How one goes about assessing the electrical consumption of a residence or business depends on whether it is an existing structure or one that’s about to be built.
Assessing Demand in Existing Structures
Assessing the electrical consumption of an existing home or business is fairly easy. Simply review monthly electric bills, going back two to three years, if possible.
If you don’t save electric bills, a telephone call to the utility company will usually yield the information you need. Most utilities will gladly provide data on energy use to their customers. Electrical consumption is listed as kilowatt-hours. Many companies also post the data online. All you need is your customer number.
If you recently purchased an existing home, you can ask the previous owners to share their utility bills. If they have not saved them, they may be willing to contact the utility to request the information. Remember, however, a house doesn’t consume electricity, its occupants do, and we all use energy differently. If you are frugal and the previous owners were not, their past energy usage data may be of little value to you in predicting your usage.
Once you have secured energy bills, calculate the total annual electrical consumption. Do this by adding the monthly totals. If you have data from two or more years, calculate a yearly average. If several years’ data are available, it is helpful to determine the range — that is, the lowest year and the highest year. It’s important to look for trends, too. For example, is energy use on the rise, staying constant, or declining from year to year? If you notice a dramatic
increase in electrical energy demand in recent years, exclude earlier data, which lowers the annual average. More recent data more closely reflects your electrical consumption. If, on the other hand, electrical energy consumption has declined, earlier data will inflate electrical demand.
If you are installing a grid-connected system, this is all the data you’ll need. You or an installer can size your system based on the annual electrical consumption and the percentage of electricity you would like to generate by your wind turbine.
If you are installing an off-grid system, you need to calculate monthly averages — that is, how much electricity is used, on average, during each month of the year. To do this, simply add up household electrical consumption for each month, and then divide by the number of years’ worth of data you have. If your records go back three years, add the electrical consumption for all three Januarys, then divide by three. Do the same for February and the remaining months. Record monthly averages on a table.
After you have calculated monthly averages, look for patterns in energy use. Are there months or seasons during which electrical demand is higher or lower than others? If you live in a hot, humid climate, for example, electrical consumption may peak in the summer when the air conditioner is operating full tilt. If you live in a colder climate in which air conditioning is not required, electrical consumption may peak in the winter because lights are on the
longest and the heating system is being used. Understanding seasonal demand is extremely important when sizing an off-grid system because you need to size your system to meet your demand during that period.
Assessing Demand in New Structures
Estimating electrical consumption in a new building — one that’s just been built or is about to be built — is more difficult, because here’s no historical energy consumption data.
One way to estimate future electrical consumption in such instances is to base it on usage in your current home. If you are building a new home that’s the same size as your current home and has the same amenities, electrical consumption may be similar to that in your current abode. However, if you are building a more energy-efficient home and installing much more energy-efficient lighting and appliances, electrical consumption could easily be 30 to 50 percent lower, perhaps even 75 percent below that in your current home. If that’s the case, adjust current electrical consumption to reflect the savings you’ll enjoy.
Another technique is a load analysis. To perform a load analysis, list all the appliances, lights and electronic devices you are installing in your new home or business. Then determine how much electricity each one uses and how many hours per day each one operates. From this data, you can approximate demand in kilowatt-hours.
Electrical load analysis is pretty straightforward. It’s made even simpler by worksheets. You can find them online. Professional renewable energy installers also often supply customers with worksheets, and may even help customers fill them out.
Begin by listing all the electrical devices in your new home or business. Rather than list every light bulb separately, however, you may want to lump them together. Once you’ve completed the list, you need to determine the amount of electricity each item consumes. This can be determined by consulting charts that list typical wattages for common electrical devices. You can find detailed charts in various books, such as Solar Energy International’s Photovoltaics:
Design and Installation Manual, or John Schaeffer’s Solar Living Source Book.You can also look at the website of WE Energies, a Wisconsin utility. It contains a chart that will help you determine the approximate electrical consumption of appliances and other devices. Their website is: webapps.we-energies.com/appliances/apply_calc.cfm.
Energy consumption in a new home can be more accurately determined by consulting the nameplate on the back or bottom of electrical devices. It typically lists the unit’s power consumption in watts. It also typically lists amperage and voltage. Unfortunately, there’s no universal standard for reporting this information. Voltage may be listed as 120 volts, 120 V, 120 volts AC, or 120 VAC. They are all the same. In some cases, the stickers only list voltage and current
(amperage). To calculate watts, simply multiply the two (watts = amps x volts). This will provide an accurate value of watts for devices such as electric heaters and incandescent light bulbs. For motors and fluorescent light fixtures, amps x volts overestimates wattage. Multiply by a factor of about 0.7 to get a better estimate of watts for these devices.
Load analysis requires use of the “run wattage,” that is, the maximum wattage an appliance draws when in operation. However, the run wattage of an appliance typically represents a worst case estimate — for example, the wattage of a TV at full volume. In such instances, you can reduce the nameplate wattage by about 25 percent (multiply wattage by 0.75).
Another way to determine wattage is to measure it with a watt meter like those shown in Figure 4.1. The meters are plugged into electrical outlets and electronic devices are then plugged directly into them. The meter indicates instantaneous power (watts).
After you have recorded the run wattage of each device, you need to estimate the number of hours each device is used each day. Multiplying the two yields watt-hours. You then need to determine the number of days each device is used during a typical week in your home or business. From this information, you calculate the weekly energy consumption of all your devices — lights, appliances,
Fig. 4.1: The cleverly named Kill A Watt meter (a) and Watts Up? Meter (b) are valuable tools for measuring the wattage of electronic devices and also ferreting out phantom loads.
stereos, tools, etc. — in watt-hours. You then divide this number by 7 to obtain the average daily consumption in watt-hours.
While straightforward, this system is tedious and subject to error. That’s because it is difficult for many of us to accurately estimate how long appliances and lights run on a typical day. For example, how many hours do you operate your toaster each day? Is it three minutes, five minutes, or ten? And how often and how long do you run the blender? How long is the kitchen light on? How long does the refrigerator run?
Another problem is that electric consumption varies by season. Electrical lighting, for example, is used much less in the summer than the winter, because days are longer in the summer and people often spend more time outdoors. In addition, furnace blowers operate a lot during the winter, but very little in the late spring and early fall and not at all in the summer. So what’s the average daily run time of the furnace blower?
Another problem with this approach is that many electronic devices draw power when they’re off. These are known as phantom loads. Television sets, VCRs, satellite receivers, cell phone and laptop computer chargers, and a host of other devices all draw power after you’ve turned them off — sometimes nearly as much as when they’re operating. Phantom loads typically account for five to ten percent of the monthly electrical consumption in US homes and, therefore, can add to the overall load, although they are rarely factored into a load analysis.
Because of these problems, homeowners often grossly underestimate their consumption of electricity. If you use this technique, be careful.
To avoid underestimating electrical demand, it’s not a bad idea to compare projected energy use determined by a load analysis to the energy consumption in your existing home or business. If your estimate and actual energy usage differ considerably, and you can’t explain why, you may want to start over.
