When you receive your energy bill each month, you may not understand exactly how the total amount has been calculated. Every device or appliance in your household contributes to the total sum of the bill. To figure out which appliances and devices are using the most energy, you can estimate the total power requirements for each unit. This estimate also is valuable for estimating the power requirements for an alternative or backup power system.
Estimating the energy requirements and costs for powering an electronic device or appliance is very straightforward. There is a label at the back of each device that lists the watts that it uses. This is the number that you will need to figure out your energy usage and requirements. Along with the watts, you will need to estimate the number of hours each day that device or appliance is used. If you prefer to not check all your devices manually, you can purchase a device to help you to estimate energy usage. These devices range from simple wattage measuring devices to sophisticated home monitoring solutions. In this post, we will assume that you do not have access to a sophisticated home monitoring solution. If you are new to reading the energy labels on the back of your appliances and devices, please review the next few background sections so that you can obtain an understanding of electrical basics and terminology.
To understand the electrical terms listed on appliance or device labels, there are several electrical terms that need to be understood. The basic terms are voltage, current, and resistance:
• Voltage (Volt): The difference in potential energy (charge) between two points in a circuit. One point has more energy than the other point and the difference between the points is called voltage. Voltage is measured in volts.
• Current (Ampere): The flow of electrons (charge) between two points in a circuit. Current is measured in amps.
• Resistance (Ohms): Resistance is the electrical resistance (difficulty) between two points in a conductor. Resistance is measured in ohms.
Voltage, current and resistance are related through an equation called Ohm’s Law, which is:
V = I x R
where V is volts, I is current, and R is the resistance. When describing voltage, current and resistance, a common analogy that is used is “water flowing in a pipe”. The current is analogous to water flow, and the voltage is the pressure in a pipe. When the voltage (pressure) is higher, a stronger current will flow. Figure 1 shows the water analogy with (a) pressure (voltage) no current, and (b) pressure (voltage) and current.
Figure 1. Mechanical Illustration of Voltage and Current.
Electricity can also be expressed in units of power called watts. A watt is a unit of electrical power represented by one ampere of current through a circuit with a potential difference of 1 volt. Power is related to voltage and current through the following equation:
P = I x V
where P is power, I is current, and V is volts. Power (electrical energy) is measured in watts or kilowatts. It can also be measured over time. For example, a 60 W light bulb consumes 60 W at a particular instance in time. A kilowatt-hour (kWh) is the electrical energy equal to the power supplied by one kilowatt for one hour.
The electrical concepts that we described so far are examples of direct current (DC). Direct current (DC) is electrical current that flows linearly in a constant direction. There is also another type of current called alternating current (AC), which differs from DC because it changes directions. Figure 2 illustrates the difference between these two concepts. As shown, DC voltage is constant over time. AC voltage has a sinusoidal wave, which means that it varies with time.
Figure 2. Visual Difference Between DC and AC Voltage.
We can use the previous water analogy to describe AC; instead of the water flowing straight through the pipe, the water in the pipe is moved back and forth using a crank connected to a piston. Figure 3 shows an illustration of this concept. The sloshing of the liquid can be very fast at 50 or 60 cycles per second (50 or 60 Hz). Devices that are powered by fuel cells or batteries use DC power; however, devices that plug into the wall in our homes use AC power.
Figure 3. AC Analogy of pressure (voltage) and current.
To estimate your household energy usage, the following sources can help:
• Electric bills
• Equipment ratings
• Expected load profiles
By looking at your electrical bill, you can see how your watts are varying from month-to-month during the year. Your energy usage is seasonal and depends upon where you live. For example, if you live in a cold climate, your winter bills may be much higher than your summer bills due to the need for running heat during the winter. In hot climates, your summer bill may be much higher than your winter bill due to running the air conditioner during the entire summer.
Each appliance or electronic device has a nameplate that specifies the voltage, current, frequency and wattage. These are usually located on the back panel of the device. These ratings represent the maximum amount of power that can be supplied; therefore, the nameplate rating is theoretically at 100 percent utilization. Many devices do not run at 100 percent utilization; therefore, using nameplate ratings can result in inflated power requirements. An example nameplate is shown in Figure 4.
Figure 4. Electronic Device Nameplates.
Although energy usage can be calculated from your electric bills and device nameplates, actual measurements will provide more accurate data. Actual load measurements can be obtained using watt-hour meters. These actual load measurements are often used to design PV, fuel cell, and battery backup systems. The actual load is required to determine the size and cost of the alternative energy system. Often system designers will recommend the customer change their energy practices to minimize their energy usage so that the PV system can be designed to accommodate those requirements instead of installing a larger system to compensate for peak usage.
The total amount of energy used by your household can be calculated simply by using six easy steps:
1. List the number of watts for each appliance or electronic device (this is called a “load” for each device). All existing and planned electrical loads should be identified.
2. Estimate the average daily use (the number of hours each day that the appliance or electronic device is running)
3. Multiply the unit's wattage by the number of hours you use it (this will give you a certain amount of “watt-hours”). For example, if you use a 120-watt television for two hours per day. You can multiply the wattage by the number of hours used per day to obtain 240 watt-hours per day.
4. On your electric bill, the electricity is in kilowatt-hours. To compare the energy consumption in kilowatt-hours, we will need to convert watt-hours to kilowatt-hours. Since 1 kilowatt is equal to 1,000 watts, divide by 1000 to convert from watt-hours (Wh) to kilowatt-hours (kWh):
240 Wh / 1000 = 0.24 kWh
5. To compare these numbers to your electric bill, we need to convert this number to the number of hours that the appliance or device uses per month. For example, 0.24 kWh x 30 days = 7.2 kWh per month.
6. To calculate electric costs and compare the costs with your electric bill, look at your electric bill to determine how much you pay per kWh. If your bill says that you pay $0.12 per kilowatt-hour, the cost can be estimated as follows: 7.2 kWh per month x $0.12 per kWh = $0.86 per month.
You can organize these numbers as in Table 1 below.
|Electrical Load||Power (W)||Average Daily Use (hr)||Average Daily Energy (watt-hours)||Average Daily Energy (kilowatt-hours)||Average Monthly Energy (kilowatt-hours)||Cost Per Month ($)|
Table 1. Table for Calculating Average Daily Energy.
By filling out Table 1, you can obtain a good estimate of the amount of power that you use each month and the associated costs. To size an energy storage system, you will also want to look at the peak wattage required (the maximum amount of energy that could be needed in a day) and the duration of the average wattage (the longest length of time that the average wattage is needed). The average power consumption defines the total energy drawn daily.
In this post, we reviewed basic electrical terms such as voltage, current, resistance, power, direct current (DC), and alternating current (AC). We then used these concepts to calculate the energy requirements for an appliance or device. These energy requirements can be used to estimate the total energy draw and the associated costs of that energy load. The calculation of these requirements can help you to reduce your electric bill and help you to size a PV, battery backup or other alternative energy system.