Understanding key units of electrical measurement empowers you to better grasp electricity concepts for work, research, or personal projects. This article clearly explains the difference between two foundational units – the kilowatt hour and amp hour. Continue reading as we contrast these measurements with insightful examples and analysis.
Overview: Kilowatt Hours vs. Amp Hours
At a basic level, kilowatt hours measure accumulated energy usage over time, while amp hours measure electric charge capacity provided over time from a source like a battery.
But there are deeper differences and use cases worth exploring. We’ll distinguish what defines each unit, everyday examples, how scientific history shaped them, when to use each one properly, and more key distinctions between kilowatt hours and amp hours.
Whether you’re an aspiring electrician, are exploring solar batteries, or just want to better understand your electricity bill – these important electrical units will unlock new concepts. Let’s get started!
Kilowatt Hour vs. Amp Hour: Side-By-Side Comparison
Before digging deeper, here is a birds-eye view comparison:
| Kilowatt Hour | Ampere Hour |
|---|---|
| Unit of power (energy transfer) used over time | Unit of electric current (charge flow) used over time |
| Total energy used = Power x Time | Total charge capacity = Current x Time |
| Used for electricity billing, energy usage analysis | Used for battery capacity ratings |
| Based on watts (power transfer rate) | Based on amps (charge flow rate) |
Now let’s fully define kilowatt hours first before explaining amp hours and some key differences.
What is a Kilowatt Hour?
A kilowatt hour (kWh) is the total energy used at a rate of one kilowatt for one hour or more. For example, a kilowatt hour would be using a 1000 watt appliance for one hour, a 500 watt appliance for two hours, and so on.
Kilowatt hours measure accumulated energy usage over time, which gets widely used for electricity billing. Power companies measure your home’s total kWh usage, then charge for that energy at a set rate (e.g. $0.12 per kWh).
But from a technical perspective, where did this standard unit originate from? Let’s look back at some history.
The History Behind Kilowatt Hour Measurement
The “kilowatt” part owes credit to Scottish inventor James Watt, a steam engine pioneer who helped optimize early engines in the late 1700s. He standardized methods for measuring horsepower and the rate of work over time.
Later in the 1882, the Institute of Electrical Engineers proposed naming the power unit after Watt in recognition. Today we define one watt (W) as the rate of energy transfer equivalent to one joule (J) per second (1 W = 1 J/s).
The “hour” portion of kilowatt hours builds on the history of standardized timekeeping, with a widely agreed upon definition of one hour being 3,600 seconds.
Combining “kilowatts” and “hours” became an easy way for the electrical industry to measure energy use over time, especially with homes containing many appliances running at different wattages. Thus the ubiquitous kilowatt hour was born!
How Kilowatt Hours Get Used
Below are some examples of how kilowatt hours get used in everyday life:
Electricity Billing
As mentioned earlier, the most common application is for electricity billing from power companies. They summarize your home or building‘s energy usage in kWh for the month, then bill you based on that.
For example, say your home used 875 kWh last month. At a rate of $0.15 per kWh, your bill would be:
- 875 kWh x $0.15 per kWh = $131.25
Without kilowatt hours, calculating electrical costs would be extremely tedious!
Appliance Energy Rating Labels
Many appliances now include an estimated yearly kWh rating on their Energy Guide labels to help consumers compare energy efficiency.
For instance, an energy efficient refrigerator may use 300 kWh per year, while a less efficient model uses 500 kWh per year. So kilowatt hours become an easy metric for annual energy consumption comparisons while shopping.
Solar Production Monitoring
Home solar panel systems often monitor and report the total kilowatt hours produced by your solar array. This helps track your investment value over time.
Most solar monitoring apps and inverters show data like:
- 435 kWh produced last month
- 2,865 kWh produced this year
Racking up self-generated kilowatt hours helps solar owners quantify their energy savings.
Electric Vehicle Range Estimates
Electric vehicles use kWh per 100 miles (or 100 km) to rate battery capacity. The Nissan Leaf for example is rated at 30 kWh per 100 miles. This gives prospective owners an estimate for how far they can drive on a full charge.
So whether you’re tracking home energy use, comparing appliances, monitoring solar panels, or reviewing electric cars – kilowatt hours appear often as the standard energy measurement unit.
