What Is A WattHour For Dummies?
A watthour is a unit of energy that measures the amount of electricity used over time. It quantifies the total amount of power consumed or produced, expressed in watthours. Specifically, a watthour is equal to one watt of power sustained for one hour.
For example, a 100watt light bulb that is left on for 10 hours will consume 1000 watthours of electricity (100 watts x 10 hours). So in simple terms, a watthour is a way to measure electrical energy usage.
The “watt” unit indicates the rate of energy consumption at any given moment, while “hour” signifies the duration. Multiplying the two together (watts x hours) provides the total energy used.
Watts vs Watthours
Watts and watthours measure two different but related aspects of electricity – power and energy. The key difference is that watts measure power, which is the rate at which electricity is being used at any given moment. Watthours measure energy, which is the total amount of electricity used over time.
An analogy can help explain the difference. Power is like the speed of a vehicle, while energy is like the total distance traveled. A car traveling at 50 miles per hour has a certain power level instantaneously. But over a 5 hour trip at that speed, the car uses 250 miles worth of energy.
In electricity:
 A 100watt light bulb uses 100 watts of power at any moment when turned on. This is its “instantaneous” power draw.
 If that 100watt bulb is left on for 10 hours, it will use 1,000 watthours (100 watts x 10 hours) of electrical energy over that time period.
So in summary, watts are a rate of power usage at any given moment, while watthours measure total energy used over time. The “hours” in watthours refers to the duration the power draw occurs.
Calculating Watthours
A watthour is calculated by multiplying power (watts) by time (hours). This gives you energy usage over time.
The formula is:
Watthours = Watts x Hours
Here are some examples of using this formula:
 A 60W light bulb used for 5 hours = 60 x 5 = 300 watthours
 A 300W computer used for 2 hours = 300 x 2 = 600 watthours
 A 1200W microwave used for 0.5 hours (30 minutes) = 1200 x 0.5 = 600 watthours
So in these examples, the total energy usage calculated in watthours is 300, 600, and 600 respectively. This shows that power usage over time equals total energy consumption.
Watthour Usage
Watthours are commonly used to measure electric energy consumption of appliances and devices. Here are some examples of typical watthour usage:
 Lightbulb – A 60W lightbulb used for 1 hour consumes 60 watthours of energy.
 TV – A plasma TV with a power rating of 150W used for 5 hours consumes 750 watthours.
 Computer – A desktop computer rated at 400W used for 3 hours consumes 1200 watthours.
 Air Conditioner – A window AC unit rated at 1000W running for 8 hours uses 8000 watthours.
 Electric oven – An oven rated at 2500W used for 1 hour to cook a meal consumes 2500 watthours.
 Hair dryer – A 1200W hair dryer used for 15 minutes (0.25 hours) uses 300 watthours.
By looking at the wattage ratings of appliances and how many hours they are used, you can estimate their energy consumption in watthours.
Electricity Bills
Watthours are directly related to electricity bills and how utility companies charge for power consumption. Your electricity bill shows how many kilowatthours (kWh) of electricity you used during the billing period, which is usually one month.
A kilowatthour represents 1,000 watthours. So if you use 1 kWh of electricity, that’s equal to using 1,000 watts for one hour, or using a 100watt lightbulb for 10 hours (100 x 10 = 1,000 watthours). The more kilowatthours used, the higher your electricity bill will be.
Utility companies use your kWh usage along with your rate per kWh to calculate your total bill. For example, if you used 500 kWh of electricity and your rate is $0.15 per kWh, your bill would be 500 x $0.15 = $75. Knowing how many watthours are in a kWh helps explain how your usage translates into costs on your electricity bill.
Watthours vs Kilowatthours
Watthours (Wh) and kilowatthours (kWh) are both units of energy that measure power used over time. However, they represent different magnitudes of energy:
– 1 watthour = 1 watt x 1 hour
– 1 kilowatthour = 1,000 watts x 1 hour
So a kilowatthour is equal to 1,000 watthours. This means that a kilowatthour is a larger unit of energy than a watthour.
To convert between the two units:
– To convert watthours to kilowatthours: Divide the watthours by 1,000.
– To convert kilowatthours to watthours: Multiply the kilowatthours by 1,000.
For example:
– 500 Wh / 1000 = 0.5 kWh
– 1.5 kWh x 1000 = 1500 Wh
Kilowatthours are more commonly used for billing electricity usage, while watthours are more often used for smaller measurements like battery capacity. But both units are useful for quantifying electric power consumption over time.
Energy Efficiency
Watthours are a useful metric for comparing the energy efficiency of devices and appliances. The number of watthours a device consumes over a period of time indicates how much power it uses. Lower watthour consumption means an appliance or gadget is more energy efficient.
For example, two different LED light bulbs may be rated for the same brightness (lumens), but one may consume 9 watthours over the course of an hour while the other uses only 6 watthours. The 6watt bulb would be considered more energy efficient since it uses less power. When multiplied over the lifetime of a device, small reductions in watthours can lead to significant energy savings.
Watthours are also helpful for calculating how much it costs to run appliances over time. More efficient devices with lower watthour usage will be cheaper to operate. By choosing energy efficient products with lower watthour consumption, consumers can reduce their electricity costs and environmental impact.
Watthour Meters
A watthour meter is an electrical meter that measures energy consumption in watthours. It is commonly used by electric utilities to bill customers for their electricity usage.
Watthour meters work by continuously measuring the instantaneous voltage and current being drawn by a load. By multiplying the voltage and current together at each instant, the power consumption in watts is calculated. The meter integrates or totals up all of these instantaneous power measurements over time to determine energy consumption in watthours.
For example, if a 100 watt light bulb is left on for 1 hour, it will consume 100 watthours of energy (100 watts x 1 hour). This accumulated energy usage is displayed on the watthour meter and is used for billing purposes.
Watthour meters have dials or numerical displays that advance in proportion to the amount of energy consumed. The meters record cumulative total consumption, so the utility provider can read the meter periodically to determine how many kilowatthours or megawatthours were used in a given time interval.
Modern electrical meters are digital rather than mechanical, but function on the same basic principle of measuring instantaneous usage over time. Smart meters have even more capabilities for twoway communication and automatic meter readings by the utility company. But their fundamental purpose remains measuring watthours for billing.
Applications
Watthours are commonly used to measure energy consumption in industrial and scientific applications. Here are some examples:

