Generator Size Calculator
Calculate what size generator you need for home emergency backup, RV camping, or jobsite tools. Accounts for continuous running load, motor startup surges, and planning headroom.
Important Planning Model Limitation:
This practical planning model assumes that only one significant motor-driven load starts at a time. If multiple large motors can start simultaneously, generator sizing may require a more detailed manufacturer or engineering analysis.
Quick Scenario Presets
Select an example outage scenario or build your load list from scratch.
Selected Appliances & Equipment (5)
Default wattages are typical estimates. Check equipment nameplates and edit values as needed.
| Appliance | Qty | Running (W) | Starting (W) | Subtotal | |
|---|---|---|---|---|---|
Refrigerator / Freezer (Modern Energy Star)Surge Driver kitchen | 1 | 180 W | |||
Sump Pump (1/3 HP) water pumps | 1 | 800 W | |||
Wi-Fi Router & Modem electronics | 1 | 25 W | |||
LED Home Lighting (Per Room, 4 Bulbs) electronics | 4 | 160 W | |||
Smart Phone Charger (USB-C Fast Charger) electronics | 1 | 15 W |
Add Appliances from Library
Click any appliance to add it to your calculation table.
Central Air Conditioner (4-Ton / 48,000 BTU)
Central Air Conditioner (3-Ton / 36,000 BTU)
Window Air Conditioner (12,000 BTU)
Portable Electric Space Heater
Window Air Conditioner (8,000 BTU)
Furnace Fan Blower (Gas/Oil Heat, 1/2 HP)
Electric Range / Cooktop (Single Burner)
Dishwasher
Microwave Oven (1,000W Cooking Power)
Electric Coffee Maker (Drip)
Toaster (2-Slice)
Older Refrigerator / Deep Freezer
Refrigerator / Freezer (Modern Energy Star)
Electric Water Heater (40-50 Gallon)
Submersible Well Pump (1 HP, 240V)
Submersible Well Pump (1/2 HP, 240V)
Sump Pump (1/2 HP)
Sump Pump (1/3 HP)
Garage Door Opener (1/2 HP)
Desktop Gaming PC & Monitor
55"-65" LED Television
Laptop / Workstation Computer
Medical CPAP Machine
LED Home Lighting (Per Room, 4 Bulbs)
Wi-Fi Router & Modem
Smart Phone Charger (USB-C Fast Charger)
RV Rooftop Air Conditioner (15,000 BTU)
RV Rooftop Air Conditioner (13,500 BTU)
RV Converter / Battery Charger (45A DC)
RV Absorption Refrigerator (Electric Mode)
Table Saw (10", 15 Amp)
Circular Saw (7-1/4", 15 Amp)
Air Compressor (1.5 HP Portable)
Cordless Power Tool Dual Charger
Add Custom Equipment / Unlisted Appliance
Enter specific nameplate wattages for appliances not listed in the library. We do not apply arbitrary multipliers.
How We Calculated Your Generator Size
1,180 W
Sum of all operational running watts.
+1,020 W
Driver: Refrigerator / Freezer (Modern Energy Star)
2,200 W
Running + single largest surge.
2,750 W
Peak demand × 1.25 headroom factor.
Engineering Notice: Sizing with the 25% planning headroom factor prevents operating your generator at 100% continuous output, ensuring reserve capacity for engine wear, altitude derating, and voltage stability during motor inrush events.
2,750 Watts
(2.75 kW Planning Capacity)
Electrical Sizing Metrics
1,180 W
1.18 kW continuous2,200 W
2.20 kW momentary3.44 kVA
Contractor & Electrician Summary
Copy this summary to share your calculated load with an electrician or contractor for equipment selection and transfer switch planning.
Includes continuous load, surge drivers, voltage notes, and safety disclaimers.
How to Match Generator Spec Sheets
When shopping for portable or standby generators, compare your calculated figures against manufacturer nameplates:
- Rated (Running) Watts: Must meet or exceed 2,750 W to operate continuously within safe engine duty limits.
- Surge (Starting) Watts: Must meet or exceed 2,200 W to start your largest motor without stalling the alternator.
Always verify actual equipment specifications before purchasing or renting.
Need help understanding running watts, starting surge, and sizing? Read our House Generator Guide or our dedicated Refrigerator Sizing Guide.
The Four-Step Generator Sizing Methodology
Generator planning capacity is calculated as total continuous running wattage plus the single largest motor startup surge delta, multiplied by the 25% CalcMyPower planning headroom factor.
