What Size Or How Big A Generator Do I Need? The Best Overview

The regular electric utility supply produces a stable and reliable current which we use without giving it a second thought. Power cuts become inevitable as the power utilities become more constrained, either by meeting environmental targets or by increasing the number of customers. 

The size generator you need will be determined by the wattage of the appliances you intend to run from the machine. If you want to keep your home running seamlessly when the main supply is cut, you will need a much bigger generator than one required for emergencies only. 


With generators ranging in power capability from 2KW to more than 35KW, and prices that vary similarly, you must spend some effort calculating the size you will need before purchasing a generator.

How To Size A Generator Correctly?

Generators are not calibrated by the physical size of the machine but rather by the electrical power (measured in watts) which they can produce.

Whether you intend to run only essential equipment from the generator or choose to keep the entire home powered up, you will need to know how much power the generator will need to produce.

If the generator is too small, it will not be able to supply the power you require.

It is not just the average watts consumed by the equipment you need to consider but also the “starting watts” that appliances such as refrigerators, freezers, and air conditioners need.

These appliances may indicate a watt requirement of 100 watts, but at startup, the device has a coil that sends a burst of up to 3 times (300watts) the required watts to get it running.

As they have thermostats measuring the temperature and switching them on and off as required, you must consider this additional wattage.

There are two methods to calculate the average watts your home uses.

Your Utility Account 

Your home utility account will include a kilowatt/hour consumption number. Be careful which amount you use because several different items may use kilowatt/hours to calculate the charge depending on the area you live in.

The item on the account you are looking for will indicate the actual energy charge.

The account will reflect the value in kilowatt-hours. To convert this to watts, you use the following formula. 

watts = (kWh × 1,000) ÷ hrs.    


  1. The billing period was 31 days, then the number of hours was 744, 
  2. 6,809 Kw/h were consumed.

It would be calculated as follows.

Watts = (5,000 kWh*1000)/744

Watts = 5,000,000/744

Watts = 6,720 watts

Remember, it is the average power used by your home. 

Hidden in this figure are high consumption periods when several high-energy appliances are working together: such as air conditioners, fridges, freezers, welding machines, neon lights,  electric cookers,  electric water geysers, etc.

It also hides the low energy periods in the early morning hours, where the high consumption appliances may be restricted to fridges, freezers, etc.

Even though you now have this average, it is still a good idea to audit the home to work out the maximum possible and maximum likely consumption.

The “maximum possible” is when every electrical consumer is switched on simultaneously. In contrast, the “maximum likely” is the more realistic situation of the appliances that are more likely to be switched on simultaneously.

Audit Your Home

The audit will produce a table of consumers, and the associated wattage each will use. It may look something like the following table.

55 Inch LED TV116116YesNo
Bulbs (60w *20)12001200NoNo
9″ disc sander12001200NoNo
Air Cooler6565YesNo
Air Fryer15001500NoNo
Air Purifier2525NoNo
Apple TV33YesNo
AV Receiver450450YesNo
Coffee Maker8001600NoNo
Cooker Hood2040NoNo
Corded Drill600900NoNo
Curling Iron2537.5NoNo
Electric Blanket200200NoNo
Electric Heater Fan20003000YesNo
Electric Kettle12001200YesYes
Electric stove20002000YesNo
Electric Toothbrush Charger66YesNo
Espresso Coffee Machine13001950YesNo
Game Console120240YesNo
Garage Door Opener300450NoYes
Hair Blow Dryer18002700YesNo
Home Air Conditioner10004000YesNo
Home Internet Router515YesNo
Hot Water Geyser30003000YesNo
Pedestal Fan5050YesYes
Sandwich Maker7001050YesNo
Smoke Detector00YesNo
Space Heater20006000YesNo
Steam Iron22003300NoNo
Vacuum Cleaner450900NoNo
Wall Fan4567.5YesYes
Washing Machine500500YesYes

How Does This Help To Calculate The Generator Size?

The above table is used purely as a demonstration, and the actual values your appliances consume may be different.

It shows the watts used by the appliances once they are running and the power required to start them. These amounts are summarised as

The maximum possible power categorizes these if everything in the house runs simultaneously, the amount used by appliances that are most likely to run together. Finally, the minimum amount keeps items like the fridge and freezers running in an “emergency” situation.

KilowattsNormal RunningKilowattsStart Up
Maximum Watts32.246.8
Likely Watts 22.734.5
Emergency Watts4.25.7

You Need To Size Your Generator Correctly

Before choosing the generator, you need to decide what you will use it for.

  1. Do you need a generator to run essential equipment if the main electrical supply is disrupted?
  2. Perhaps you need it to run equipment off-site, such as traveling in an RV or using a welding machine on a work site.
  3. You need to run the entire household without any disruption.

Whatever the reason for purchasing a generator, it is essential to size it correctly. 

If It Is Bigger Than You Need 

You don’t want to purchase a machine that is too big.

It Will Be A Waste Of Money

You will waste money on the purchasing price.

It Will Cost Too Much To Run

The fuel consumption will be higher, which will cost more.

If It Is Too Small For Your Needs

If it’s too small, then you will have the following problems.

It Won’t Be Able To Run All The Equipment 

If the generator produces fewer watts, then your system requires it will not cope.

The gas engine will constantly slow down as more power is drawn,  throttling the current to your appliances. 

When this happens, you may experience flickering or dimming lights, AC units pumping slower or other symptoms resulting from your appliances not having enough power to run correctly. The generator’s circuit breaker will ultimately trip if the systems are working correctly.

You Risk Damaging The Generator

If too much power is required from the generator, it may start to run loudly or overheat.

Black soot on the generators’ exhaust shows that too much power is being drawn from the machine.

You risk an explosion if the generator is overloaded and stands next to a flammable material (such as gas). It is good practice to never store gas or other flammable items next to a running internal combustion engine.

Consistently drawing too much power from a generator will result in the life expectancy of the machine being shortened.

You May Damage You Electrical Appliances

If the generator is running at or near capacity and a fridge starts running, the fridge will draw three times the usual power to create the compressor; this only lasts for a few seconds before it reduces the draw to normal levels.

If the generator is not in poor condition, the ongoing increases and decreases in power draw will cause the gas engine to speed up and slow in turn. Even though the machine has a voltage regulator, the result may be surging in electricity, damaging more sensitive electronic equipment such as PCs, televisions, and audio equipment.


Gas (diesel) powered generators are very convenient. Although they are not generally suited for long-term permanent use, they are a great backup to the utility power supply or your solar system on cloudy days. 

You must size the equipment carefully to ensure that it is neither too large nor too small. If you can access the consumption from your utility bill, this is a good start; you will still need to factor in the variations in consumption during the day.




How Many Watts Do You Need to Run the Average Home and Appliances?