Motor energy cost
What a motor costs to run for a year — kilowatts, kilowatt-hours, and dollars from horsepower, load, hours, rate, and efficiency. Energy is the biggest cost in a motor's life, and three points of efficiency add up fast.
Motor energy cost · annual kWh & dollars
$/yr = HP · 0.746 · load ÷ eff × hours × rate
What this gives you
This is what an electric motor actually costs to run for a year — in kilowatts, kilowatt-hours, and dollars. Energy is by far the largest line item in a motor's life; over a few years the electricity bill dwarfs the purchase price, so this is the number that decides whether a rewind, a repair, or a premium-efficiency replacement pays for itself. Enter the horsepower, how hard the motor is loaded, how many hours a year it runs, your electricity rate, and the nameplate efficiency.
The formula
Start at the shaft and work back to the wall. One horsepower is 0.746 kW, so from a horsepower nameplate the electrical draw is kW = HP × 0.746 × (load ÷ 100) ÷ (efficiency ÷ 100). If you already know the motor's kW rating, switch the power input toggle to kW and the 0.746 drops out — the draw is simply kW = kW × (load ÷ 100) ÷ (efficiency ÷ 100). Multiply by the hours run per year for annual energy, then by the rate for annual cost: $/yr = kW × hours × $/kWh. The load factor scales the draw for a motor that isn't working at its full nameplate rating; efficiency sits in the denominator because the losses inside the motor are energy you pay for but never get to the shaft.
Premium-efficiency payback
Enter a higher premium efficiency and the motor's price premium and the tool works the replacement economics for you. It re-runs the same cost formula at the premium efficiency, reports the annual savings (base cost − premium cost), and divides the price premium by the monthly savings to give a simple payback in months: payback = price premium ÷ (annual savings ÷ 12). The premium case also draws as a second, shorter cost bar on the schematic so you can see the gap at a glance.
Field note — efficiency is small on paper and large on the bill
A jump from 90 to 94 percent efficiency looks like almost nothing, but it is a real cut in the energy the motor wastes, and on a machine that runs around the clock it can save hundreds of dollars a year. That is the whole economic case for premium-efficiency motors: the sticker costs more, but the running cost is lower for the entire service life, and the simple payback shows how fast that price premium comes back. Run this with the nameplate efficiency and the real load, not a round number, before you decide to rewind an old motor rather than replace it.
Worked example
A 10 HP motor at full load, running continuously (8,760 hours) at 90 percent efficiency on $0.12 /kWh power: the draw is 10 × 0.746 ÷ 0.90 = 8.29 kW, which over a year is 72,611 kWh and costs about $8,713. Specify a 94 percent premium motor for the same duty and the cost falls to about $8,342 — a saving of roughly $371 a year. If that premium motor costs $400 more to buy, the payback is 400 ÷ (371 ÷ 12) ≈ 12.9 months — and every year after that is money saved.