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How to Size a Generator for Home [Watt List]

DISCLAIMER: AS AN AMAZON ASSOCIATE I EARN FROM QUALIFYING PURCHASES. THIS POST CONTAINS AFFILIATE LINKS THAT WILL REWARD ME MONETARILY OR OTHERWISE WHEN YOU USE THEM TO MAKE QUALIFYING PURCHASES. FOR MORE INFORMATION, PLEASE READ MY EARNINGS DISCLAIMER.

Purchasing a generator is usually a large investment for a homeowner or business owner. It’s also something most people don’t do very often and have limited knowledge of. What fuel to use (gasoline, diesel, natural gas, propane), which company to pick, what size generator to use? – Are all common questions that need to be answered.

To size your generator properly, your best bet will be to have a licensed electrician check your requirements out and recommend what you need. If this is not an option for you, you can find out the approximate wattage needed by looking at equipment labels and size it by doing some calculations. All you have to do is add all the wattage that your appliances require to use at the same time and add 10-20% on top of that. This number will be the generator capacity that you need.

Why add a 25% “cushion”? This will allow you more flexibility if you decide to upgrade your equipment or if the generator starts underperforming, you will still have your equipment going. Of course, you can get away with getting a generator size that matches your needs, but very often, it does not work out.

It is very common for people to choose a generator that is too small for their needs (due to improper calculations in the first place) and since this is the equipment that you usually rely on in the event of an emergency, it may fail you at a VERY wrong time!

It is A LOT BETTER (and more practical) to choose a generator that is too strong rather than too weak. You will not have an overload problem and its operational life will be extended. Also, there will always be an option to use more power if needed.

If you want to keep your generator working in abnormal operating conditions, such as extreme temperatures or high altitudes, you will need a 10-20% than your needs to “cover-up” possible underperformance. There is also surge-producing equipment that you need to be covered for.

You are always at risk of losing electrical power to your appliances and overloading your generator if somehow your wattage is miscalculated, or appliances simply got old and started demanding more output. If the operating or surge wattage of a generator does not fulfill your demands, it will not work properly for you.

Surge watts vs running watts

When some motors are being turned ON, they may require a large initial “surge” of energy. This is referred to as “surge watts”. Surge watts are substantially greater than operating watts, which are needed to keep an appliance functioning after it has been switched on.

Running watts” is a measurement of how much energy an appliance consumes when it is turned on. Devices that do not have motors (that must be initiated with an electrical surge), will only need to count this type of rating.

Typically, a generator’s operating watts are rated at roughly 90% of its surge watt capability. Even though it makes sense, sometimes how products are sold, can be misleading.

Watch very carefully what you are buying. For example, this generator is rated for 4000 surge watts (which means it will only give your 4000-watts for a short period of time, few seconds at best, check manufacturer’s docs), but it only has 3550 of running watts:

This is your REAL number of watts:

Now you understand the difference between the generator’s operating and surge watts. This is not complicated, as long as you are aware of these two numbers.

Generator sizing

Now, let’s look at the sizing. Choosing a right-size generator may appear difficult, but it is actually fairly simple. All you have to do is add the wattage of all equipment that you want to power at the same time.

Let’s say that when electricity is out, you would like to use your biggest and most critical equipment during summertime – a refrigerator and an air conditioner. If you size your generator just to accommodate these two items when you start a microwave (when they are working), you may experience a problem with your generator and it may shut down.

This is why you need to DECIDE what you will be using as a generator load. Here are approximate wattages for most common household or shop equipment (just to give you an idea, so you wouldn’t miss anything):

Lights

 Devices
Watts (running) Watts (surge)
Light Bulb (60 watts) 60
Light Bulb (75 watts) 75
Outdoor Light String 250
Outdoor Bug light 100
Christmas lights (50-string) 20
Tube Light (1500mm) 22
Quartz Halogen Work Light (300w) 300
Quartz Halogen Work Light (500w) 500
Quartz Halogen Work Light (1000w) 1,000
Lights (incandescent)
100
 
