Replacing a Household Device: The Run-Cost Math That Decides It

A 1,500-watt space heater on a 12-hour timer costs about $5.50 a day at the U.S. average of $0.17/kWh (EIA, 2026), while a 150-watt ceiling fan on the same timer costs roughly 55 cents. That tenfold spread is the whole decision, and it is visible before you measure anything.

Once you have the free online calculator open, the sizing arithmetic is the easy part. This guide covers the numbers that actually drive the outcome: wattage, run hours, duty cycle, thermostatic cycling, and how much installed cost you should accept before electricity savings stop making sense.

Start With Watts, Not With Brand Names

Every domestic heating, cooling and flow device has a nameplate wattage, and that single number explains most of the cost gap. In this guide “device replacement” means exactly that: you are swapping a fan, an LED fixture, a pool pump, a HEPA filter unit, a baseboard heater or a hair dryer, and you want to know what it costs to run and whether the new one is worth the purchase price.

Typical nameplate ranges for common household devices:

Device	Typical wattage	Hours/day	Cost/month at $0.17/kWh
Ceiling fan (medium)	60–90 W	8	$2.50–$3.70
LED bulb equivalent to 60 W incandescent	9 W	5	$0.23
Pool pump (1 HP, variable speed, low setting)	250–450 W	6	$8.70–$15.70
HEPA air purifier (large room)	60–120 W	24	$7.50–$14.90
Baseboard heater (6 ft)	1,000 W	8	$41
Space heater	1,500 W	8	$62
Electric clothes dryer	5,000 W	1	$26

Two devices can draw the same wattage and still cost wildly different amounts because of how long they run. A 1,500 W hair dryer used for 10 minutes costs less than 5 cents; the same 1,500 W space heater on for 8 hours costs 20 times more in a single evening.

The Formula, and Where People Get It Wrong

The monthly cost of any device is wattage × hours per day × 30 ÷ 1,000 × your $/kWh rate. For a 900 W pump running 6 hours at $0.17/kWh: 900 × 6 × 30 ÷ 1,000 = 162 kWh, and 162 × 0.17 = $27.54 per month.

The mistake that shows up in almost every DIY bill estimate is using 24 hours when the device cycles on a thermostat. A space heater rated 1,500 W does not pull 1,500 W continuously: in a room held at 68°F (20°C) with an outdoor temperature of 35°F (2°C), a typical unit reaches a duty cycle of 40–60%. Use 0.5 in place of 1 as your hours multiplier and your estimate lands within about 10% of the meter.

If the device has multiple settings, run the formula once per setting. A pool pump that runs 8 hours at high speed and 4 at low speed is not one calculation — it is two, summed. A hair dryer on medium heat is 900 W, not 1,875 W.

Duty Cycle: The Number Nobody Measures

Duty cycle is the fraction of wall-clock time a device is actually drawing power. It matters most for thermostatically controlled equipment — heaters, air conditioners, dehumidifiers, refrigerators, water heaters — and it is the reason two identical nameplates can produce bills 50% apart.

You can estimate duty cycle without instrumentation. Note the start time and end time of a full on-off-on cycle with a stopwatch. If the compressor runs 12 minutes out of every 30, the duty cycle is 0.40. Take three readings across a day and average them. For new equipment, a $25 plug-in energy monitor gives you a 24-hour kWh total directly, removing the guesswork entirely.

Working an Example: Replacing a 1 HP Pool Pump

A homeowner has a 1 HP single-speed pump drawing 1,500 W, running 8 hours a day, May through September. At $0.17/kWh that is 1,500 × 8 × 30 ÷ 1,000 = 360 kWh, or $61/month, roughly $305 across the season.

A variable-speed replacement runs 12 hours, split 4 hours at 1,200 W and 8 hours at 300 W. Monthly: (1,200 × 4 + 300 × 8) × 30 ÷ 1,000 = 216 kWh, or $36.70 — a saving of about $24/month, or $120 across the season. If the pump costs $900 installed, the payback is about seven and a half pool seasons. That is a reasonable but not compelling number, and it is exactly the kind of result that a bill-only calculation would have missed.

Option	Installed cost	Season electricity	Payback
Keep single-speed pump	$0	~$305	—
Variable-speed replacement	$900	~$183	~7.5 seasons
Same, plus 6 hr/day schedule cut on old pump	$0	~$229	immediate

When to Replace and When to Repair

The rule of thumb that holds up under a calculator is simple: if the annual electricity saving multiplied by the remaining expected life of the new device exceeds the installed cost by a factor of 1.5 or better, replace. That 1.5 margin covers the risk of an imperfect estimate and the fact that new equipment sometimes performs below its label.

Repair wins when the fault is one component and the device’s efficiency is already near the best available. A failing capacitor in a fan is $12; the energy gap between a five-year-old fan and a new one is a dollar or two a month, so the fan stays. A failing compressor in a 15-year-old dehumidifier is a different calculation — that repair typically costs 60–70% of a new unit while the replacement may draw 30% less.

Frequently Asked Questions

Does a higher-wattage device always cost more to run? Only if the run hours are the same. A 1,500 W heater on a 20% duty cycle uses 300 W averaged over time, which is less than an 800 W device running continuously.

Is my electricity rate the average or the marginal rate? Use the marginal rate — the per-kWh price you pay for the next block of usage. Tiered tariffs can push the last 200 kWh of a month to two or three times the baseline rate, which is why heavy devices often look more expensive on a real bill than a flat-rate calculation predicts.

Can I trust the nameplate? It is an upper bound, not a measurement. Manufacturer testing tolerates a small overage, and motors draw more at start-up. For anything above 1,000 W, a plug-in meter is worth the $25.

Build the Comparison Before You Shop

Write down four numbers for your current device — wattage, observed run hours, duty cycle, and your marginal rate — then do the same for each candidate replacement. The only figure that should drive the purchase decision is the annual difference, compared against the installed price and expected life.

Keep the calculator open while you read spec sheets: the free online calculator handles the multiplication and division instantly, and the payback arithmetic is simple division into the percentage saving you will actually notice on a bill. Neither number is complicated — the discipline is in measuring run hours rather than assuming them.