At AP Electric & Generators, we want to make it easy for you find the perfect generator for your needs. As such, we have developed this guide to walk you through the process.
Residential Standby Generator
Home Standby Generators were recently dubbed the "Next Must-Have Appliance" for the home. These technologically advanced machines protect homes and families by supplying power that keeps the lights on, the air conditioner or furnace running, and food in the refrigerator and freezer safe from spoilage. They power sump pumps, medical equipment, computers, lights, and kitchen appliances.
A power outage is detected the moment it happens and the generator automatically starts. After a few seconds to reach operating speed, the automatic transfer switch isolates the home from the utility and connects it to the generator which protects both your home and the utility from unexpected line hazards. In just seconds, power is restored and life goes on uninterrupted.
Choosing the right generator is important, and the first step determines how much power you actually need. Too small a unit may not handle a central air conditioner and keep the pumps running that keep your basement dry. Using too large of a generator is inefficient and wastes fuel.
Generators are rated in watts or in kilowatts (kW), with 1,000 watts equaling 1 kilowatt. Appliances are often rated in amps, amperes, volt-amperes or sometimes watts and kilowatts. Deciding which electrical circuits and appliances to power in an emergency is the first step to sizing a standby generator for your home.
Sizing A Residential Standby Generator
First, select your generator according to its strength by deciding what appliances you need to power.
If you want your unit to run your air conditioner, then you need something strong, because a home’s air conditioner requires significant power. You can choose a generator that powers your whole home, or just the basics. So, take note of your A/C unit specifications; this will tell you the wattage required to run it. You can usually find this information on the unit, or you can estimate it based on your circuit breaker’s size.
A standby generator for your home needs to run everything during an outage that your normal electricity source would. It is possible to match the maximum amount of amps that your circuit breaker can handle, but it is not necessary. You just need enough to get you and your home through an outage.
Methods for Sizing a Residential Generator
Full Load Capacity by Measurement
Estimate the required generator capacity by taking full-load current measurements during peak usage at the service panel.
- Use a clamp-on ammeter on each leg of the electrical service and add the measurements together. This provides the total amps used by the facility.
- Divide the total amps by three for three-phase current, and by two for single-phase current, then multiply the result by the supply voltage, and again by 1,000 for Kilowatts (kW) required.
- Add the power in kilowatts used by each emergency safety system according to articles 700, 701, 702 and 708 of the NEC to the kilowatts required to obtain Full Load Kilowatts.
- Full Load Kilowatts = Total Amps x Supply Voltage / 1,000.
- Reserve Capacity = Full Load Kilowatts x 0.25.
- For 100 percent power, generator size = Full Load Kilowatts + Reserve Capacity.
Full Load Capacity by History
Utilize your utility company's billing system to find your maximum power usage.
- Most companies will include your peak demand on your monthly bill.
- Analyze the power usage over the previous year to find the highest peak demand and then add 25 percent for reserve capacity.
Full Load Capacity with Extensive Motor Use
- Find the starting current for the largest motor that turns on and off and multiply it by the voltage for the number of watts required.
- For all other motor and non-motor load, multiply the current by the voltage for watts.
- Total the watts required for the largest motor and all remaining motor and non-motor loads for the total power and multiply by 1000 for kilowatts.
- Add 25 percent for reserve/surge capacity and size the generator according the result.