Variables & Constants
W_running (Watts (W))ΔW_max (Watts (W))W_peak (Watts (W))1.25 (Multiplier)Worked Example: Mixed Residential Power Outage
Continuous load represents the steady-state electrical power needed to keep all active appliances operating simultaneously.
Under standard asynchronous operation, only one significant motor draws inrush current at any given instant. Sizing for all surges simultaneously would severely oversize the generator.
This momentary peak represents the maximum electrical inrush event the generator alternator must support.
The 25% planning margin ensures the generator does not operate at 100% capacity, providing fuel economy, engine protection, and transient reserve.
Comparing Generator Technologies
Compare portable inverter, open-frame, dual-fuel, and standby generators by power quality (THD), noise level, portability, and fuel storage.
| Technology | Power Quality (THD) | Noise Level | Portability | Fuel Flexibility | Typical Application |
|---|---|---|---|---|---|
| Portable Inverter | Clean (<3% THD) | Quiet (50 to 65 dBA) | High (40 to 120 lbs) | Gasoline / Dual-Fuel | Electronics, camping, RVs, essential home circuits |
| Conventional Open-Frame | Distorted (10% to 25% THD) | Loud (68 to 80+ dBA) | Moderate (Wheeled) | Gasoline | Jobsite power tools, compressors, sump pumps |
| Dual-Fuel / Tri-Fuel | Varies by alternator | Moderate to loud | Moderate to heavy | Gas, Propane (LPG), Natural Gas | Storm prep; propane stores indefinitely without carburetor clogging |
| Home Standby (Stationary) | Utility grade (<5% THD) | Baffled (60 to 70 dBA) | Permanent outdoor pad | Piped Natural Gas / LP | Whole-house automatic backup via Automatic Transfer Switch |
Critical Generator Safety & Installation Rules
Portable generators produce lethal carbon monoxide gas and severe electrical hazards if misused.
Carbon Monoxide (CO) Poisoning (CDC & CPSC)
NEVER operate a generator indoors, in a garage, in a basement, shed, or crawlspace, even if doors and windows are left open.
Operate generators outdoors only, at least 20 feet away from all windows, doors, and fresh air intake vents, with the engine exhaust directed away from living spaces per current CDC and CPSC guidelines. Install working battery-powered CO alarms inside your home.
Anti-Backfeeding & Transfer Equipment
NEVER connect a generator directly to a wall outlet or dryer plug. This illegal practice ("backfeeding") energizes outdoor power lines, creating lethal electrocution hazards for utility lineworkers.
Use properly installed transfer equipment or an approved interlock arrangement where applicable to prevent unintended interconnection with utility power. Follow applicable NEC and local code requirements and use qualified electrical professionals for installation.
Calculation Assumptions & Engineering Planning Model
Transparent electrical assumptions used in this generator sizing calculator:
| Parameter | Model Default | Typical Field Range | Effect on Generator Sizing |
|---|---|---|---|
| Motor Startup Sequencing | Single Largest Surge | 1 to 2 motors simultaneously | Assumes only one major motor starts at any instant; simultaneous starts require larger commercial sizing. |
| CalcMyPower Planning Headroom | 25% (1.25×) | 10% to 30% | Maintains engine operation near 80% rated continuous duty for thermal reserve and longevity. |
| Apparent Power Factor (PF) | 0.80 Lagging | 0.80 (Standby) to 1.0 (Inverter) | Used for kVA apparent power estimates on standby and prime commercial generator sets. |
| Appliance Wattage Defaults | DOE / Energy Star Benchmarks | Varies by manufacturer | Serves as editable planning estimates; nameplate ratings should always be confirmed. |
Generator Sizing & Safety Disclaimer
- The generator capacity figures provided by this calculator are planning estimates for consumer guidance only.
- Electrical loads, motor inrush characteristics (Locked Rotor Amps), altitude deratings, and local electrical codes vary. Sizing estimates do not substitute for professional engineering analysis, manufacturer-specific sizing software, or licensed electrical contractor assessment.
- Always install equipment in compliance with applicable National Electrical Code (NEC) articles and local building codes.
- Never operate portable generators indoors, in garages, or near open windows due to lethal carbon monoxide hazards.
Frequently Asked Questions
What size generator do I need to run a refrigerator and a freezer?
Can a 5,000-Watt generator run a whole house?
What is the difference between starting watts and running watts?
How do I connect a generator to my house safely without backfeeding?
What size generator is needed for a 30-amp RV?
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Refrigerator Generator Sizing Guide
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Watts to Amps Electrical Calculator
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NEC Residential Generator Sizing & Motor Inrush Surge Guide
Properly sizing an emergency home backup or jobsite generator requires calculating total running continuous wattage, accommodating motor starting surge inrush, and balancing split-phase electrical lines.
