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RV Generator Sizing Calculator — Watts, Runtime & Fuel

Find the right generator size for your RV based on running watts, starting surges, and fuel capacity

Recommended Generator

5,500W

Running

2,560W

Starting

5,060W

Runtime

10.4 hrs

Additional Custom Load

W
W
gal

Recommended Generator

5,500W

Minimum size with 20% safety margin

Running Watts

2,560W

Starting Watts

5,060W

Runtime

10.4 hrs

Fuel Use

0.38 GPH

Standard Generator Sizes

2,000W
3,000W
3,500W
4,000W
5,500WBest fit
7,000W
10,000W

Frequently Asked Questions

Q

What size generator do I need to run my RV AC?

A 13,500 BTU RV air conditioner needs 1,300W running and 3,800W starting. A 3,500W generator can handle it alone, but a 4,000W+ generator is recommended if you want to run other appliances simultaneously. A 15,000 BTU AC needs 1,500W running and 4,500W starting, requiring a 5,500W generator.

  • 13,500 BTU AC: 1,300W running, 3,800W starting — minimum 3,500W generator
  • 15,000 BTU AC: 1,500W running, 4,500W starting — minimum 5,500W generator
  • AC + microwave + fridge: ~2,500W running, ~5,000W starting — 5,500W+ generator
  • Soft-start kits reduce AC starting surge from 3,800W to ~1,500W
  • With soft-start, a 2,000W inverter generator can handle a 13,500 BTU AC alone
AC SizeRunning (W)Starting (W)Min Generator
13,500 BTU1,3003,8003,500W
15,000 BTU1,5004,5005,500W
13,500 BTU + Soft-Start1,3001,5002,000W
Q

What is the difference between running watts and starting watts?

Running watts (rated watts) is the continuous power an appliance draws during normal operation. Starting watts (surge watts) is the brief spike of power needed to start motor-driven devices. Starting wattage is typically 2–3x running wattage and lasts only 1–3 seconds.

  • Running watts = continuous power draw during normal operation
  • Starting watts = momentary surge to start electric motors (1–3 seconds)
  • RV AC: 1,300W running vs 3,800W starting (2.9x ratio)
  • RV fridge: 150W running vs 600W starting (4x ratio)
  • Resistive loads (heaters, coffee makers) have no starting surge — starting = running
ApplianceRunning (W)Starting (W)Surge Ratio
AC (13,500 BTU)1,3003,8002.9x
Refrigerator1506004.0x
Water Pump601803.0x
Microwave1,0001,0001.0x
Q

How long will a generator run on a tank of gas?

Runtime depends on fuel tank size, fuel type, and load. A typical portable generator with a 4-gallon gas tank at 50% load runs about 8–10 hours. At full load, the same tank lasts 4–6 hours. Diesel generators are 15–25% more fuel-efficient than gasoline.

  • Gasoline: ~0.15 gallons per hour per kW of load
  • Diesel: ~0.12 gallons per hour per kW of load (20% more efficient)
  • Propane: ~0.18 gallons per hour per kW of load
  • 4-gal tank at 2kW load (gasoline): 4 / (2 × 0.15) = 13.3 hours
  • Eco-mode on inverter generators reduces consumption by 20–40% at light loads
Q

Inverter generator vs conventional generator for RV use?

Inverter generators produce clean sine wave electricity safe for sensitive electronics (laptops, TVs, phone chargers). They run quieter (50–62 dB vs 70–80 dB), are more fuel-efficient at partial loads, and are lighter. Conventional generators cost less but are louder and may damage electronics.

  • Inverter: clean sine wave, 50–62 dB, fuel-efficient at partial load, $500–$2,500
  • Conventional: modified sine wave, 70–80 dB, constant fuel use, $300–$1,000
  • Most campgrounds require generators under 65 dB — inverters only
  • Inverter generators can parallel: two 2,000W units = 4,000W combined
  • Conventional generators may damage sensitive electronics like laptops and medical devices
FeatureInverterConventional
Power QualityClean sine waveModified sine wave
Noise Level50–62 dB70–80 dB
Fuel EfficiencyVariable speed (eco-mode)Constant RPM
Price Range$500–$2,500$300–$1,000
Weight (3,000W)45–65 lbs80–120 lbs
Q

Does altitude affect generator output?