Kilowatt Hour Equivalents & Conversions
Here are some common kilowatt hour conversions:
| 1 kWh = 3,600,000 joules |
| 1 kWh = 3.6 megajoules |
| 1 MWh = 1,000 kWh |
| 1 kWh = 3412 BTUs |
| 1 horsepower hour = 2.655 kWh |
Joules, megajoules, BTUs, and horsepower can all be converted to/from kilowatt hours. But the most relevant conversion is often transposing electric usage billing amounts between kilowatt hours and megawatt hours.
Let’s move on to amp hours now!
What is an Amp Hour?
An amp hour (Ah) is the total electric charge delivered by a steady one amp current flowing for one hour. Another way to define it is the number of hours a battery can provide a particular current (measured in amps).
For example, a 5000 milliamp hour battery could theoretically sustain 5 amps for 1 hour, 1 amp for 5 hours, or 50 milliamps for 100 hours (though real-world conditions vary).
In practice, batteries use amp hour ratings to convey capacity. Higher amp hour batteries can source more current over time. Let’s examine the history and scientific origin of amps and amp hours:
The Experiments Behind Amp Hour Measurement
André-Marie Ampère was a French physicist who explored the relationship between electricity and magnetism in the early 1800s.
Through experiments with wire coils and compass needles, he observed patterns between the direction of current flows and the orientation of magnetic forces they created.
These experiments laying the foundation for electrodynamics – the physics behind electricity and magnetism. Ampère recognized the need for standardized electrical measurement.
Later in 1881, the unit of electric current was named the ampere (amp) in Ampère‘s honor. An amp was defined as equivalent to one coulomb (a standardized unit of electric charge) per one second.
This formed the basis of current measurement. By extension, amp hours measure current capacity over time. Building on over 200 years of science and discovery!
Where Amp Hour Battery Ratings Get Used
Now let’s examine some common places you’re likely to encounter amp hour ratings:
Electric Vehicles
The Nissan Leaf electric car for example uses a lithium-ion battery with a capacity of 40 kilowatt hours (kWh). Given its battery voltage of 350-400 volts, this equates to around 100 amp hours of capacity.
Knowing both the kWh and Ah ratings helps assess an EV’s charge capacity from different angles. The kilowatt hour rating focuses on total energy, while the amp hour rating allows comparing charge flow over time.
Power Banks
Small power banks that recharge personal devices often have amp hour capacities in the 1-4 Ah range. The Anker PowerCore Slim for example has a 10,000 mAh or 10 Ah rating.
Device manufacturers lean more on milliamp hours (mAh) for compact consumer products. But it still tells you runtime capabilities – just in smaller values.
Solar Batteries
Whether for RVs, cabins, or home solar electric systems – paired battery banks are often rated in amp hours. Trojan’s 6V golf cart batteries popular for solar use rate at 225 Ah each for example.
Strings of such batteries can deliver substantial currents. Knowing both total amp hours (charge capacity) and kilowatts (power delivery) aids solar energy system sizing.
Cordless Power Tools
Cordless drills, saws, sanders and other devices often show amp hour ratings for their lithium ion battery packs. The Bosch Bulldog SDS-Plus Rotary Hammers runs on an 8 Ah battery pack for instance.
As with electric vehicles, combining both the wattage and amp hour estimates helps approximate total runtime. Watts x hours = watt hours, just as amps x hours = amp hours.
In summary, any device running on DC battery power often provides an amp hour rating to convey capacity. The higher the amp hours, the more current it can theoretically provide over time.
Milliamp Hour (mAh) Equivalents
For smaller electronics, battery capacities get expressed in milliamp hours (one-thousandth of an amp hour) rather than the higher amp hour values used for vehicles and solar batteries. For reference:
| 100 mAh = 0.1 Ah |
| 10,000 mAh = 10 Ah |
| 1,000 mAh = 1 Ah |
So a 5000 mAh smartphone battery could also be said to have a 5 Ah rating. Milliamp hours just allow finer rating precision for lower capacities.
In everyday use you’ll encounter both milliamp hours and amp hours depending on the type of device and its battery size.
