LED bulb against blue background

Knowing a few basic but commonly misunderstood energy terms can help you make informed choices to reduce your energy use and electricity bill.

Just a bit of knowledge and awareness can empower every customer, said Daniel Hogan, Tucson Electrical Power’s Manager of Demand Side Management Programs and Services.

The unit of measurement most commonly used to track electric use is the kilowatt hour (kWh). This represents one kilowatt of electricity – or 1,000 watts – supplied for one hour. Kilowatt hours are used to calculate parts of your monthly bill. Leaving a 100-watt light bulb on for 10 hours will consume 1 kWh of energy.

The average TEP residential customer uses about 10,000 kWh per year. Because each kWh costs about 11.5 cents per kWh, average customers pay about $3 per day to power everything in their home.

TEP has a variety of programs to help customers lower their energy usage, such as rebates on energy-efficient air conditioners and heat pumps and variable-speed pool pumps and discounts on LED bulbs and shade trees.

Light Bulbs

Several terms are used to describe the energy usage, power and color associated with a light bulb.

The number of watts indicates the power consumption. Common incandescent bulbs are 40-watt, 60-watt and 75-watt. A 13-watt LED bulb or a 23-watt CFL bulb is equivalent to a 75-watt incandescent bulb.

Lighting power, or output, is measured in lumens. More lumens means a brighter bulb. A 60-watt soft white incandescent bulb provides about 840 lumens.

The color emitted by a light source is measured in degrees of Kelvin (K). As an object gets hotter, its glowing color shifts, moving from deep reds to oranges and yellows and, ultimately, white. Match flames and low-pressure sodium lamps are 1,700K, while incandescent bulbs range between 2,700K and 3,300K. Outdoor light on an overcast day measures at 6,500K.

Air Conditioners

An air conditioner’s Seasonal Energy Efficiency Ratio (SEER) indicates how much cooling it produces for each unit of energy it consumes, with a higher number standing for greater efficiency.

When purchasing an air conditioner, you should take into account both the overall power and the efficiency of the unit. It might be helpful to think of these ratings as you would the more familiar terms associated with automobiles.

A SEER rating, which can range from the federal minimum of 13 to the current high of 23, measures operating efficiency – just like a car’s miles-per-gallon rating.  The amount of heat an air conditioner can remove each hour is rated in tons – typically 2 to 5 for residential units – which is similar to the horsepower or cylinder count of a car’s engine.


The rate of heat loss through windows is indicated by the U-factor. The lower the number, the greater the window’s resistance to heat loss.

The Solar Heat Gain Coefficient (SHGC) of a window is best described as a ratio of the amount of solar heat allowed through a window on a scale of 0 to 1, with 1 equaling the maximum amount and 0 equaling the least amount. A SHGC of .30 means 30 percent of the available solar heat can pass through a window.

High performance double-pane windows can have a U-factor of 0.30. For comparison, an old metal casement window might have a U-factor of 1.3.


The thermal resistance of insulating materials used in building and construction is represented by their R-value. A higher value means more efficient insulation.

The insulation value for traditional walls typically ranges between R13 and R19, while the insulation value for the top of a house typically is between R30 and R38.