Do you need a generator for your home or business?
Most generators have two wattage ratings – starting and running watts. But what do these numbers mean? And how do you know which one to choose?
We’ll help you identify your power requirements and calculate the total watts of appliances you need to run. Once you know your appliances’ running and starting watts, selecting the correct rating of the generator is easy.
Check the complete article for more information on choosing the correct wattage generator!
Work, Power, And Wattage
You might already be aware of these terms, but let me briefly introduce them once again, particularly in electrical parlance.
Two electrical charges of the same polarity repel each other while opposite charges attract. If you want to bring a charge +q2 near another bigger charge +q1, you must apply a force to counteract the repulsion force (known as the Coulomb force) between these two charges.
This statement can be alternately stated as you are applying force to carry out external work in moving charge +q2 against the electric field of charge +q1. The unit of measurement of charges is called Coulomb, denoted by C.
The work done (W) in moving one Coulomb of electrical charge from one point to another is known as the voltage or potential difference (V) between these two points. The work done is measured in the units of Joules (J)
That means V = W / Q, Alternatively W = QV.
A moving charge constitutes current (I), which is defined as the rate of flow of charge through a specified area.
Hence I = Q/t, or Q = It
Power & Wattage
As shown below, let us consider a circuit with a battery power source feeding a load, connected across the terminals c and d. Let I be the current flowing through it, and V is the voltage drop across these terminals.
Circuit Diagram
As discussed above, a charge q will require a work of W to be done to move between terminals c and d having a voltage drop V, where W = Vq. Also, The amount of charge moving in time t is given by q = It.
From these two equations, we get W = V . I . t
Hence the work done is the product of potential difference (V), current, and time. Power is defined as the time rate of doing any work, which implies, it is the ratio of work done with time. It is measured in the units of joules per second, also known as watts.
Hence P = W / t or P = V . I
Electrical power may also be defined as the electrical energy transferred in any electrical circuit. We say the power consumed is one watt if one Joule of electrical energy is transferred in one second.
Introduction – Starting & Running watts
Electrical motors form the largest power-consuming pool of equipment in today’s world. Three-phase squirrel cage induction motors are used in the majority of commercial and industrial applications, while single-phase inductance or reluctance motors are extensively used in almost every household appliance from any fan, refrigerator, air conditioner, or washing machine, coffee maker, air compressor, etc. The development of modern electronic controllers has further widened their base.
How do Induction motors develop power?
Let us try to understand the concepts behind how an induction electric motor develops power and why there are two wattage values. To run any motor, power is fed to the stator winding, which produces a rotating magnetic field in the case of a three phase motor and a stationary pulsating field in the case of a single-phase induction motor.
The pulsating field in single-phase motors is made up of two magnetic fields rotating in opposite directions.
This rotating magnetic field induces an EMF E2 in the rotor resulting in a current I2. The frequency of this EMF is proportional to the relative speed between the rotating magnetic field and the rotor speed. Since the rotor is stationary, the frequency of E2 is 60 Hz matching the supply frequency.
This current interacts with the magnetic field to produce torque to start turning. As the motor speed increases, the relative speed between the magnetic field and the rotor continues to decrease till the motor reaches its final operating speed, which is usually 3 to 6% below the speed of the magnetic field. This relative speed between the two is known as the slip of the motor, described by the following equation:
S = (Ns – N) / Ns = 1 – (N/Ns), where
- S is the slip of the motor,
- Ns is the synchronous speed or the speed of the rotating magnetic field,
- N is the speed of the rotor
The rotor can never match the synchronous speed, as the relative speed will become zero, and no further torque will be developed by the motor.
The current in the rotor I2 is given by the relation, I2 = E2 /{sqrt [(R2 / S)2 + X2]} amps, where
R2 and X2 are the rotor winding’s resistance and reactance, and S is the slip.
Let us assume E2 as 240V, R2 = 2 ohms, and X2 = 8 Ohms and slip is 5% (0.05) for any theoretical motor and calculate the current at the starting of the motor when slip, S = 1 and when the motor is running at operating speed with slip S = 0.05.