The Single-Largest-Surge Generator Sizing Formula
Never add the starting surge watts of every appliance together. In practical outage operation, motorized appliances start one at a time. Sizing accounts for the sum of all continuous running loads plus the single largest startup surge, derated by a 25% safety margin.
Running Watts
The power continuously consumed while an appliance operates normally. Heating elements, light bulbs, and televisions draw constant running wattage with zero starting surge.
Starting Surge Watts
Electric motors (refrigerators, well pumps, AC compressors) require 2 to 4 times their running wattage for 1 to 3 seconds to overcome mechanical inertia from a standstill.
Split-Phase Balancing (120V/240V)
Standard 240V generator output consists of two 120V legs (L1 and L2). Connecting too many 120V loads to a single leg trips the main circuit breaker even when total wattage seems sufficient.
Master Appliance Running & Starting Wattage Reference Chart
Comprehensive reference values across 35+ residential appliances, workshop power tools, and essential circuits:
| Appliance / Equipment | Category | Running Watts | Starting Watts | Surge Delta | Voltage |
|---|---|---|---|---|---|
| Standard Refrigerator / Freezer (18-22 cu ft) | Kitchen & Refrigeration | 700W | 1500W | +800W | 120V |
| Chest Freezer (15 cu ft) | Kitchen & Refrigeration | 500W | 1200W | +700W | 120V |
| Microwave Oven (1,000W Cooking Output) | Kitchen & Refrigeration | 1500W | 1500W | +0W | 120V |
| Drip Coffee Maker | Kitchen & Refrigeration | 1200W | 1200W | +0W | 120V |
| Toaster (2-Slice to 4-Slice) | Kitchen & Refrigeration | 1100W | 1100W | +0W | 120V |
| Dishwasher (Washing & Heated Dry) | Kitchen & Refrigeration | 1500W | 2000W | +500W | 120V |
| Garbage Disposal (1/2 HP) | Kitchen & Refrigeration | 600W | 1400W | +800W | 120V |
| Electric Range Stove Burner (Single 6-Inch) | Kitchen & Refrigeration | 1500W | 1500W | +0W | 240V |
| Central Air Conditioner (3-Ton, 14 SEER) | Heating & Cooling | 3500W | 9500W | +6000W | 240V |
| Central AC (3-Ton with Electronic Soft Starter) | Heating & Cooling | 3500W | 5500W | +2000W | 240V |
| Window Air Conditioner (5,000 BTU) | Heating & Cooling | 500W | 1200W | +700W | 120V |
| Window Air Conditioner (12,000 BTU) | Heating & Cooling | 1200W | 2800W | +1600W | 120V |
| Gas / Propane Furnace Blower Motor (1/2 HP) | Heating & Cooling | 800W | 1900W | +1100W | 120V |
| Oil Furnace (Burner & Blower Combined) | Heating & Cooling | 1200W | 2400W | +1200W | 120V |
| Portable Electric Space Heater (High Setting) | Heating & Cooling | 1500W | 1500W | +0W | 120V |
| Electric Storage Water Heater (40-50 Gallon) | Heating & Cooling | 4500W | 4500W | +0W | 240V |
| Box Fan / Oscillating Stand Fan | Heating & Cooling | 75W | 150W | +75W | 120V |
| Deep Well Submersible Pump (1/2 HP) | Pumps & Water | 1000W | 2500W | +1500W | 240V |
| Deep Well Submersible Pump (1 HP) | Pumps & Water | 1800W | 4500W | +2700W | 240V |
| Basement Sump Pump (1/3 HP) | Pumps & Water | 600W | 1400W | +800W | 120V |
| Basement Sump Pump (1/2 HP Heavy Duty) | Pumps & Water | 900W | 2100W | +1200W | 120V |
| Sewage Ejector Pump (1/2 HP Grinder) | Pumps & Water | 1100W | 2600W | +1500W | 120V / 240V |
| Pancake / Hot-Dog Air Compressor (1.5 HP) | Tools & Workshop | 1600W | 3500W | +1900W | 120V |
| Circular Saw (7-1/4 Inch, 15 Amp) | Tools & Workshop | 1400W | 2500W | +1100W | 120V |
| Jobsite Table Saw (10-Inch, 15 Amp) | Tools & Workshop | 1800W | 3800W | +2000W | 120V |
| Wet/Dry Shop Vacuum (12-Gallon, 5.0 Peak HP) | Tools & Workshop | 1200W | 2000W | +800W | 120V |
| Automotive / Marine 12V Battery Charger (15A) | Tools & Workshop | 250W | 250W | +0W | 120V |
