How Do I Calculate What Size Generator I Need?

When it comes to generators, size matters a lot. A generator too big for the application can lead to power wastage, thus unnecessary expenses. Meanwhile, a generator too small for the application may fail to power all your appliances and devices.

For the above reasons, you must determine the correct size generator based on the task at hand. The following is a 3-step guide to help you pick just the right size.

1. Calculate your load size

The first thing that you should have in mind is why you’re buying the generator in the first place. Where will you use it, and which appliances and devices will it power once you bring it home?

The only way to find out is to list everything you plan to power with your generator. This varies widely from one user to another and one home to the next.

How much power do you need

For instance, if shopping for whole-home generators, two users may need completely different generator sizes if they have different home sizes. The user with a larger home will need a bigger generator. Two users may also need different generator sizes based on appliance sizes. A bigger refrigerator, for instance, draws more power.

Another critical factor is the number of appliances. You need a larger generator to power more appliances. For instance, if you desire a backup generator to power your computer systems during a blackout, the ideal size will directly depend on the number of computer devices.

The following is a summary of typical power ratings for common electricity-powered equipment and appliances.

RECREATION & CAMPING ESTIMATED RUNNING WATTS ADDITIONAL STARTING WATTS
AM/FM Radio 100
Box Fan 100
Cell Phone Battery Charger 25
Inflator Pump 50 150
RV Air Conditioner (13,500 BTU) 700 1500
GENERAL PURPOSE & EMERGENCY BACKUP ESTIMATED RUNNING WATTS ADDITIONAL STARTING WATTS
Coffee Maker 1000
Computer with a 17″ Monitor

800
Microwave – 1000 Watts

1000
Refrigerator/Freezer 700 2200
Sump Pump ½ HP 1050 2150
Television – 27″ 500
Space Heater 1800
Window AC – 12,000 BTU 3250 3950
Humidifier 175
JOBSITE & CONSTRUCTION ESTIMATED RUNNING WATTS ADDITIONAL STARTING WATTS
Air Compressor – 1 HP

1400 3600
Airless Paint Sprayer – 1/3 HP

600 1200
Belt Sander – 3″

1200 2400
Bench Grinder – 8″

1400 1100
Circular Saw – 7-¼”

1400 2300
Hand Drill – ½”

600 900
Miter Saw – 10″ 

1800 3300
Quartz Halogen Work Light

1000
Reciprocating Saw

960
Table Saw, 10″ 2000 2000

The total wattage will tell you the amount of electrical power needed, and from there, you can work out the minimum electrical input required from a generator. Fortunately, you can get appliance wattage information from the equipment’s nameplate or the manufacturer’s guide.

Example calculation
With this example you need a generator that produces at least
3950 Running Watts and 6350 Starting Watts.

On the same note, keep in mind that electrical appliances may have different starting and running wattage requirements. Typically, it takes more energy to start motor-powered equipment than to run the motor.

Make sure to factor in the initial surge when sizing the generator. Otherwise, your generator might fail to jumpstart your systems.

2. Converting kilowatts to kilovolt-amperes

When you sum up the maximum required wattage for your equipment, you will end up with the total sum in kilowatts. This is the actual power the load needs to produce a useful working output.

However, generators are rated in kilo-volt-amperes, not kilowatts. A kilovolt-ampere is the measure of apparent power which means the total amount of power in use in a system.

In an efficient system, 1kW=1kVa. But, unfortunately, no system is 100% efficient. Therefore, not all the system’s apparent power goes into producing useful work output.

For this reason, you need to convert kilowatts to kilovolt-amperes depending on the efficiency of the electrical system. Electrical efficiency is expressed as a power factor ranging from 0 to 1. The closer the power factor to 1, the more efficient the system.

The power factor is crucial for matching the size of your load to a generator. For instance, a generator rated at 100kVa and a 0.8 power factor will be too small to power a device that requires 100KW of actual power. Instead, the device would need at least 100/0.8 = 125KVA.

Choosing the proper generator size is easy once you have a rough estimate of the required power in kilovolt amperes. Just make sure to go for a generator with some allowance. This will give you some space for adding an appliance or two in the future. It also helps manage occasional underperformance due to adverse operating conditions, such as extreme temperatures, high altitudes, and dusty environments.

3. Running requirements

Finally, you should also choose the size based on running requirements. For instance, consider whether the generator will be the primary source of power or not. This is very important since you don’t want a generator running continuously at maximum capacity for more than 30 hours.

Therefore, if you will use it as your primary power source, buy a 70%-80% bigger capacity. Also, leave a small margin for safety precaution and performance challenges.

Another important consideration is whether the generator has multiple motors and how many motors you intend to engage at a time. For example, if you intend to engage two or more motors on the same generator, you can do with a smaller size.

Lastly, some generators can be paralleled to create a bigger power source. Generator paralleling means connecting two or more generators in parallel. Doing so enables you to create a more potent power source from smaller-sized generators, meaning you can pick several smaller generators even for a major application.

Whole-House Energy Monitoring Systems

If you want more detailed data on your home’s energy use (as well as the ability to measure the energy use of 240-volt appliances), you might consider installing a whole-house energy monitoring system. The features of these systems vary, and the cost and complexity depends on the number of circuits you want to monitor, the level of detail of the data, and the features available. The monitor can be as simple as a  “plug load” monitor that plugs into an outlet; then you plug the device/appliance into the monitor. Typically, an LED screen displays the consumption.

Here are some recommended products to monitor your energy use:

Emporia Smart Home Energy Monitor with 16 50A Circuit Level...
Sense Energy Monitor – Track Electricity Usage in Real Time and...
Eyedro Home Solar & Energy Monitor - Track, React, Save Money -...
Emporia Smart Home Energy Monitor with 16 50A Circuit Level...
Sense Energy Monitor – Track Electricity Usage in Real Time and...
Eyedro Home Solar & Energy Monitor - Track, React, Save Money -...
1,349 Reviews
2,091 Reviews
439 Reviews
$149.99
$300.13
$149.00
Emporia Smart Home Energy Monitor with 16 50A Circuit Level...
Emporia Smart Home Energy Monitor with 16 50A Circuit Level...
1,349 Reviews
$149.99
Sense Energy Monitor – Track Electricity Usage in Real Time and...
Sense Energy Monitor – Track Electricity Usage in Real Time and...
2,091 Reviews
$300.13
Eyedro Home Solar & Energy Monitor - Track, React, Save Money -...
Eyedro Home Solar & Energy Monitor - Track, React, Save Money -...
439 Reviews
$149.00

Summary

Hopefully, you now know how to pick the right power generator for your next application. Contact us if you need further guidance.

How Do I Calculate What Size Generator I Need?

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