What size generator do I need to run a refrigerator and a freezer?

A modern Energy Star refrigerator runs on approximately 180 Watts but requires 1,200 Watts during compressor startup. A separate freezer requires about 180 to 400 Watts running and 1,200 to 1,600 Watts starting. Total continuous running load is about 360 to 580 Watts, with the largest single startup surge contributing roughly 1,200 Watts above running draw, yielding a peak demand of about 1,560 Watts. Under the CalcMyPower planning model with a 25% headroom factor, a 2,000 to 2,500 Watt inverter generator provides reliable operation.

Can a 5,000-Watt generator run a whole house?

A 5,000W generator can easily power essential household circuits (refrigerator, sump pump, gas furnace blower, lights, Wi-Fi, television, and microwave) with planning capacity to spare. However, it cannot run large 240V central air conditioning compressors (3 to 5 tons) or electric water heaters simultaneously, which require large standby generators (14kW to 22kW).

What is the difference between starting watts and running watts?

Running watts (rated watts) is the continuous power an appliance consumes while operating normally. Starting watts (surge watts) is the temporary extra power (up to 3 times running watts) needed for 1 to 3 seconds by motor-driven equipment (refrigerators, pumps, air conditioners) to overcome mechanical inertia during startup.

What size generator is needed for a 30-amp RV?

A 30-amp RV service operates at 120 Volts, representing a maximum electrical capacity of 3,600 Watts (30A × 120V). Sizing a generator with 3,500W to 4,500W starting surge and at least 3,000W continuous capacity allows you to start and run a 13,500 or 15,000 BTU rooftop air conditioner while running the internal RV converter charger and residential electronics.

Generator Sizing•Electrical Sizing Tool•Updated September 2026

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.

Input Parameters

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.

Example Scenario — editable: Pre-loaded with common essential home circuits. Adjust quantities below or add custom loads.

Selected Appliances & Equipment (5)

Default wattages are typical estimates. Check equipment nameplates and edit values as needed.

Appliance	Qty	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)

4500W run+9000W surge

Central Air Conditioner (3-Ton / 36,000 BTU)

3500W run+6500W surge

Window Air Conditioner (12,000 BTU)

1200W run+1600W surge

Portable Electric Space Heater

1500W runno surge

Window Air Conditioner (8,000 BTU)

800W run+1000W surge

Furnace Fan Blower (Gas/Oil Heat, 1/2 HP)

800W run+1500W surge

Electric Range / Cooktop (Single Burner)

1500W runno surge

Dishwasher

1500W run+500W surge

Microwave Oven (1,000W Cooking Power)

1200W run+300W surge

Electric Coffee Maker (Drip)

1000W runno surge

Toaster (2-Slice)

850W runno surge

Older Refrigerator / Deep Freezer

400W run+1200W surge

Refrigerator / Freezer (Modern Energy Star)

180W run+1020W surge

Electric Water Heater (40–50 Gallon)

4500W runno surge

Submersible Well Pump (1 HP, 240V)

2000W run+2500W surge

Submersible Well Pump (1/2 HP, 240V)

1000W run+1500W surge

Sump Pump (1/2 HP)

1050W run+1150W surge

Sump Pump (1/3 HP)

800W run+1000W surge

Garage Door Opener (1/2 HP)

550W run+850W surge

Desktop Gaming PC & Monitor

450W runno surge

55"–65" LED Television

120W runno surge

Laptop / Workstation Computer

100W runno surge

Medical CPAP Machine

60W runno surge

LED Home Lighting (Per Room, 4 Bulbs)

40W runno surge

Wi-Fi Router & Modem

25W runno surge

Smart Phone Charger (USB-C Fast Charger)

15W runno surge

RV Rooftop Air Conditioner (15,000 BTU)

1800W run+1700W surge

RV Rooftop Air Conditioner (13,500 BTU)

1500W run+1700W surge

RV Converter / Battery Charger (45A DC)

600W runno surge

RV Absorption Refrigerator (Electric Mode)

350W runno surge

Table Saw (10", 15 Amp)

1800W run+2200W surge

Circular Saw (7-1/4", 15 Amp)

1800W run+1000W surge

Air Compressor (1.5 HP Portable)

1500W run+2000W surge

Cordless Power Tool Dual Charger

150W runno surge

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

Step 1: Running Load

1,180 W

Sum of all operational running watts.