While starting, I2 = 240 / {sqrt [(2)2 + (8)2] } = 240/8.246 = 29.10 A.
At operational speed, I2 = 240 / {sqrt [(2/0.05)2 + (8)2] }
= 240 / {sqrt [(40)2 + (8)2] } = 240/40.792 = 5.88 A
This example shows that the starting current is approximately five times the normal running current when the motor operates continuously. The starting current is also referred to as the locked rotor current (LRA) because it flows even when the rotor has not started turning.
Why Do Generators Have Two Ratings?
These starting and running currents of the motors translate into the starting Watts and running watts of portable generators. Generator manufacturers specify both these values for any generator they market. The main difference between running watts and the starting watts is:
The running watts are also known as the continuous watts. They refer to the wattage continuously produced by a portable generator when the large motor loads imposed on it have attained near full speed at around 95% of the synchronous speed, which implies a slip of about 4 to 6%.
The starting watts refers to the amount of power required from the generator for a very short startup period when a big motor (like an air conditioner) or several medium-size motors are started at the same time, let’s say by the operation of a transfer switch. As the generator provides additional starting watts over the continuous watts, the difference is sometimes referred to as the surge watts.
Many manufacturers indicate the starting watts as their generator capacity or rated watts. This is misleading, and you should always consider the running watts as the maximum watts that the generator will be able to supply continuously.
How To Choose The Correct Wattage?
If you want to arrive at a reasonably accurate estimate of the generator size, you need to put in the effort in the planning stage and do a fairly detailed exercise identifying the watts required from portable generators. This will ensure your generator can provide enough power during outages or natural disasters.
Identify Your Power Requirements
- List all the essential appliances you want to power during an emergency. Depending on your budget, if you want to power the entire house, you can go in for a standby generator with an automatic transfer switch.
- Many generator sizing guides, manufacturers, and retailers provide a chart with approximate running watts and surge watts for different types of equipment. A sample chart can be obtained from this site.
- Fill in the list of equipment that you have selected to supply power during an emergency with the details of running and starting watts from the above charts.
- Usually, this information is also available on the data plate of the appliance. Substitute the actual value in the chart for the appliances you are able to obtain the data.
Calculate Total Watts
Having prepared the chart in the previous section, carry out the following steps:
- Total all the running watts of the appliances you have selected.
- To this total, add the additional highest starting watts (or the highest surge watts) of your selected appliances.
- The total will give you the maximum starting wattage temporarily required from the generator. While selecting any generator, ensure this value does not exceed the starting of the surge watts rating of the selected generator’s model.
- If the rating is going high and you want to reduce your costs, you can consider starting your appliances in sequence, where the highest starting current appliance is first. The next appliance started after the previous one had attained a steady speed. It
The most accurate Estimate
The above exercise will give you an indicative generator size. It does not consider the actual wattage for many of the equipment based on the actual make and models. It does not consider the load imbalance on various house wiring circuits, which may have a huge impact on the assessment of electrical loading.
The best method includes the measurement of the actual current in each of the circuits in your house wiring near the circuit breaker with a clamp-on-ammeter. To estimate your running wattage accurately, you need to multiply the largest measured current by 240. Above this, consider the additional power at the startup, similar to the previous section
In our separate article, we have explained in detail with topics like the whole house generator sizing with sizing procedure, sizing a generator for partial loads, sizing a generator or central air-conditioning units with sizing procedure, sizing a portable generator, power needs based on application, etc. You may go through to have a good understanding of the subject.
Conclusion
What are the takeaways when it comes to starting watts versus running watts? First and foremost, you need to identify your power needs during the emergency scenario. Once you know how many watts your appliances require, add them all up to get your total running wattage.
Then, list down the motor loads requiring higher watts for a very short period and find the starting watts for these selected appliances. Substitute these watts in the previous list to find the total starting watts. The sum of the running watts, the largest starting watts will, and the total of these two provide you with the size of your generator. If you have any questions or want more information on generators, please let me know in the comments section below!