Lights (fluorescent) 90
125
Lights (flood) 500
 

Personal

 Devices Watts (running) Watts (surge)
Hair Dryer 1,000 2,000
Curling iron 1,500
Iron 1,000
CPAP machine 30-60
Treadmill 280 900

 Electronics

 Devices Watts (running) Watts (surge)
TV (22” LED) 17
TV (49” LED) 85
TV (82” LED) 230
DVD / Blue Ray 30
Radio
70
Computer (desktop)
80
Computer (laptop)
200
Monitor 75
Printer (laser) 400
Printer (inkjet) 35
Fax machine
105
UPS (small)
500
850
UPS (large) 3,000
6,500
Game console 40
Phone charger 25
Satellite Receiver 30-250
Router 600
Home Sound System 95

Air

 Devices Watts (running) Watts (surge)
Air Purifier 25
Air Cleaner 250
Humidifier (tabletop) 177
Dehumidifier 390

Water

 Devices Watts (running) Watts (surge)
Water heater 4,000-4,500
Water Heater (Immersion) 3,000
Water Heater (Electric) 2,000 3,000
Water Heater (Tankless) 6,600 2,200
Hot tub pump (1hp) 900
Hot tub heater 6,000
Filter Pump (swimming pool) 1,200-1,400
Sweep Pump (swimming pool) 900
Waterbed heater 5,000
Aquarium (20 gal.) 150

Heating

 Devices Watts (running) Watts (surge)
Gas/Oil Furnace Fan Blower (1/8 HP) 300 500
Gas/Oil Furnace Fan Blower (1/6 HP) 500 750
Gas/Oil Furnace Fan Blower (1/4 HP) 600 1,000
Gas/Oil Furnace Fan Blower (2/5 HP) 700 1,400
Gas/Oil Furnace Fan Blower (3/5 HP) 875 2,350
Central Electric Furnace (15 kw) 15,350
Central Electric Furnace (20 kw) 20,490
Central Electric Furnace (25 kw) 25,670
Heat Pump 4,800 10,800
Electric Heater 1,000 2,000
Space Heater (small) 800
Space Heater (large) 1,800
Oil Filled Radiator (750w) 750
Electric Thermal Radiator 750
Electric Fireplace (heating mode) 1,500
Electric blanket 200

Cooling

 Devices Watts (running) Watts (surge)
Attic Fan 370
Ceiling Fan 150
Box Fan (20-inch) 180
Air conditioner (7k BTU) 1,000 2,200
Window AC (10k BTU) 1,200 3,600
Window AC (12k BTU) 3,250 9,750
Central AC (10k BTU) 1,500 2,200
Central AC (20k BTU) 2,500 3,300
Central AC (24k BTU) 3,800 4,950
Central AC (32k BTU) 5,000 6,500
Central AC (40k BTU) 6,000 6,700

House (major appliances)

Devices Watts (running) Watts (surge)
Refrigerator/Freezer (side-by-side) 1,120 2,200
Fridge/Freezer (18 cu. ft, frost-free) 720
Fridge/Freezer (24 cu. ft, frost-free) 810
Refrigerator/Freezer (18 cu. ft) 630
Refrigerator/Freezer (24 cu. ft) 720
Freezer only (12 cu. ft) 650
Freezer only (24 cu. ft) 845
Electric Oven 3,410
Microwave 1,450
Vacuum cleaner (regular) 800
Vacuum cleaner (heavy-duty) 1,225 2,500
Washing machine 1,150 3,400
Electric dryer (clothes) 750 1,800
Gas dryer (clothes) 650 720
Dishwasher (cool dry) 700 1,400
Dishwasher (hot dry) 1,450
Electric stove (8” range) 2,100
Electric stove (6” range) 1,500
Electric stove (1 element) 1,500
Electric range (4 elements) 12,500
Fan (whole house) 500 650
Fan (Pedestal) 50 60
Fan (Ceiling) 60 70
Fan (Table) 10 15

House (small kitchen appliances)

