How to Size a Generator for Home [Watt List]
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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:
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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:
- 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)
- Portable. This can be either inverter-generator or regular generator and it’s the best for construction sites and RV traveling
- 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:
- Do you live in a region where rolling blackouts are normal and anticipated? If you know about it, might as well prepare for it.
- Do you live in an area where natural disasters, like heavy storms and hurricanes, are frequent? *@#! Happens! Life does not stop during bad weather!
- 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!
Click on the white button above to find your electrician!
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