Key Differences Between kWh and Ah
Now that we’ve explored kilowatt hours and amp hours in-depth individually, let’s compare some of the key differences:
Purpose
- Kilowatt hours quantify energy usage
- Amp hours convey charge capacity
Unit Origins
- Kilowatts developed from early steam engine power measurements
- Amps developed from early electrodynamics experiments
Common Usage
- Kilowatt hours used for electricity billing
- Amp hours used for battery ratings
Measurement Scope
- Kilowatt hours measure total energy used by multiple devices
- Amp hours measure capacity of a single specific power source
So in summary:
- Kilowatt hours work well for system-wide energy use and billing
- Amp hours suit device-specific battery sizing and runtime context
Understanding when each unit excels makes selecting the right measurement for your electrical project easier.
Kilowatt Hours vs. Amp Hours in Practice
While the definitions and historical technical differences are important, how do kilowatt hours and amp hours actually come into play in everyday electrical engineering and construction situations?
Here are two examples contrasting real-world uses cases:
Kilowatt Hours for Building & Home Energy Use
Architects, contractors, and homeowners alike often analyze the estimated annual kWh usage of buildings during the design process using modeling software.
The anticipated kilowatt usage sets parameters for properly sizing:
- Utility electrical hookup needs
- Photovoltaic solar arrays
- Backup home generators
- Overall energy efficiency investments
Kilowatt hours simplify running total usage across all types of electronics into one energy usage number for easier planning.
Amp hour battery bank specifications don’t provide direct guidance on overall building energy needs across heating, lighting, appliance usage, etc in the way that kilowatt usage estimates do.
Amp Hours for Off-Grid Solar Battery Banks
Alternatively, when installing standalone off-grid photovoltaic systems with battery banks – like at a rural homestead or RV – the solar panels and batteries get sized based on total amp hour capacity:
- Solar charge controller’s amp rating limits panels
- Batteries based on total amp hour storage capacity
- Inverter capacity also sized based on current (amps)
Additionally, keeping the system voltage consistent across these components enables smooth DC to AC inversion.
So in off-grid solar contexts focused just on the self-contained system – rather than tying back into the grid – amp hours become the central specification for ensuring properly matched charge controllers, battery banks, wiring ratings, and inverters.
Kilowatt hours don’t offer direct system matching guidance between solar panels, batteries, and inverters. Amp hour capacities bridge these components in off-grid configurations.
Frequently Asked Questions
Can I convert between kilowatt hours and amp hours?
Unfortunately no, there is no direct conversion between kWh and Ah – they measure different attributes as we’ve covered.
You cannot covert kilowatt hours of energy usage to amp hours of charge capacity – it’s an apples to oranges comparison.
How long will a 100 Ah battery last?
On its own, an amp hour rating only provides the charge capacity – it does not indicate runtime by itself. You’d also need to know:
- Operating voltage (to determine watt hours)
- Connected load power draw in watts
- Efficiency losses
For example, a 12V 100 Ah battery has roughly 1200 watt hours (100 x 12). If running a 60 watt lights, in a perfect world that could run for 20 hours (1200 / 60).
But allowing for 80% efficiency, the runtime would be closer to 16 hours. So amp hours indicate potential, but real-world runtime varies.
What uses more power – amps or watts?
Watts measure power, amps measure current. So technically watts “use” power and amps “use” current.
But from an overall electrical throughput perspective, higher watts and higher amps both equate to more power usage. You must multiply volts x amps to get watts.
Generally something pulling more amps OR something operating at a higher voltage will consume more watts of power.
Conclusion
Let’s recap the key differences between these two common electrical measurements:
- Kilowatt hours quantify energy use over time, commonly for electricity billing
- Amp hours convey charge capacity over time, commonly used for battery ratings
While they may seem similar at first glance, their differences stem from distinct scientific origins and practical applications.
Kilowatt hours build on early steam power measurements, later getting applied to residential energy billing. Amp hours originate from electrodynamics experiments and battery current capacities.
In practice, kilowatt hours serve broader discussions around multi-device electricity usage, while amp hours suit contexts specifically focused on battery-powered systems and standalone current capacities.
So next time you encounter kilowatt hours or amp hours in an electrical context, consider the usage scenario to determine which measurement best applies. Both provide value within relevant applications.
I hope this guide gave you a thorough understanding of kilowatt hour and amp hour measurement units! Let me know if you have any other questions. Applying these fundamental electrical concepts will give you confidence for personal projects or professional work.