Devices Watts (running) Watts (surge)
Blender 300 900
Coffee Grinder 1,500
Coffeemaker 550 1,000
Espresso Coffee Machine 1,300
Electric Fry Pan 1,300
Griddle/Deep Fryer 1,200
Slow Cooker 200
Rice Cooker 200 500
Bread Maker
600
2,300
Sandwich Grill 1,160
Barbeque Grill
1,350
Food Processor 350 500
Electric kettle 1,200 3,000
Percolator 800
Toaster (2-slice) 800 1,600
Toaster (4-slice) 1,150
 
Toaster oven 1,440
Waffle Iron
1,120
Air Fryer 1,500
Electric Can Opener 168
Food Dehydrator 800
Broiler 1,350
Hot Plate
660
Induction Hob 1,400
Garbage Disposal
700

 Garage

Devices Watts (running) Watts (surge)
Security system 500
Garage Door Opener (1/4 HP)
550 1,100
Garage Door Opener (1/3 HP) 775
1,400
Garage Door Opener (1/2 HP) 857 2,350
Trash disposal 450
Pump Sump at 1/2 hp 1,050 2,150
Pump Sump at 1/3 hp 800 1,300
Inflator Pump 50 150
Winch 1,800 5,400
Engine Block Heater 150-1,000
EV Home Charger 1,600 1,800
Drain Cleaner 250

 Yard

Devices Watts (running) Watts (surge)
Pressure washer (1.0HP) 1,200 3,600
Electric Lawn Mower 1,440 4,320
Weed Trimmer 500
Edger Trimmer 960 2,400
Hedge Trimmer 450

Farm equipment

Devices Watts (running) Watts (surge)
Well Pump (1/3 HP) 750 1,500
Well Pump (1/2 HP) 900 2,000
Well Pump (3/4 HP) 1,500 3,000
Well Pump (1 HP) 2,000 4,000
Well Pump (1.5 HP) 2,500 5,000
Well Pump (2 HP) 3,700 7,500
Well Pump (3 HP) 5,000 10,000
Well Pump (5 HP) 7,500 15,000
Well Pump (7.5 HP) 10,000 20,000
Well Pump (10 HP) 15,000 30,000
Water Pump (1/3 hp) 250
Water Pump (1/2 hp) 1,120
Electric Fence (25 miles) 2,500
Stock Tank Water Heater 1,150
Heat Lamp 250

Shop or worksite

Devices Watts (running) Watts (surge)
Motor (1/4 HP) 600 1,000
Motor (1/2 HP) 875 2,300
Airless Sprayer (1/3 HP) 600 1,200
Air Compressor (1/4 HP) 975 1,600
Air Compressor (1 HP) 600 4,500
Electric drill (1”) 720-1,000 1,800
Electric Drill (1/2” @ 5.4 Amps) 600-700 900
Electric Drill (3/8” @ 4 Amps) 440 600
Hammer drill 1,000 3000
Impact Wrench (1”) 1,200 1,400
Cordless Drill Charger 380-5,750
Belt Sander 1,200 2,400
Belt Sander (3”) 1,000
Disc Sander (9”) 1,200
Orbital Sander 1,200 2,600
Disc Grinder 2,000 4000
Bench Grinder (8”) 1,400 2,500
Electric Welder 7,800-9000 10,100-11,700
Table Saw (10”) 1,800 4,500
Radial Arm Saw 2,000
Reciprocating Saw 960
Circular Saw (6 1/2”) 1,000 2,000
Circular Saw (7 1/4”) 1,500 3,000
Circular Saw (8 1/4”) 1,800 3600
Band Saw (14”) 1,100 1,100
Chain Saw (12”) 1,100 1,100
Miter Saw (10″) 1,800 1,800
Jig Saw 300
Rebar Cutter 2,800
Hammer (Demolition) 1,260
Hammer (Rotary) 1,200
Concrete Vibrator (0.75 HP) 850 1,900
Concrete Vibrator (1.00 HP) 1,100 2,500
Concrete Vibrator (2.00 HP) 1,800 3,600
Jointer / Planer (6”) 1,800 1,800
12′ Concrete Cutter 1,800 3,600
Vacuum pump 2 HP 1,000
Heater (kerosene) 400-625 600-1,000

Travel (RV, boat, van, truck, etc.)

