.
Wow! Absent heating/cooling the house bounces between .5 and 1.5kWh depending on what happens to be on at any given time. Knowing that nothing else big was running at the time it was easy to pick out the times the heater was drawing power. It’s sucking up about 4X what the entire rest of the house was using! From the graph is easy to see that the power to keep the house at 67’ during the day is considerably less than the higher temp (5′ more, and against cooler outside temperatures too). There are three distinct large bands of usage that looks to be the effort to get the house warmed up, which tapers off a little once it stabilizes and then continues to cycle in order to maintain 72′.
12 Hour graph of electricity usage
Obviously I am somewhat guessing that this is what is going on since my measuring/monitoring capabilities are so primitive, but given I was around the house all day and KNOW that other major appliances were not running at the time it seems pretty reasonable. It would be impossible to for me regularly track what the HVAC is doing on an ongoing basis unfortunately.
But what does this mean in terms of cost? I exported the data and looked at the 12 hour period from 9am to 9pm. By looking at 5-10 minute average before the spikes I estimated the incremental consumption of the heater, and added both up. Heating and cooling is generally thought to be about 30-40% for most people. The power to just run the house during this period was about 9.6kWh. The incremental heating was nearly twice as much at 18.7kWh, or two thirds!
Now we can’t extrapolate 12 hours into the entire season, but this is certainly interesting. My HVAC has been serviced several times in the past couple years, but is it running efficiently? Is the amount of cycling and the energy consumed reasonable given the weather outside? Have I done everything I can to make sure I am using the resulting heat efficiently? My electric bill was pretty cheap last month as I was paying a lot of attention to power usage and not running the HVAC. I’m not too optimistic for this month!
