A car power window motor is a small DC electric motor that draws roughly 50 to 100 watts (W) at 12 volts, which equals about 0.07 to 0.13 mechanical horsepower (hp) and around 170 to 340 BTU per hour (BTU/h). Converting those watts into horsepower and BTU/h matters because a manual window conversion removes the motor, regulator, and wiring from the door, and you need to know the electrical load you are deleting in the same units you would use to rate the manual crank you are installing. The Power Converter handles every step of that math: type the wattage, pick the source and target units, and read the result in watts, kilowatts (kW), megawatts (MW), mechanical hp, metric horsepower (PS), BTU/h, foot-pounds-force per second (ft·lbf/s), calories per second (cal/s), or kilocalories per hour (kcal/h). Open the Power Converter and the calculation is done in your browser with nothing uploaded. This guide walks through the conversion process for the actual door work and explains the unit math behind every step.
What Converting Power Windows to Manual Actually Involves
Swapping an electric window for a hand-cranked one is mostly a mechanical job. You pull the door panel, unbolt the regulator from the door shell, disconnect the motor harness, and lift the glass out. Then a manual regulator goes in, the glass re-seats in its run channels, and the crank handle is fitted to the trim. Junkyard regulators are common because the same body style usually accepts either setup, but every swap has one quiet step that often gets skipped: reading the motor's electrical rating before you cut the wires. That rating is printed on the motor's data plate or in the vehicle's service manual, and it is almost always given in watts, amps, or both. Knowing the wattage tells you how much load the wiring, switch, and fuse were sized for, which is the same number you need to confirm the new manual install is safe — no live circuit left dangling, no fuse protecting a circuit that no longer exists.
The mechanical side of the conversion does not change when you know the wattage, but the electrical side does. You typically tape off or cap the motor harness, leave the door switch disconnected, and verify with a multimeter that no voltage is present at the connector. Reading the motor label and converting its watts into BTU/h or horsepower first is a fast sanity check on what kind of small motor you are dealing with, and it lines up with the same units you will use when you compare it against a human hand cranking the same window.
Why the Motor's Wattage Still Matters
Even though a manual regulator has no electrical load, the motor's nameplate wattage is the only reliable number for the energy the system was designed to handle. Most factory window motors sit between 50 W and 100 W, with some larger four-door motors closer to 120 W. That is small compared to a 1,500 W space heater or a 250,000 W (250 kW) car engine, but it is not zero. Leaving a hot wire bundled into the door after pulling the motor is a corrosion and short-circuit risk, and the wiring gauge and fuse were chosen for that specific load.
Two practical uses for the wattage number during the conversion: first, you can confirm the motor you pulled actually matches what the service manual lists, because some aftermarket replacements draw slightly more current at the same speed. Second, you can convert that wattage into BTU/h to compare it against the heat dissipation the door cavity used to absorb. 1 BTU/h is about 0.293 W, so a 90 W motor turns into roughly 307 BTU/h — small compared to the 12,000 BTU/h of a window air conditioner, but useful as a reference point when the door is sealed up and you want a quick sense of the heat that used to be generated there.
Watts to Horsepower for a Car Window Motor
The most common reason to convert during a manual window conversion is to put the motor's output in the same units as a human cranking the window. Horsepower is the unit the question gets framed in — "how much power does the motor really have?" — and there are two distinct definitions to keep straight.
| Unit | Symbol | Definition | Value in watts |
|---|---|---|---|
| Mechanical (imperial) horsepower | hp | 550 ft·lbf/s | 745.6998715822702 |
| Metric horsepower | PS / CV / cv | 75 kgf·m/s | 735.49875 |
| Kilowatt | kW | 1,000 W | 1,000 |
| BTU per hour | BTU/h | 1 BTU per 3,600 s | ≈ 0.293 |
Because 1 mechanical hp equals 745.6998715822702 W and 1 metric hp equals 735.49875 W, the two horsepowers differ by about 1.4 percent. That is small enough that most backyard conversions ignore it, but it is large enough to matter when you cross-reference a US spec sheet against a European parts catalogue. A 100 PS figure on a European window-motor listing is roughly 98.6 mechanical hp, not 100. The Power Converter keeps them as separate units so the result you read is exactly the one you asked for.
How to Convert Motor Wattage With the Power Converter
- Type the motor's rated wattage into the power value field. Most factory window motors are stamped with a watt or amp figure; if only amps are given, multiply by the nominal 12 V system voltage to get an approximate wattage.
- Choose the unit you are converting from — watts (W) is the safe default — and pick the unit you want the answer in. For comparing against a manual crank, choose mechanical hp; for matching against an AC or heater datasheet, choose BTU/h.
- Read the converted result instantly. The tool returns the value in your chosen target unit, accurate to the factors used by the SI definition of the watt.
- Tick "Show all units at once" to see watts, kW, MW, mechanical hp, metric hp (PS), BTU/h, ft·lbf/s, cal/s and kcal/h on the same screen. That view is useful when you want to check the motor's output against a few reference points at once.
- Repeat with the manual crank's expected load in watts or hp to see where the two meet. For values at the extreme ends — milliwatts to megawatts — the result switches to scientific notation so significant figures are preserved.
One worked example: a typical 90 W window motor. Dividing by the exact mechanical horsepower factor: 90 W ÷ 745.6998715822702 = 0.1207 mechanical hp. Switching the target to metric horsepower: 90 W ÷ 735.49875 = 0.1224 PS. Switching again to BTU/h using the 1 BTU/h ≈ 0.293 W factor: 90 W ÷ 0.293 ≈ 307 BTU/h. Three results, same motor, three different reference frames — and the Power Converter gives them all from one input.
Two Horsepowers and One Common Reading Mistake
The single biggest trap when reading window-motor specs is that "hp" means different things in different markets. Mechanical (or imperial) horsepower is the unit used in the United States and the United Kingdom and is defined as exactly 550 ft·lbf/s = 745.6998715822702 W. Metric horsepower — written PS (Pferdestärke), CV or cv, and common on European and Asian spec sheets — is defined as 75 kgf·m/s = 735.49875 W. The two differ by about 1.4 percent, so a 100 PS European window motor is roughly 98.6 mechanical hp.
US material almost always quotes mechanical hp when it says "hp". When the source just says "horsepower" with no qualifier and you are looking at a UK or US spec sheet, treat it as mechanical hp. When you see PS, CV, or cv, treat it as metric hp. The Power Converter lists both as separate units, so the conversion you run is always the one that matches the spec sheet in front of you.
Plan the Manual Crank's Load the Same Way
The same converter that sizes the motor you are pulling can size the manual crank you are putting in. Human arm power output peaks at roughly 75 to 100 W during a short burst, which is roughly the same range as a factory window motor. In horsepower terms that is about 0.10 to 0.13 mechanical hp — close enough to the 0.12 hp figure for a 90 W motor that the manual crank feels familiar, not alien, when you first turn the handle.
For the broader HVAC or thermal numbers, BTU/h is the right unit. A 90 W motor becomes about 307 BTU/h, a 12,000 BTU/h window air conditioner is about 3.5 kW, and the same Power Converter handles both pairs because they share the same SI base, the watt. Every input becomes watts first, then is divided into the target unit, so the pairing stays consistent no matter which units you start and end with. For a related vehicle-power calculation, the guide at How to Convert Power Wheels to DeWalt Battery: Power Math uses the same converter for battery and motor math on a smaller scale.
Because every step runs in your browser, you can keep the door open, run the numbers, and never upload the motor's serial or the wiring diagram. Open the Power Converter, type the wattage, and read the result in the unit that matches the spec sheet in front of you.