Devices Watts (running) Watts (surge)
RV Roof-top AC (7K BTU) 600 1,700
RV Roof-top AC (10K BTU) 700 2,000
RV Roof-top AC (13.5K BTU) 1,250 2,750
RV Roof-top AC (15K BTU) 1,500 3,500
RV refrigerator 180  
Mini freezer (chest) 350-600
Electric Water Heater (6 gal.) 1,440
Furnace Fan (1/3 HP) 700 1,400
Portable heater 1,500  
Portable fan 10-250 30-450
Electric grill 1,650

** Note! These numbers are approximate, check with your equipment manufacturer (you can use this handy pdf file with a checklist to write down your power needs)

** Note #2! Devices that are plugged into your generator (even if you turn them OFF) will still draw power. Refrigerators, even though they are ON all the time, work in a standby mode. They cycle on and off as needed to keep interior temperatures consistent.

Calculations

Now, select all of the equipment you want to power in no time. IF your appliance or any device is showing only amps and not watts, here is a formula to convert them:

Amps x Volts = Watts

The good news? Your generator does NOT have to accommodate ALL the equipment that you need, only whatever you will use at exactly the same time. Keep your own wattage list handy and pay attention to what you are turning ON during generator usage times.

So, what’s the main rule for choosing generator wattage:

Main appliance and electronics that you will use at the SAME time + 10-20% = continuous power of your generator (in watts)

** Important! Make sure to watch your surge power requirements as well!

Which generator is right for you?

Let’s start with what are your plans for your generator? Do you want a generator that can be used in an emergency or for camping excursions and remote construction sites? Maybe you just want the best quality generator for your electronics (during short trips)?

There are three basic types of generators out there:

  1. Handheld. This is very popular as an inverter-generator version, which is the best for your electronic equipment (as long as you get a pure sinewave version)
  2. Portable. This can be either inverter-generator or regular generator and it’s the best for construction sites and RV traveling
  3. Standby. These are good generators for powering your whole house or shop when electricity from utilities is not available.

Extreme oversizing is not a good idea as well, since bigger units do require extra fuel and maintenance. Not to mention, why would you want to spend more than necessary anyway? The main question to ask yourself in this case is:

  1. Do you live in a region where rolling blackouts are normal and anticipated? If you know about it, might as well prepare for it.
  2. Do you live in an area where natural disasters, like heavy storms and hurricanes, are frequent? *@#! Happens! Life does not stop during bad weather!
  3. Are you providing a service that cannot handle ANY lack of power? In this case, using a UPS (or Uninterrupted Power Supply) system in conjunction with your generator is highly recommended.

If you say YES to any of these, then, a regular standby generator will do a good job for you. If you seldom get blackouts, you might want to acquire a smaller generator and use it in case of emergency.

Don’t forget that larger generators consume more fuel than smaller generators. It’s important to keep this in mind when estimating the cost of a new or used generator. Not only would you have to refill larger generators more often, but you will also need to store extra fuel on-site (or installing additional storage tanks.)

Should you parallel a generator?

If your power requirements are substantial, you might be considering paralleling several generators to get the right output. What will that do for you?

  • Having multiple generators is like a hedge against a disrupted power supply that can cause a delay or halt in your business’s operations. You will be able to “delegate the burden” to another generator if one of them fails.
  • Once you exceed a certain generator capacity, paralleling becomes more cost-effective than using a single device.
  • It might be easier to fit two or three parallel generators into a room than one oversized device.
  • Load sharing between two or more generators will prolong their lives and reduce the risk of overburdening a single device.

Here is a really cool device available from Amazon, if anybody is interested! Kill-a-Watt (paid link) will take a reading of your wattage usage and give you a precise number that you need to run your whole house!


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