What Size Generator Do I Need?

What Size Generator Do I Need

When purchasing a generator, you want to find the ‘goldilocks zone’. You need to have a generator big enough that you have the power to run everything you want safely and without damaging your generator. Still, you don’t want a generator that is bigger than required either as that will hit your bank balance hard. 

Rather like packing for your summer holidays, it is easy to get it wrong either way and end up with too much that you’ll never need or not have enough to get by. It often takes a few tries to get it right when you are packing for a holiday – you can always tell people who go away a lot because they manage to travel light but still have everything they need.

However, when you are a first-time buyer of a generator, the consequences and costs involved in getting the sizing wrong can be worse than just forgetting your speedos. 

Thankfully, this useful guide will show you how to size a generator so you have the right generator for all your needs without overspending 

What Size Generator Do You Need? 

The first question you need to ask yourself is what will your generator be used for and how often is it going to be used. These questions can also be thought of in terms of whether your generator is for home or work use.

Residential vs Industrial

Generators vary in size, that is the capacity or power output, enormously. Generators for home or residential use start at around 5KW and go up to 50KW of power output. Industrial generators start at 50KW and can go up to a huge 3000KW. 

So, when thinking about the size of a generator, you need to first decide on the use of the generator, whether it is for industrial or residential use. 

Process Of Sizing 

Once you know whether the generator you need is for industrial or home use, you then need to begin to think about what your generator will be powering. 

List Of Items That Will Be Powered

First of all, make a list of everything that your generator will need to provide power for. It is important to include and think of everything. If it’s for home use, be sure to include all essential items required for day-to-day living – water, heat, light and kitchen use, such as fridge/freezer and cooker as well as other essentials such as phone chargers and laptops. 

Determine Starting Wattage 

As a rule, electrical equipment, whether it is your electrical appliances at home or your power tools or other electrical equipment at work, will have two wattage numbers of their labelling or manufacturer’s guide (if the label is for whatever reason unavailable). There is a starting wattage, which is a higher number and a running wattage. 

Your generator MUST be able to manage the starting wattage of all the appliances it is expected to run. 

Generator Label Checks 

It is always advisable to buy a generator from a reputable source, such as ourselves at Generator Pro, to be sure that you are getting a safe, well-running generator and also that it is the size you believe you are buying.

You can also check the power output of your generator by looking at its labelling. This is essential for knowing how much you can run from it at any one time. 

Generators should have three types of tags or labels visible: data tags, which will have the output and other engine specifications, caution labels will help you ensure you are running your generator properly and warning/danger labels will help you ensure you are running your generator safely to not endanger yourself or others. 

What If I Can’t Find The Label?

All appliances are different so if you can’t find a label on the product itself, we would always advise googling the model number. If you are unlucky with your search and still can’t find a power rating, here is a rough idea of the power consumption of different products:


Approximate Starting Wattage
Approximate Running Wattage
240V required?
Refrigerator or Freezer (Energy Star)1200132-192 
Microwave Oven
650 watts10001000 
800 watts13001300 
1000 watts15001500 
Incandescent Lightsas indicated on bulb (i.e. 60W)as indicated on bulb (i.e. 60W) 
Furnace Fan, gas or fuel oil 
 1/8 Horsepower   500300Y
 1/6 Horsepower750500Y
 1/4 Horsepower1000600Y
 1/3 Horsepower1400700Y
 1/2 Horsepower2350875Y
 Tube type300300 
 Flat Screen (20”)120120 
 Flat Screen (46″)190190 
 Coffee Maker (4 cup)600600 
 Dishwasher (Cool Dry)540216 
 Electric Fry Pan15001500 
 Electric Range (8-inch element)21002100Y
 Automatic Washer12001200 
 Clothes Dryer (Electric)67505400Y
 Radio50 to 20050 to 200 
 Sump Pump 
 1/3 Horsepower1300800Y
 1/2 Horsepower21501050Y
 Window Air Conditioner (10,000 BTU)22001500 
 Monitor (LCD style)3030 
 Hot Water Heater45004500Y
 Garage Door Opener1420720 
Approximate Starting Wattage
Approximate Running Wattage
240V required?
 Tube type300300 
 Flat Screen120120 
 RV Air Conditioner   
 11000 BTU1600 1010  
 13500 BTU28001800 
 15000 BTU 3300 2000 
 RV Refrigerator 600180 
 Electric Grill (tabletop) 1650 1650 
 Slow Cooker170-270170-270 
 Hair Dryer (1600 watts)19001800 
 Microwave Oven (650 watts)10001000 
 Coffee Maker600600 
 Radiant Heater13001300 
 Laptop computer200-250200-250 
 Satelite Receiver 250 250 
 Radio50 to 20050 to 200 
 Two-Way Radio   
 Fan (portable)120 40 
 DVD Player350 350 

