As a rule of thumb, 1 electrical HP = 0.746 kW, so a 1 HP electric motor would consume 0.746 kW per hour with a full load and 100% efficiency. However, the latter seldom happens in real life, so actual electricity consumption can be calculated by using the following formula:
- P (kW) = HP x (0.746/Efficiency) x Load x Time
But if you don’t want to bother doing the calculations, this post is for you.
For your convenience, we at PowerSportsGuide have compiled the most common conversion numbers into one chart!
How Much Power Does a Motor Consume?
The power consumption of an electric motor depends on many factors, including:
- Performance
- Efficiency
- Load
- Time
There’s no question that the main factor affecting power consumption is engine performance.
The general rule is that 1 electrical HP = 0.746 kW; therefore, a one HP electric motor would (theoretically) consume 0.746 kW per hour.
Why theoretically?
First, each electric engine has its own efficiency factor, which has to be taken into account. And depending on the make and model, the efficiency of those units varies widely.
Are you wondering what the typical efficiency is of an electric motor?
As a rule of thumb, the estimated efficiency of many electric engines falls into the range of 70% – 90%.
This means that a fully loaded 1 HP electric motor would consume anywhere from 0.746/0.7 – 0.746/0.9 kW, which translates to 0.829 – 1.066 kW per hour.
Unfortunately, no hard number can be universally used for efficiency since efficiency varies from one model to the next.
Besides efficiency, the load is no less important.
Let’s face it; electric motors rarely operate at their fully rated load. Instead, they typically run somewhere between 50-99% of their peak performance.
According to Energy.gov, most of these engines are designed to provide their best performance at about 75% of their rated load.
“Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range of 5 to 10 hp; peak efficiency is at 7.5 hp. A motor’s efficiency tends to decrease dramatically below about 50% load.”
Assume a 1 HP motor operates at 75 % of full load, the input power would be calculated as follows:
P (kW) = 0.746 x 0.75 = 0.56 kW per hour (considering 100% efficiency)
Last but not least, don’t forget that energy = time x power, so the running time of the motor has to also be taken into account.
Motor Power Consumption Formula
Based on the above, the power consumption formula of electric motors is as follows:
- P (kW) = HP x (0.746/E) x Load x Time
Where:
- P = Input power
- HP = Rated horsepower
- E = Efficiency
- Load = load of the engine
- Time = Duration of engine operation in hours
Here are some examples!
Example 1:
- Engine power = 3 HP
- Efficiency = 80%
- Load = 75%
- Time = 2 hours
P (kW) = 3 x (0.746 / (80/100)) x 0.75 x 2 = 4.196 kW
The power consumption of this 3 HP engine would be 4.196 kW in two hours, considering 80% efficiency and an 75 % load.
Example 2:
- Engine power = 5 HP
- Efficiency = 90%
- Load = 70%
- Time = 0.5 hours
P (kW) = 5 x (0.746 / (90/100)) x 0.7 x 0.5 = 1.451 kW
The power consumption of this 5 HP engine would be 1.451 kW in half an hour, considering 90% efficiency and a 70 % load.
Motor Power Consumption Chart
For your convenience, we’ve compiled the most common power consumption figures of electric motors at full load into this chart:
| Motor HP | Energy usage (kw) [Efficiency: 100%] | Energy usage (kw) [Efficiency: 75%] |
| 0.5 | 0.373 | 0.497 |
| 1 | 0.746 | 0.829 |
| 1.5 | 1.119 | 1.243 |
| 2 | 1.492 | 1.658 |
| 2.5 | 1.865 | 2.072 |
| 3 | 2.238 | 2.487 |
| 3.5 | 2.611 | 2.901 |
| 4 | 2.984 | 3.316 |
| 4.5 | 3.357 | 3.730 |
| 5 | 3.730 | 4.144 |
| 5.5 | 4.103 | 4.559 |
| 6 | 4.476 | 4.973 |
| 6.5 | 4.849 | 5.388 |
| 7 | 5.222 | 5.802 |
| 7.5 | 5.595 | 6.217 |
| 8 | 5.968 | 6.631 |
| 8.5 | 6.341 | 7.046 |
| 9 | 6.714 | 7.460 |
| 9.5 | 7.087 | 7.874 |
| 10 | 7.460 | 8.289 |
| 15 | 11.190 | 12.433 |
| 20 | 14.920 | 16.578 |
| 25 | 18.650 | 20.722 |
| 30 | 22.380 | 24.867 |
| 35 | 26.110 | 29.011 |
| 40 | 29.840 | 33.156 |
| 45 | 33.570 | 37.300 |
| 50 | 37.300 | 41.444 |
| 55 | 41.030 | 45.589 |
| 60 | 44.760 | 49.733 |
| 65 | 48.490 | 53.878 |
| 70 | 52.220 | 58.022 |
| 75 | 55.950 | 62.167 |
| 80 | 59.680 | 66.311 |
| 85 | 63.410 | 70.456 |
| 90 | 67.140 | 74.600 |
| 95 | 70.870 | 78.744 |
| 100 | 74.600 | 82.889 |
Disclaimer: This post is for informational purposes only.
References:
https://www.unitconverters.net/power/hp-to-kw.htm
https://www.calculatorsconversion.com/en/hp-to-kw-calculator-examples-table-formula/