Manufacturing plants track their hourly energy usage in watthours to calculate electricity costs and identify opportunities for efficiency improvements.

Data centers measure their massive power draws in watthours. A single data center can use over 1 billion watthours per year.

Scientists measure laser output power in watthours for experiments. Highenergy laser facilities can output megajoulelevel pulses (millions of watthours).

Energy storage systems like batteries and flywheels are rated in watthour capacity, which determines how much energy they can discharge over their lifetime.

Power plants generate electricity in watthours. Their output must match up with the watthour demand on the electric grid.

Research labs measure the energy consumption of prototypes and devices in watthours during product testing.
In these industrial and scientific contexts, watthours provide a standardized way to quantify massive amounts of energy usage, capacity, and production.
Summary
To recap the main points about watthours:
– A watthour is a unit of energy that measures power consumption over time. It refers to the energy consumed by a 1 watt device running for 1 hour.
– Watts measure the rate of energy consumption at any given moment. Watthours measure total energy used over time.
– To calculate watthours, multiply the power rating of a device (in watts) by the time it runs (in hours). For example, a 60W light bulb running for 5 hours uses 300 watthours (60 x 5 = 300 Wh).
– Electricity bills charge by the kilowatthour (kWh), which equals 1,000 watthours. This allows utilities to easily bill for large amounts of energy usage.
– Understanding watthours helps consumers compare device energy efficiency and estimate costs. It also assists engineers in properly sizing systems to meet energy needs.
– Watthour meters track energy consumption over time. They can monitor a whole building, circuit, or even individual devices.
– In summary, the watthour is a useful standardized unit for measuring electrical energy consumption.