Power Tools:

Approximate Starting WattageApproximate Running Wattage
 Air Compressor
 1/2 hp1600975
 1 hp45001600
 Bench Grinder (8 in.)25001400
 Circular Saw (Heavy Duty, 7 1/4 in.)23001400
 Concrete Vibrator
 1/2 hp840 (avg.)840 (avg.)
 1 hp1080 (avg.)1080 (avg.)
 2 hp1560 (avg.)1560 (avg.)
 3 hp2400 (avg.)2400 (avg.)
 Demolition hammer1260 (avg.)1260 (avg.)
 Drain cleaner250 (avg.)250 (avg.)
 3/8 inch, 4 amps600440
 1/2 inch, 5.4 amps900600
 Electric Chain Saw (14 inches, 2 hp)11001100
 Hand Drill (1/2 in.)900600
 High-pressure Washer (1 hp)36001200
 Rotary hammer1200 (avg.)1200 (avg.)
 Table Saw (10 in.)45001800
 Industrial Motors  
 Split Phase
 1/8 Horsepower1200275
 1/4 Horsepower1700400
 1/3 Horsepower1950450
 1/2 Horsepower2600600
 Capacitor Start Induction Run
 1/8 Horsepower850275
 1/4 Horsepower1050400
 1/3 Horsepower1350450
 1/2 Horsepower1800600
 3/4 Horsepower2600850
 1 Horsepower30001000
 1 1/2 Horsepower42001600
 2 Horsepower51002000
 3 Horsepower68003000
 4 Horsepower98004800
 Capacitor Start Capacitor Run
 1/8 Horsepower600275
 1/4 Horsepower850400
 1/3 Horsepower975450
 3/4 Horsepower1900850
 1 Horsepower23001000
 1 1/2 Horsepower32001600
 2 Horsepower39002000
 3 Horsepower52003000
 4 Horsepower75004800
 Fan Duty
 1/4 Horsepower1200650


Approximate Starting Wattage
Approximate Running Wattage
  Electric Fence, 25 miles250250
  Milk Cooler18001100
  Milker (vacuum pump, 2hp)23001000
  Portable Heater (kerosene, diesel fuel)
  50,000 BTU600400
  90,000 BTU725500
  150,000 BTU1000625
  Battery Charger
  15 amp380380
  60 amp with 250-amp boost1500/57501500/5750

Total Power Requirements 

The time your generator is going to be used for will also affect the total power requirement that you need. As a general rule, to keep your generator running efficiently for years to come, you don’t want to be running it at its maximum capacity for more than half an hour. 

Therefore, you need to work out how long your generator is going to be put to work. 

If it is going to be for short bursts of time, i.e. for power tools that will be used sporadically or for charging and brief use while camping, then running at maximum capacity shouldn’t be too much of a problem. 

If it is to power your house in the case of a power outage that could last a day or more, or any other circumstance where it needs to be running for long periods, you need to allow for a 20-30% margin of excess capacity. 

Consult Our Experts

Generators are not a short-lived purchase. You want to be sure that when you make this investment that you are making the right choice. 

At Generator Pro, we are always happy to help you make the right choice for your needs using our in-house expertise and experience. Whether you are replacing a generator or buying one for the first time, we can help. If you are a generator newbie, try our first-time buyer’s tool to get you started. 

We are genuinely passionate about two things: generators (of course) and saving YOU money. So, why would you go anywhere else!

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