Step 2: Largest Surge

+1,020 W

Driver: Refrigerator / Freezer (Modern Energy Star)

Step 3: Peak Demand

2,200 W

Running + single largest surge.

Step 4: Planning (+25%)

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.

Recommended CapacityCalcMyPower Engine

2,750 Watts

(2.75 kW Planning Capacity)

Sized to support a 1,180W running load and absorb a 1,020W motor surge with 25% planning headroom.

Electrical Sizing Metrics

Total Running Load

1,180 W

1.18 kW continuous

Peak Starting Demand

2,200 W

2.20 kW momentary

Apparent Power (kVA)PF = 0.80 (Assumption)

3.44 kVA

Running: 1.47 kVAPeak: 2.75 kVA

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.

Recommended Equipment Categories

To connect and operate emergency generator power safely, standard installation hardware is required:

Dual-Fuel Inverter Generators (3,500W – 4,500W)Clean power (<3% THD) for electronics; runs on gasoline or long-storing propane.

Outdoor Power Inlet Boxes (30A / 50A)NEMA 3R weatherproof through-wall connection; eliminates running cords through windows.

Manual Transfer Switch & Interlock KitsCode-compliant break-before-make subpanels preventing deadly utility backfeeding.

Heavy-Duty 4-Prong Generator Cords (L14-30P)10 AWG all-weather copper extension cords designed for 120V/240V generator transfer.

As an Amazon Associate, CalcMyPower earns from qualifying purchases. We never recommend untested hardware.

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.

Formula

W_planning = (W_running + ΔW_max) × 1.25

Variables & Constants

Total Running WattsW_running (Watts (W))

Sum of continuous active power across all operational equipment (Σ Q_i × W_{r,i}).

Largest Additional Starting WattsΔW_max (Watts (W))

The maximum surge delta among active loads: max(Starting Watts - Running Watts).

Peak Starting DemandW_peak (Watts (W))

The momentary electrical surge demand when the single largest motor cycles on: W_running + ΔW_max.

CalcMyPower Planning Headroom Factor1.25 (Multiplier)

A 25% safety margin ensuring the generator operates within ~80% of its rated capacity for engine protection.

Worked Example: Mixed Residential Power Outage

Example Scenario: A homeowner needs emergency backup power during a winter storm for a modern refrigerator (180W running / 1,200W starting), a 1/2 HP gas furnace blower (800W running / 2,300W starting), a 1/3 HP sump pump (800W running / 1,800W starting), Wi-Fi router (25W), and 4 rooms of LED lighting (160W).

Sum Total Continuous Running Watts

W_running = 180 + 800 + 800 + 25 + 160 = 1,965 Watts

Continuous load represents the steady-state electrical power needed to keep all active appliances operating simultaneously.

Determine the Largest Single Motor Surge Delta

Fridge delta = 1,200 - 180 = 1,020W; Sump delta = 1,800 - 800 = 1,000W; Furnace Blower delta = 2,300 - 800 = 1,500W. Max delta = 1,500 Watts (Furnace Blower).

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.

Compute Peak Starting Demand

W_peak = 1,965 W + 1,500 W = 3,465 Watts

This momentary peak represents the maximum electrical inrush event the generator alternator must support.

Apply the CalcMyPower Planning Headroom Factor

W_planning = 3,465 W × 1.25 = 4,331.25 Watts (rounded to 4,331 W)

The 25% planning margin ensures the generator does not operate at 100% capacity, providing fuel economy, engine protection, and transient reserve.

Result: Based on this 1,965W continuous load and 3,465W peak demand, a generator with at least 4,350W rated running watts and 3,500W surge watts provides comfortable, safe emergency backup capacity.

Comparing Generator Technologies

Understanding the trade-offs between generator types based on engineering characteristics rather than arbitrary wattages.

Technology	Power Quality (THD)	Noise Level	Portability	Fuel Flexibility	Typical Application
Portable Inverter	Clean (<3% THD)	Quiet (50–65 dBA)	High (40–120 lbs)	Gasoline / Dual-Fuel	Electronics, camping, RVs, essential home circuits
Conventional Open-Frame	Distorted (10–25% THD)	Loud (68–80+ 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 (60–70 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	Impact on Runtime
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

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