Power and Energy Primer: Part 1 - The Light Bulb and the Electric Bill
What's the difference between a kilowatt and a kilowatt-hour? And why does it matter? Whatever your background may be, it's good to review the basics. Our Power and Energy Primer provides an overview of the building blocks of our energy system.
Definitions like “energy is the capacity for doing work” and “power is the rate of doing work or the rate of using energy” don’t do much to differentiate between a kilowatt and a kilowatt-hour. Last week’s Introduction to our Power and Energy Primer lays out the difference between power and energy. This part shows how the difference plays out in a real-world example: the light bulbs in your home.
What is the difference between the power demand of a light bulb versus the light bulb energy use? Consider this example.
To perform a day’s “work” of providing of light, a 60 Watt (W) incandescent lamp consumes:
60 W of power
* 8 hours
0.48 kWh of energy.
Your electricity utility charges you for your energy consumption, or power * time.
The typical American household uses 15 lights (out of the 40 fixtures a house normally has). If each requires 60 W of power, you’ll have:
15 incandescent lamps
* 60 W
0.9 kW peak power demand for lighting
and the lamps combined would consume
* 8 hours
7.2 kWh of electric energy each day.
If electricity costs 10¢/ kWh, you’ll pay 72¢ a day, or $263 per year, for the energy these lights consume.
To pay less to your electric utility, you could reduce your energy consumption and/or supply your own power.
You pay your electric utility to supply power to your house and are charged for each unit of energy your technologies consume to perform their work. So if each of your lamps provides light more efficiently, it will consume less energy.
A CFL lamp can provide the same amount of lumens, or perform the same amount of work, with about one fourth as much power as an incandescent. So if you replaced each of your 60 W incandescents with a 15 W CFL, you’d reduce your power needs to:
15 CFL lamps
* 15 W
0.225 kW of peak power demand for lighting
and the CFLs would consume:
* 8 hours
1.8 kWhs of electric energy each day.
At 10¢/kWh, your electricity bill for lighting would be 18¢ a day, or $66 per year. So by replacing your incandescent bulbs with energy efficient CFLs, you’d achieve:
- $ 66
$197 worth of energy savings each year.
And if you don’t want to pay your electric utility at all? If you’d rather keep your incandescent bulbs and power them with a solar PV panels?
A solar system’s installation and materials cost $5 per watt, and a 1 ft2 solar panel provides 8 W of power. So a system to meet the peak power demand of your incandescent lamps would cost:
$5 (per W) * 900 W = $4,500 and take up 900 W x 1 ft2/8 W = 112.5 ft2
But if you installed CFLs, you’d spend:
$5 (per W) * 225 W = $1,125 and use 225 W x 1 ft2/8 W = 28.1 ft2
of roof space to meet your peak power demand for lighting. The example above demonstrates why you should consider investing in energy efficiency before installing a solar PV system. Both approaches save energy and money, and they can be combined to maximize savings.
Reducing your power demands by improving energy efficiency increases the feasibility and cost-effectiveness of a renewable energy investment.
Photos courtesy of DOE/NREL, Credit- Dennis Schroeder, Warren Gretz, and Susan Bilo