| LED Household Lighting (10 Bulbs, 60W Equivalent) | Lighting & Essentials | 90W | 90W | +0W | 120V |
| Incandescent Lighting Circuit (10 Bulbs, 60W each) | Lighting & Essentials | 600W | 600W | +0W | 120V |
| Garage Door Opener (1/2 HP) | Lighting & Essentials | 500W | 1400W | +900W | 120V |
| Home Security System & 8-Camera DVR | Lighting & Essentials | 100W | 100W | +0W | 120V |
| Wi-Fi Router & Cable / Fiber Modem | Electronics & Medical | 25W | 25W | +0W | 120V |
| Smart LED Television (55 to 65-Inch) | Electronics & Medical | 120W | 120W | +0W | 120V |
| Desktop PC Workstation with Dual Monitors | Electronics & Medical | 300W | 300W | +0W | 120V |
| Laptop Computer USB-C Charger (65W to 100W) | Electronics & Medical | 65W | 65W | +0W | 120V |
| CPAP Machine (with Heated Humidifier) | Electronics & Medical | 70W | 70W | +0W | 120V |
Generator Amperage Output Chart (120V & 240V by Generator Size)
Check the available continuous and surge current for common portable and home standby generator sizes:
| Generator Size (Watts) | 120V Total Current | 240V Transfer Current | 80% Continuous Limit (240V) | Receptacle / Hookup |
|---|---|---|---|---|
| 1,000W (1 kW) | 8.3A | N/A (120V) | N/A | Camping, tailgating, phone charging, LED lights, laptops |
| 1,500W (1.5 kW) | 12.5A | N/A (120V) | N/A | Small refrigerator, television, internet router, CPAP machine |
| 2,000W (2 kW) | 16.7A | N/A (120V) | N/A | Standard household refrigerator, TV, microwave (one at a time) |
| 2,500W (2.5 kW) | 20.8A | N/A (120V) | N/A | RV 30A plug, refrigerator, small window air conditioner |
| 3,000W (3 kW) | 25A | N/A (120V) | N/A | RV 13,500 BTU rooftop AC, refrigerator, power tools |
| 3,500W (3.5 kW) | 29.2A | N/A (120V) | N/A | Full RV trailer power, jobsite air compressor, furnace blower |
| 4,000W (4 kW) | 33.3A | 16.7A | 13.3A | Gas furnace, refrigerator, sump pump, lights, basic circuits |
| 5,000W (5 kW) | 41.7A | 20.8A | 16.7A | Home backup via manual transfer switch, 1/2 HP well pump, freezer |
| 6,500W (6.5 kW) | 54.2A | 27.1A | 21.7A | Essential circuits, 240V deep-well pump, gas water heater, furnace |
| 7,500W (7.5 kW) | 62.5A | 31.3A | 25A | Standard 30A home inlet box, multiple pumps, furnace, refrigeration |
| 8,500W (8.5 kW) | 70.8A | 35.4A | 28.3A | Heavy emergency home backup, small central AC with soft starter |
| 10,000W (10 kW) | 83.3A | 41.7A | 33.3A | 50A transfer switch inlet, 3-ton central AC, whole-house subpanel |
| 12,000W (12 kW) | 100A | 50A | 40A | Full 50A utility service backup, central heat pump, water heater |
| 15,000W (15 kW) | 125A | 62.5A | 50A | Whole-home standby generator with automatic transfer switch |
| 18,000W (18 kW) | 150A | 75A | 60A | Whole-home automatic standby, 4-ton AC, electric range, water heater |
| 20,000W (20 kW) | 166.7A | 83.3A | 66.7A | Most common residential whole-home standby size (100A service feed) |
| 22,000W (22 kW) | 183.3A | 91.7A | 73.3A | Full residential 100A whole-home coverage, two central AC units |
| 24,000W (24 kW) | 200A | 100A | 80A | Large home whole-house standby, multiple heat pumps, electric utilities |
| 26,000W (26 kW) | 216.7A | 108.3A | 86.7A | Large luxury home or light commercial standby installation |
Carbon Monoxide Safety & Transfer Switch Requirements
Never operate a combustion generator indoors, inside a garage, or near open windows. Always place the unit outdoors at least 20 feet away from living spaces with the exhaust directed away.
When powering home circuits, NEC Article 702 mandates an approved manual transfer switch or mechanical interlock. This equipment prevents dangerous electrical backfeeding into utility power lines during an outage.