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What tracking electricity can illuminate about energy usage

Photo credit to Rodolfo Clix


These figures come from my complete accounting records.

Leaving the lights on

It is important to have a clear estimate of the drivers of electricity usage in the home. Without accurate estimates, discussions veer off into noise level discussions about who left which lights on. If we left every light in the house on 24/7 it would cost less than $15 per month (using LED bulbs). The drivers of electricity usage are the appliances which change the temperature of something: the refrigerator, washer/dryer, and heater/air conditioner. Everything else is noise level. People who are serious about saving their money or the earth focus on those three things exclusively. People who are serious about letting their emotional immaturity ruin their interpersonal relationships focus on who left the lights on.

It is important to acknowledge this was not always the case. In the era of incandescent light bulbs, leaving every light in my house on would have cost $120-$160 instead of $15 (at today's electricity prices). A collection of incandescent bulbs left on would be a significant driver of a home's electricity use.

To help build intuition about what drives the electric bill, consider the limiting case for a single appliance. The circuit breakers in an American home should trip at 15 A. Recalling the power equation \(P\, \text{[W]} = \left(I\, \text{[A]}\right) \left( V \, \left[\text{V}\right] \right) \), the upper bound on the power draw for one circuit is 1,650 W (or it will trip the breaker). There are exceptions - refrigerators and garages are typically on 20 A circuit breakers which will only flip above 2,200 W. Houses without gas appliances will have a few other major draws from the 220 V circuits which are typically 20 or 30 A and deliver up to 4400-6600 W. The highest load items are those which often trip circuit breakers in old houses or which run on 220V circuits: dryer, air conditioner, stove, oven, vacuums, refrigerators, power tools, etc. The drivers of the electricity bill are the subset of commonly used items from that list: kitchen appliances (refrigerator, stove, oven), dryers (if electric), heating/furnace (if electric) and air conditioning.

Device Power draw when on Approximate daily use (when used)
Central heating 5,000-15,000 W Winter 30-100 kWh
Welder (50A @ 220V) 10,000 W ----
AC 3,000-10,000 W Summer 30-40 kWh/day
Dryer 2,000-5,000 W 2-5 kWh/day
Refrigerator 1,500-2,500 W 1-2 kWh/day
Dishwasher 1,200-2,400 W 1-2 kWh/day
Air compressor 1,500 W ----
Electric oven 1,000-5,000 W 1-5 kWh/day (when used)
Electric stove burner 1,000-3,000 W 1 kWh/day
Power hand tools (circular saw, etc) 500-1,500 W ----
Vacuum 500-1,500 W ----
All the light bulbs in the house 200 W <3 kWh/day
Desktop computer 100 W 2 kWh/day
Light bulb (LEDs use less than 15% of the power of conventional lights) 10 W 0.2 kWh/day
Electricity bill vs time plot

[Caption] Price per kilowatt-hour computed by dividing the total bill by the total usage. The bill is composed of a service charge which is not a function of usage, and a per kilowatt-hour charge. This causes the per kilowatt-hour bill to be lower during higher use months. I live near the south coast of the United States where the summer air conditioning increases the home power consumption by a factor of 3x to 4x. Apartment and house, same city 3.5 miles apart, same electric company. Both places had electric ovens/stoves and AC units but gas water heaters and furnaces so the heating costs do not appear on the electricity bill.

The only serious variation in the electricity bill is the seasonal oscillation when the AC unit drives power use. The outliers, both in the apartment and in the house, occurred when the AC unit was malfunctioning. In both cases, the AC unit would run 24/7 and never lower the temperature below 78-80°F at night. My old apartment was a slum and the AC never was fixed so I eventually bought a wall unit and settled for cooling my bedroom.

Lessons in home insulation

The house AC unit had a malfunctioning TXV valve in 2016 which was a $1,000 repair. After fixing the TXV valve, we were able to lower the house temperature to 80°F during the day and 76°F at night. Two years later we did a major remodel project: removed all the siding down to the studs, replaced all the windows with double-pane windows (and eliminated one entirely), installed insulation, and installed new siding. Prior to summer 2018, there was no insulation of any kind in any exterior wall on the house, just an empty air gap between the drywall and exterior siding. The former windows were original 1950s single-pane windows. Given all we did, $8,000 in Lowes receipts, we expected a significant shift in power use but this has been less dramatic than we hoped. We are, however, more comfortable at 75°F during the day and 72°F at night. The energy savings are compensated for by our cooler home temperature.

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© MC Byington