Electricity and Fuel: Where Your Running Costs Come From

A one-time purchase has a price tag; a habit has a drip. The space heater you forget to turn off and the commute you drive every week are "running costs" — and they obey one boring, beautiful formula that makes every one of them comparable.

The one formula behind every running cost

Whether the utility is electricity or gasoline, the math is the same three moves:

usage = rate_of_use × time
cost  = usage × price_per_unit

For electricity, the "rate of use" is power in kilowatts and the "time" is hours, giving kilowatt-hours. For a car, the rate of use is fuel per mile and the time is miles driven, giving gallons. Multiply usage by the price of a unit and you have dollars. Once you see costs this way, every appliance and every trip becomes the same kind of arithmetic — which is why a single month of tracking usually reveals more savings than any gadget swap.

Electricity: watts to kilowatt-hours to dollars

Power labels are in watts, but you're billed in kilowatt-hours, so the first step is always dividing by 1,000. A 1,500-watt space heater is 1.5 kW. Run it 5 hours a day and it burns 1.5 × 5 = 7.5 kWh per day. Over a 30-day month that's 225 kWh, and at a typical $0.15 per kWh the bill line looks like:

7.5 kWh/day × 30 days × $0.15/kWh = $33.75/month

Keep the heater going all winter and the same habit costs about $410 a year — which reframes whether that spare room really needs to be toasty. Two details people miss: first, anything that makes heat (heaters, dryers, ovens, old bulbs) dominates a bill, because it takes a lot of power for a lot of hours. Second, small-but-always-on loads add up quietly: a single watt left on around the clock is about 8.8 kWh a year, roughly $1.30 at $0.15/kWh — trivial alone, meaningful when a dozen chargers and standby lights do it together.

Fuel: efficiency lives in the denominator

Gasoline has the same shape, with one twist: "miles per gallon" is a rate of distance per fuel, so to get fuel you flip it. Drive 400 miles and burn 14.3 gallons between fill-ups and your true economy is 400 / 14.3 ≈ 28 mpg. At $3.40 per gallon the trip cost 14.3 × $3.40 ≈ $48.60, which is about 12 cents per mile — the single most useful number for comparing cars, commutes, and "should I drive or fly" decisions.

The denominator effect is why mpg improvements feel nonlinear. Going from 15 to 20 mpg saves more fuel per mile than going from 30 to 35 mpg, even though both are "5 mpg better." That's also why the trip-cost side of the story — legs, detours, price shopping between stations — gets its own treatment; the fuel cost calculation guide covers the trip-planning math, so here we'll stay on the ownership side.

Measuring your car's real mpg

Dashboard trip computers tend to be optimists, often reading 5–10 percent high. The honest method is the fill-to-fill log: fill the tank, reset the trip odometer, drive normally, and at the next fill divide miles driven by gallons pumped. Two or three cycles smooth out the noise from idling, weather, and short trips. Converting is easy too — a car at 28 mpg is at 8.4 L/100km, since the conversion is 235.2 ÷ mpg (the constant is 235.2 in metric-friendly form: 100 km is 62.14 miles and a gallon is 3.785 litres).

Cost per hour, cost per mile

Normalize before you compare. Electricity cost per hour is just kilowatts times the rate: that heater costs 1.5 × $0.15 = $0.225 per hour — about the same as driving a 28 mpg car at $3.40 gas for two miles. Fuel cost per mile is the price divided by mpg: $3.40 / 28 ≈ $0.12. These per-unit numbers are what let you answer real questions: is it cheaper to run the dryer at night on an off-peak rate? Does the fuel-efficient car justify its price premium given your 12,000 miles a year? (At 12,000 miles, the gap between 22 and 32 mpg is about 170 gallons — over $570 a year at $3.40.)

Where the savings actually hide

The formula tells you the levers: usage, time, and unit price. Behavior changes usually beat hardware swaps — turning off the 1,500 W heater saves more than upgrading a 10 W bulb to 8 W. Timers and thermostats shrink the time factor automatically. And for fuel, the biggest lever isn't the brand of gas, it's the miles: combining trips and keeping tires inflated changes the denominator more than any station choice.

Try it yourself

The Electricity Cost Calculator turns watts, hours per day, and your rate into daily, monthly, and annual cost — ideal for auditing the heater-in-the-basement question. The Fuel Economy Calculator converts a fill-to-fill log into mpg, L/100km, and cost per mile or kilometre. And the Fuel Cost Calculator prices a single trip from distance, efficiency, and gas price.