What Is Horsepower hour (hp·h)?
A horsepower hour is a unit of energy. It tells you how much work is done when an engine keeps a power of one horsepower running for one full hour.
So if a motor produces one horsepower of power and does that for sixty minutes, the total work it does in that time is one horsepower hour.
Definition
Horsepower hour is an energy unit based on the power unit called horsepower and the time unit hour.
More exactly:
- Power part: 1 mechanical horsepower is about 745.7 watts.
- Time part: 1 hour is 3,600 seconds.
Energy equals power multiplied by time. So:
- 1 hp·h is the energy from 1 horsepower used for 1 hour.
- 1 hp·h is about 2,684,520 joules of energy.
- 1 hp·h is about 0.7457 kilowatt hours.
This means a horsepower hour measures the total work or energy, not just how strong the engine is at one moment.
History / Origin
The idea of horsepower was introduced in the late 1700s by James Watt, a Scottish engineer. He wanted a simple way to compare his steam engines to horses, which were the common source of work at that time.
He decided that one horsepower would be a certain amount of work a strong horse could do over time, such as pulling a load. From this, engineers later used horsepower over a period of time, like one hour, to measure the total work done. That total work became known as a horsepower hour.
Before modern units like the joule and kilowatt hour were popular, horsepower hour was useful for engineers who worked with engines, steam machines, and early machinery.
Symbol & Abbreviation
The main symbol for horsepower hour is:
- hp·h
In older books or documents you may also see:
- hp h
- hph
- HP h or HPh in all capital letters
All of these mean the same thing, energy from one horsepower used for one hour. The small dot in hp·h simply shows multiplication of horsepower by hour.
Current Use Around the World
Today, scientists and most engineers use the joule and the kilowatt hour to measure energy. These are official SI based units. Because of this, horsepower hour is not a standard unit anymore.
Even so, horsepower hour can still appear in some places:
- Older engineering books and manuals for engines and machines.
- Railway and locomotive work, where engine output in horsepower hour was once common.
- Some mechanical and automotive discussions that like to use horsepower based units.
In modern power plants and home electricity bills, you will almost always see kilowatt hours, not horsepower hours.
Example Conversions
Here are some simple conversions using 1 mechanical horsepower hour:
- 1 hp·h ≈ 0.7457 kilowatt hours (kWh)
- 1 hp·h ≈ 2,684,520 joules (J)
- 1 hp·h ≈ 2.6845 megajoules (MJ)
- 1 hp·h ≈ 2,540 to 2,550 British thermal units (BTU) often rounded to about 2,545 BTU
Reverse conversions are also useful:
- 1 kWh ≈ 1 ÷ 0.7457 ≈ 1.34 hp·h
- 1 MJ ≈ 1 ÷ 2.6845 ≈ 0.37 hp·h
Example story:
- If an engine runs at 5 horsepower for 2 hours, total energy in hp·h is 5 × 2 = 10 hp·h.
- That 10 hp·h is about 10 × 0.7457 = 7.457 kWh.
Related Units
Horsepower hour is closely linked to these other units of energy and power:
- Horsepower (hp) power unit that shows how fast work is done.
- Watt (W) SI unit of power, 1 hp is about 745.7 W.
- Joule (J) SI unit of energy, 1 hp·h is about 2.68 million J.
- Watt hour (Wh) energy unit, 1 hp·h is about 745.7 Wh.
- Kilowatt hour (kWh) 1,000 Wh, used on electricity bills, 1 hp·h is about 0.7457 kWh.
- British thermal unit (BTU) energy unit often used for heating and air conditioning, 1 hp·h is about 2,545 BTU.
FAQs
Is horsepower hour a unit of power or energy?
Horsepower hour is a unit of energy. Horsepower alone is power. When you multiply horsepower by time in hours, you get an energy unit.
Why is horsepower hour not used much today?
Modern science and engineering use joules and kilowatt hours because they fit the SI system and are easier to compare across many fields. Horsepower hour is now mostly historical or used in special cases.
How do I change horsepower hour into kilowatt hours?
Multiply the number of horsepower hours by 0.7457. For example, 3 hp·h × 0.7457 ≈ 2.2371 kWh.
How do I change horsepower hour into joules?
Multiply the number of horsepower hours by about 2,684,520. For example, 0.5 hp·h is about 1,342,260 joules.
What is the difference between hp and hp·h?
Hp is power, like how strong an engine is at a moment. Hp·h is energy, like how much total work that engine does over some time.
Can I still use horsepower hour in calculations?
Yes, you can, as long as you clearly show your units and convert correctly to standard units like joules or kilowatt hours when needed.