Unit & Measurement Tables
Power Conversion Table
Power is energy per unit time, and almost every confusion about it comes from two things: the two horsepower definitions and the difference between power and energy. A 1,500 W heater running for two hours does not use 1,500 W of electricity, it uses 3 kWh. The wattage is the rate; the kilowatt hour is the amount.
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The horsepower problem is older. James Watt defined one horsepower as 550 foot-pounds per second to sell steam engines against draft horses, and that number survives in US and UK engineering. Continental Europe used a metric horse instead: 75 kilogram-force metres per second, or 735.5 W. The two differ by 1.387%, which is why a car sold as 300 hp in one market is advertised as 296 PS in another. Neither is an SI unit, and neither is more correct than the other — they are just different conventions.
Every figure here is computed at build time from the exact definitions: 550 ft·lbf/s for mechanical horsepower, 75 kgf·m/s for metric horsepower, 1,055.055 852 62 J per BTU and 4,184 J per kilocalorie. No value is copied from a table, so the columns stay consistent with each other and with the energy conversion table on this site.
Watts to kilowatts, horsepower, BTU per hour and kilocalories per hour
| Watts | Kilowatts | Metric hp (PS) | Mechanical hp (hp) | BTU per hour | Kilocalories per hour |
|---|---|---|---|---|---|
| 1 | 0.001 | 0.001 | 0.001 | 3.4 | 0.9 |
| 10 | 0.010 | 0.014 | 0.013 | 34.1 | 8.6 |
| 60 | 0.060 | 0.082 | 0.080 | 204.7 | 51.6 |
| 100 | 0.100 | 0.136 | 0.134 | 341.2 | 86 |
| 500 | 0.500 | 0.680 | 0.671 | 1,706.1 | 430.2 |
| 1,000 | 1.000 | 1.360 | 1.341 | 3,412.1 | 860.4 |
| 1,500 | 1.500 | 2.039 | 2.012 | 5,118.2 | 1,290.6 |
| 2,000 | 2.000 | 2.719 | 2.682 | 6,824.3 | 1,720.8 |
| 3,500 | 3.500 | 4.759 | 4.694 | 11,942.5 | 3,011.5 |
| 5,000 | 5.000 | 6.798 | 6.705 | 17,060.7 | 4,302.1 |
| 10,000 | 10.000 | 13.596 | 13.410 | 34,121.4 | 8,604.2 |
| 100,000 | 100.000 | 135.962 | 134.102 | 341,214.2 | 86,042.1 |
1,000 W is 1 kW by definition. The two horsepower columns differ by 1.387% because they come from different definitions, not from rounding.
Kilowatts to the two horsepower ratings
| Kilowatts | Mechanical hp | Metric hp (PS) |
|---|---|---|
| 50 | 67.1 | 68.0 |
| 75 | 100.6 | 102.0 |
| 100 | 134.1 | 136.0 |
| 110 | 147.5 | 149.6 |
| 125 | 167.6 | 170.0 |
| 150 | 201.2 | 203.9 |
| 200 | 268.2 | 271.9 |
| 250 | 335.3 | 339.9 |
| 300 | 402.3 | 407.9 |
| 400 | 536.4 | 543.8 |
| 500 | 670.5 | 679.8 |
A car brochure quoting 110 kW is quoting about 148 hp or 150 PS. The gap widens with size: at 300 kW it is 4 hp.
Cooling capacity: BTU per hour to tons and kilowatts
| BTU per hour | Tons of cooling | Kilowatts |
|---|---|---|
| 5,000 | 0.42 | 1.47 |
| 9,000 | 0.75 | 2.64 |
| 12,000 | 1.00 | 3.52 |
| 18,000 | 1.50 | 5.28 |
| 24,000 | 2.00 | 7.03 |
| 36,000 | 3.00 | 10.55 |
| 48,000 | 4.00 | 14.07 |
| 60,000 | 5.00 | 17.58 |
One ton of cooling is 12,000 BTU/h by definition, a relic of the ice trade. 12,000 BTU/h is 3.517 kW of cooling.
Typical appliance power draw
| Appliance | Watts | Mechanical hp | kWh per hour of use |
|---|---|---|---|
| LED light bulb | 9 | 0.01 | 0.009 |
| Laptop charger | 65 | 0.09 | 0.065 |
| Television, 55 inch | 100 | 0.13 | 0.100 |
| Refrigerator, average draw | 150 | 0.20 | 0.150 |
| Microwave oven | 1,000 | 1.34 | 1.000 |
| Space heater | 1,500 | 2.01 | 1.500 |
| Electric kettle | 2,000 | 2.68 | 2.000 |
| Window air conditioner | 3,500 | 4.69 | 3.500 |
| Level 2 EV charger | 7,400 | 9.92 | 7.400 |
Nameplate watts are the maximum draw. A refrigerator rated 150 W cycles on and off and averages far less over a day.
How to Use These Tables
- Reading a car spec: if the brochure gives kW, multiply by 1.341 to get mechanical hp or by 1.360 to get PS. A 110 kW engine is 148 hp or 150 PS.
- Sizing an air conditioner: divide BTU per hour by 12,000 to get tons, or multiply by 0.2931 to get kilowatts. A 24,000 BTU/h unit is two tons, or 7.03 kW of cooling.
- Estimating an electricity bill: multiply watts by hours and divide by 1,000 to get kWh. A 2,000 W kettle used 15 minutes a day is 0.5 kWh per day.
- Comparing a gas furnace with an electric heater: gas input is often quoted in BTU/h, electric in kW. 100,000 BTU/h is 29.3 kW of input.
- Checking a motor nameplate: European motors are marked in kW, US motors in hp. A 7.5 kW motor is 10.1 hp, not 7.5 hp.
Common Questions
Is hp the same as PS?
No. Mechanical horsepower (hp) is 745.7 W, metric horsepower (PS) is 735.5 W. The difference is 1.387%, so 100 hp is 101.4 PS. Car magazines mix the two constantly.
How many watts is one horsepower?
745.699 871 582 270 2 W, exactly, because one horsepower is defined as 550 foot-pounds per second and a foot-pound per second is 1.355 817 948 331 400 4 W.
Why is an air conditioner measured in tons?
Before mechanical refrigeration, cooling was sold as ice, and one ton of ice melting over 24 hours absorbs 12,000 BTU per hour. The unit stuck even though nobody buys ice for cooling any more.
How many kW is 1,000 BTU per hour?
0.293 kW. Divide BTU per hour by 3,412 to get kW, or multiply by 0.2931. A 12,000 BTU/h air conditioner draws about 3.5 kW of cooling, though its electrical draw is lower because of the efficiency rating.
Does a 100 W appliance use 100 W every hour?
It uses 100 W while it is running, which is 0.1 kWh per hour. Over 10 hours that is 1 kWh. Wattage is a rate, and the bill is charged on the total, so both numbers are needed.
What is the difference between power and torque?
Torque is a turning force, measured in newton metres or pound-feet. Power is torque times rotational speed. An engine can make high torque at low speed and low power, which is why trucks and sports cars are geared differently.
More Unit & Measurement Tables
Sources
- NIST Special Publication 811 — units for power and heat flow rate — National Institute of Standards and Technology
- NIST Guide to the SI, Chapter 5 — non-SI units accepted for use — National Institute of Standards and Technology
- ASHRAE — refrigeration ton and cooling load definitions — American Society of Heating, Refrigerating and Air-Conditioning Engineers
Filed with Unit Conversion calculations.