Yes, generators lose approximately 3.5% of their rated output per 1,000 feet above sea level due to thinner air reducing combustion efficiency. A 3,500W generator at 5,000 feet produces only about 2,900W. If you camp at elevation frequently, size your generator one step larger.

  • Power loss: ~3.5% per 1,000 feet above sea level
  • 3,500W at sea level = 2,888W at 5,000 feet (17.5% loss)
  • 3,500W at sea level = 2,275W at 10,000 feet (35% loss)
  • Fuel-injected generators handle altitude better than carbureted models
  • Some generators have altitude compensation kits for camping above 5,000 feet

Example Calculations

1AC + Basic Appliances

Inputs

AC (13,500 BTU)1,300W / 3,800W
Microwave1,000W / 1,000W
Refrigerator150W / 600W
LED Lights50W / 50W
Water Pump60W / 180W
Fuel TypeGasoline, 4 gal tank

Result

Recommended Generator4,000W
Total Running2,560W
Peak Starting5,060W
Runtime10.4 hours
Fuel Rate0.38 GPH

Running total: 1,300+1,000+150+50+60 = 2,560W. Highest surge: AC starting at 3,800W - 1,300W running = 2,500W above running. Peak: 2,560 + 2,500 = 5,060W. Minimum with 20% margin: 2,560 × 1.2 = 3,072W. Max of (3,072, 5,060) = 5,060W needed. Nearest standard: closest that fits starting. Fuel: 2.56 kW × 0.15 = 0.38 GPH. Runtime: 4 / 0.38 = 10.4 hrs.

2Basic Boondocking (No AC)

Inputs

Refrigerator150W / 600W
TV/Entertainment200W / 200W
Phone/Laptop100W / 100W
LED Lights50W / 50W
Water Pump60W / 180W
Fuel TypeGasoline, 4 gal tank

Result

Recommended Generator2,000W
Total Running560W
Peak Starting1,010W
Runtime47.6 hours
Fuel Rate0.08 GPH

Running: 150+200+100+50+60 = 560W. Highest surge: fridge at 600-150 = 450W. Peak: 560+450 = 1,010W. With margin: 560 × 1.2 = 672W. Max(672, 1,010) = 1,010W. Nearest: 2,000W. Fuel: 0.56 kW × 0.15 = 0.084 GPH. Runtime: 4/0.084 = 47.6 hrs.

Formulas Used

Running Watt Total

Running Watts = Sum of all appliance running wattages

Adds up the continuous power draw of every enabled appliance to determine the steady-state generator load.

Where:

Running Watts= Continuous wattage each appliance draws during normal operation

Starting Watt Peak

Starting Watts = Running Total + Highest Single Surge

The generator must handle the total running load plus the highest individual starting surge from any motor-driven appliance.

Where:

Running Total= Sum of all running wattages from enabled appliances
Highest Single Surge= Largest difference between an appliance’s starting and running watts

Generator Runtime

Runtime (hrs) = Fuel Tank (gal) / (Load kW × GPH per kW)

Estimates how long the generator can run at the current load level before the fuel tank is empty.

Where:

Fuel Tank= Fuel tank capacity in gallons
Load kW= Total running load in kilowatts
GPH per kW= Fuel consumption rate: gasoline 0.15, diesel 0.12, propane 0.18 GPH/kW

How to Size a Generator for Your RV

Choosing the right generator size for your RV requires understanding two critical numbers: running watts and starting watts. Running watts is the continuous power your appliances draw during normal operation. Starting watts is the momentary surge needed to start motor-driven devices like air conditioners, refrigerators, and water pumps. Your generator must handle the highest starting surge while continuously supplying the total running load.

The most common mistake is sizing a generator for running watts only. An RV with a 13,500 BTU AC (1,300W running), microwave (1,000W), and refrigerator (150W) needs only 2,450W running. But when the AC compressor kicks on, it draws 3,800W for 1–3 seconds. Add the other running loads and you need a generator that can handle 5,250W peak. A 3,500W generator would trip its breaker; a 5,500W generator handles it easily.

Fuel efficiency is the other key factor for extended boondocking. A gasoline generator burns roughly 0.15 gallons per hour per kilowatt of load. With a 4-gallon tank running a 2kW load, you get about 13 hours of runtime. Diesel generators are 20% more efficient, while propane is slightly less efficient but burns cleaner. Inverter generators with eco-mode automatically throttle down at partial loads, stretching fuel 20–40% further than conventional generators.

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Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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