One of our most popular posts is How Much Electricity Does My AC Consume? In that post I tried to explain how to calculate AC power consumption by multiplying the max current by the voltage of the AC unit. While I think this is a good introduction, I don't think it's the best method to calculate AC power consumption. The AC unit doesn't always use the max current when operating. Luckily there is a better way; ACServiceNow has one of the best HVAC energy calculators I've found.
The only downside of this method is you may have to do a little digging on the internet to find the correct information, but once you find it your results should be more accurate. You only need to know three things:
- Equipment size (tons)
- Electricity rate ($/kWh)
You can easily find your electrical rate from your power bill (it may not be listed on there, but just divide the total amount of the bill by your total kWh consumed for the month). You may have to find out the other two pieces of information by digging up your AC unit's manual.
I had to do this for my Lennox AC units because the tonnage and SEER rating weren't listed on the label. I had to google the model number. I have a split system (multiple units for my home to allow for greater zone control) and here is what my two unit's labels look like:
I found the SEER (13) from a product brochure for my Lennox 13ACD units. Wikipedia defines the seasonal energy efficiency ration (SEER) as:
The SEER rating of a unit is the cooling output in Btu (British thermal unit) during a typical cooling-season divided by the total electric energy input in watt-hours during the same period. The higher the unit's SEER rating the more energy efficient it is.
Here is another great explanation of what SEER is.
I found the tonnage from the Lennox 13ACD product manual. The unit label shown on the left is a 2.5 ton unit, while the label on the right is for the 2 ton unit.
After entering in this information in the calculator, and clicking on my climate zone to estimate the AC run time, I calculated that my AC units cost me $446 per year to run! How accurate is that number? Well, you can get a good idea by looking at your power bill history. Any amount that occurs in the months that you use your air conditioner over your baseline energy consumption is probably due to your air conditioner. I did this type of analysis in our post Window Film Best Energy Efficiency Investment. In that post I wrote:
"If you look at the figure below from my electricity consumption from 2011, you can see that my average electric consumption before it starts getting warm, which is between April and September, is around 400 kWh or $55 per month.
If we assume that the amount over $55 between the months of April through October is from the hot air outside which forces our electric AC unit to run more we get a total of....
- April bill = $71, minus $55 => $16
- May bill = $81, minus $55 => $26
- June bill = $135, minus $55 => $80
- Jully bill = $159, minus $55 => $104
- August bill = $177, minus $55 => $122
- September bill = $120, minus $55 => $65
- October bill = $74, minus $55 => $19
.....an extra $432 dollars that I spend cooling my house in the warmer months."
So the calculator predicted my ac power consumption costs me $446 and I estimated $432 by looking at my power bills....that's pretty darn close!!!
Along with helping to motive you to turn down your thermostat (reducing the run time of your ac units), knowing how to calculate much money your air conditioner costs you will also help you in determining if it makes sense to upgrade to a newer, more efficient unit (higher SEER).