Generator Wattage Calculator
Our generator wattage calculator will help you assess the load requirements of your generator. If you're in the market for a generator (whether as a power backup or as a main power supply), the first step is determining the total load your new generator must power. In our calculator, you can choose the appliances you wish to operate with your generator and calculate the total watts your generator must supply.
In the article below, we delve into rated watts and surge watts and how to calculate appliance wattage for a generator. We'll also discuss calculating generator sizing from the estimated generator watts.
💡 If you're unfamiliar with electric power or its unit watts, we recommend our watt calculator.
Rated watts and surge watts of an appliance
An electrical appliance uses electric power to produce some output or effect:
- A refrigerator uses electric power to cool its contents.
- An air conditioner uses electricity to cool a room.
- A bulb consumes electric power to generate light.
Electric appliances have a power rating to indicate the maximum power they will consume during their operation. An appliance's rated watts or running watts is the power it consumes while operating (or "running"). Although the true power consumption will vary during the operation cycle, the rated watts reflect the maximum power consumed during its operation.
Some appliances require higher power than their rated watts when starting. This higher power needed at an appliance's start is known as surge watts or starting watts. Typically, machines that use a motor, such as an air conditioner or a refrigerator, require higher power during their start-up.
💡 Wish to calculate how much power your appliance is consuming? Our appliance wattage calculator will help you do that and more!
Calculating generator wattage
A generator must be able to power all of the appliances you wish to run simultaneously. To do this, it must generate power equal to the total of rated watts of all the appliances connected to it. In addition, it must support the surge watts required to start every piece of equipment in the circuit.
For example: suppose you need a generator as a power backup for your house, and you want it to power the following appliances during a power outage:
Electric range (8 inch element)
Light bulb (15 W×4)
LED TV (50 inch)
Your generator much be able to power all of this equipment until the power returns. So it must generate a rated wattage of 3,050 W and surge watts of 3,890 W.
Calculating generator watts is the same as calculating appliance wattage for your generator.
Is generator wattage equal to total rated watts or total surge watts?
As discussed, some appliances require higher power than usual during their start-up. If your generator can supply rated watts to run a machine but cannot produce the power needed to start it in the first place, then it's no good to you. So, surge watts must be accounted for.
That said, you might not want to start every appliance simultaneously — you could start them one after the other or in groups. In this case, you won't need to account for the total surge watts as long as the generator can account for the total surge watts of all the equipment you're starting at a time, along with the running watts of other running devices.
For example, suppose you have two motors connected to a generator. Motor A has running watts of 700 W and surge watts of 2,500 W, while Motor B has rated watts of 600 W and starting watts of 2,000 W. If you start both motors simultaneously, your generator must produce 2,500 W + 2,000 W = 4,500 W. However, if you start Motor A first and start Motor B once Motor A is running, you'll only need 700 W + 2,000 W = 2,700 W.
Regardless, not only is it safer to assume that generator wattage is equal to total surge watts — whether you start them all at once or not — any unused wattage provides a cushion for any new appliances added to the system in the future! Our generator wattage calculator assumes the desired generator wattage equals the total surge watts.
Generator sizing calculation
Estimating generator wattage is but the first step in calculating generator size:
1. Calculating apparent power
Generators also come with their own rated power, which indicates the maximum electric power they can produce. This rated power is the generator's apparent power since some of this power will be lost in the system. In an ideal generator with 100% efficiency, apparent power, and generator wattage can be equal. However, in the real world, we must account for any power loss in the generator. Otherwise, the generator will not meet our power demand. Mathematically, we have:
The term refers to the electrical efficiency, valued between 0 and 1. An international standard generator has a power factor of 0.8. For more on a generator's apparent power and power factor, head to our generator power calculator or our power factor calculator!
2. Operating capacity of the generator
Generators should not run at total capacity for long, especially if you intend to use one as a main power supply. It is advisable to operate generators at 70-80% of their full capacity.
3. Other factors
Apart from this, you'll also have to consider other factors, such as the space available on site for the generator, the feasibility of transporting and installing the generator, etc.
Estimating generator watts using this calculator
Whether you want to calculate a portable generator wattage or a standby generator for your home, our generator wattage calculator can help. All you need do is specify the rated and surge watts of the various appliances you want the generator to power. You can enter data for up to 50 appliances — more fields will appear as you need them!
Specify the category of the appliance. You can choose between:
Each category has relevant appliances you can choose from. Please select all the appliances connected to the generator, and our generator wattage will auto-fill the typical rated watts and surge watts of each appliance. You can edit these values if an appliance of yours has a different power rating.
If your appliance is not on our lists, select the Other appliance option from any list and manually enter its rated and surge watts.
The calculator will show you the total rated and surge watts under the Generator wattage section.
Our generator watts calculator will also calculate generator sizing for you! You'll find the generator's apparent power and required size under the Generation sizing section.
Advanced mode: The generator's power factor and operating capacity are set by default to 0.8 and 70%, respectively. You can customize these values by clicking on the
⚠️ Make sure you enter the rated and surge watts for each appliance! The calculator will give you the wrong result if you leave some blanks.
How do I calculate wattage for a home generator?
To calculate the wattage of a home generator, follow these steps:
- Find the rated and surge watts of all appliances you want your generator to power.
- Add together the rated watts of various appliances to get the total running watts required from your generator.
- Add together the surge watts of various appliances to get the total starting watts required from your generator.
- Verify with our generator wattage calculator.
What size generator do I need if my house needs 3,560 watts?
You need a generator of 6.357 kVA to power a 3560 W house. To calculate this yourself, follow these steps:
Assume a generator efficiency of 80%, giving us a power factor of 0.8.
Divide 3560 W by 0.8 to get the generator's apparent power:
3560/0.8 = 4450 W = 4.45 kW
Convert the apparent power from kilowatts to kilo-volt-amperes (1 kW = 1 kVA) to get:
4.45 kW = 4.45 kVA
Assume the generator operates at 70% of its max capacity.
Divide the apparent power by 0.7 to calculate the generator size:
4.45/0.7 ≈ 6.357 kVA.
How many AC can run on a 10 kVA generator?
You can run up to three room air conditioners of 10,000 BTU, which require 1,200 W rated watts and 2,200 surge watts, assuming your generator operates at maximum capacity at 80% efficiency. To manually verify this answer, follow these steps:
- Convert 10 kVA into kW (1 kVA = 1 kW):
10 kVA = 10 kW.
- Multiply 10 kW by 0.8 (80% efficiency) to determine the generator watts available:
10 kW × 0.8 = 8 kW.
- Divide 8 kW by 2,200 W surge watts of one AC to get:
8/2.2 = 3.636...
Hence, you can run 3 ACs.
Is it better to oversize a generator?
Oversizing a generator may seem appealing if you'll connect more equipment to the generator shortly. However, remember that oversizing too much can leave you with more problems than benefits — larger generators will cost more, burn more fuel, stack up carbon in their engine, etc. That's why sizing your generator correctly is essential.