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Explaining Kilowatts vs. Kilowatt-Hours

May 4, 2011

What’s the practical difference between kilowatts and kilowatt-hours? Knowing the difference can help you look smart and impress your friends.

Kilowatts (kW)– this is the rate that energy is produced or consumed. A kilowatt is 1000 watts (W). Sometimes you may see megawatt (MW), which is a million watts.

The power of a generator is specified in kW. Devices consume electricity at a rate specified in kW. A lightbulb may be 60 watts (.06 kW).

Kilowatt-hours (kWh) – amount of electricity produced or consumed. This is rate (kW) multiplied by time (hours). Your electric bill will charge you for the number of kilowatt-hours you consumed.

For example, running a .06 kW (60w) lightbulb for 1 hour consumes .06kWh of electricity. Running a 30w bulb for 2 hours also consumes .06 kWh of electricity. 

Running a .3 kW solar panel for 6 hours a day may produce 1.8 kW per day.

Power generation must match power consumption. In order to provide power to devices that cumulatively consume 10 kW, you need to cumulatively produce 10 kW. If there’s not enough electricity to meet demand, then we have blackouts. But we can’t have too much electricity either or the grid gets overloaded. For example, the Seattle Times reported that hydro plants were producing higher levels of electricity due to excess snow, and so other electricity sources were being shut down to avoid overloading the grid.

What are some actual wattage numbers?

Here’s a table showing very rough ballpark kW values for various generators.

Nuclear Power plant 1 million kW
A single wind turbine (when wind blows) 1000 kW
A single household solar panel (average during day) 0.3 kW
generator hooked up to a bicycle (while peddling at average rate) 0.1 kW

Most electronic devices will label how many watts they consume. My laptop requires 65 W. (So in theory, I could power my laptop with a bicycle-generator and get some exercise while I work. )

A hairdryer may be 1800 W.

While conventional gas cars measure efficiency in miles-per-gallon (mpg), electric cars measure efficiency in kWh-per-mile. For example, the EV1 gets 0.180 kWh/mile.

According to the Department of Energy, the U.S. consumes about 3.5 trillion kWh a year. (Here is a breakdown by sources.)

The Energy Information Administration (EIA) has the national averages: "In 2008, the average annual electricity consumption for a U.S. residential utility customer was 11,040 kWh, an average of 920 kilowatt-hours (kWh) per month."

In my own personal electric bill from a few months ago, I consumed 780 kWh. It cost about $80 (including taxes), so that’s 10.25 cents / kWh.

Math and why it matters

The ability to calculate kW and kWh is essential because it lets us evaluate the effectiveness of various proposals. Here are some examples:

Proposal #1: Say we have people peddling on bicycles connected to generators. How many such bicycle-generators would be needed to replace a 1 million kW nuclear power plant?

At .1 kW per person, it would take (1 million / .1)  = 10 million people. The power plant runs 24 hours a day. Say the person only works a standard 8-hour day. So we need 30 million people peddling bicycles full-time (8 hours per day) to replace a 1 million kW plant.

Proposal #2: How cost-effective would it be to employ people selling electricity produced from bicycle-generators?

A 100 watt bicycle generator may produce 0.8 kWh in a 8 hour work day. At 10 cents per kWh, that’s 8 cents worth of electricity produced for a day’s work peddling your bicycle generator. So we can see that manually generating electricity like this is not a very effective policy.

If the bicycle generator system cost $500, it would take $500 / ($.08 / day) = 6250 days = 17 years just to earn back the money to pay for the initial capital investment of the generator.

Proposal #3: How many bicycle-generators would be needed to provide the U.S. electricity needs?

Assume the U.S. uses 3.5 trillion kWh per year and a generator produces 100 watts.

The U.S. needs 3.5 trillion kWh per year / (365 days/year) = 9.5 billion kWh per day.

1 person can produce 0.8 kWh per day.   So that would take 11.9 billion people peddling bicycle-generators to meet the electricity demands of the U.S.

The U.S. population is about 300 million, which is about 1/40 the amount needed. So clearly such an energy source does not scale to national demand.

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  1. Hey, how about some piratical situations. Ie. Have 1kw generator, what can you power with it and how long. How about a formula?

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  1. Why the U.S. will never run exclusively on windmills « energypolicygeek

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