What Size Generator to Run an RV Furnace: Choosing the Right Wattage

The RV furnace is a critical appliance for comfort during cold-weather travel, but selecting the right generator size requires matching starting and running wattage needs with RV electrical constraints. This guide explains how to calculate furnace power requirements, account for furnace types, and choose a safe, efficient generator for reliable heating.

Furnace Type	Approx Running Watts	Approx Starting Watts	Recommended Generator Size
12V DC Standalone Furnace	60–200 W	60–200 W	1,000–2,000 W
120V AC Furnace (Electric Ignition)	200–800 W	400–1,600 W	2,000–3,500 W
Hydronic/Propane Boiler With Circulator	300–1,000 W	800–2,400 W	3,500–5,500 W

How RV Furnaces Work And Why Generator Size Matters

RV furnaces typically burn propane and use electric components such as blowers, igniters, control boards, and sometimes electric heating elements. The furnace’s electrical draw includes a continuous running load and occasional starting surges from the blower motor and ignition systems.

Choosing the correct generator size ensures the furnace starts reliably without tripping the generator’s overload protection and allows other appliances to run concurrently. Undersized generators may cause voltage drops, repeated restarts, or premature wear on both the furnace and generator.

Identify Your Furnace Type And Electrical Requirements

RV furnaces fall into three common categories: 12V DC furnaces, 120V AC furnaces, and hydronic or boiler-style heating systems. Each has different electrical profiles and implications for generator sizing.

12V DC Furnaces

12V DC furnaces are common in older and smaller RVs. They run directly off the house battery via an inverter or DC distribution and draw relatively low wattage for blowers and controls.

Typical running wattage ranges from 60 to 200 watts. Because these units do not have high starting surges, a small inverter generator or inverter coupled with the battery bank often suffices.

120V AC Furnaces

Many modern RVs use 120V AC furnaces with electric ignition and larger blowers. These require conversion to AC via an inverter or direct connection to shore power or a generator.

Running watts are typically between 200 and 800 watts, but motor startup can double or triple that briefly. Properly accounting for starting surge is essential when selecting a generator.

Hydronic And Boiler Systems

Hydronic systems use a boiler and circulator pumps and sometimes electric elements for backup. These systems can draw more power, especially at startup when pumps and control circuits engage.

Running wattage ranges from 300 to 1,000 watts, with starting surges that may exceed 2,000 watts. Larger generators are usually recommended for reliability.

Calculate Starting Versus Running Watts

Determining the right generator size requires understanding both the running and starting wattage for the furnace and other simultaneous loads. Running watts are steady-state consumption; starting watts are momentary peaks when motors start.

For motors, use an estimate of 2–3 times running watts for the startup surge unless the manufacturer provides specific locked-rotor or starting current values. If the furnace blower is controlled by a soft-start inverter, starting surge may be lower.

How To Read Your RV Furnace Nameplate And Manuals

Manufacturers provide electrical specs on the furnace nameplate or in the owner’s manual. Look for input in amps at the system voltage. Convert amps to watts with the formula: Watts = Volts × Amps.

Example: A furnace listing of 120V and 4 amps equals 480 running watts. For motor startup, plan for 2–3 times that number unless manufacturer data indicates otherwise.

Include Other RV Loads In The Total Generator Calculation

A generator must handle the furnace load plus any other loads running simultaneously. Common concurrent loads include refrigerators, lights, water pumps, microwave, TV, and battery chargers.

List essential loads and calculate combined running and starting watts. Prioritize which loads will run together to choose an appropriate generator size and avoid unnecessary oversizing.

Practical Generator Size Recommendations

Below are practical generator recommendations based on furnace type and typical RV electrical scenarios:

• Small 1,000–2,000 Watt Generators: Suitable for 12V DC furnaces and minimal accessories. Good for quiet use and fuel efficiency but limited simultaneous AC loads.
• Medium 2,000–3,500 Watt Generators: Ideal for most 120V AC RV furnaces and basic additional loads like a small refrigerator and lights. Provides cushion for motor starting surges.
• Large 3,500–5,500+ Watt Generators: Recommended for hydronic systems, larger blowers, rooftop AC units, or when multiple high-draw appliances run simultaneously.

Generator Type And Features That Matter

Choosing the right generator type and features improves furnace performance, reliability, and the RV experience.

Inverter Generators

Inverter generators provide clean, stable power with low total harmonic distortion (THD), protecting sensitive electronics and furnaces. They are often quieter and more fuel-efficient, making them well suited for RV use.

Conventional Open-Frame Generators

Open-frame generators may offer higher wattage for the price, but they are louder and typically have higher THD. They work for heavy loads but may be less suitable near campsites with noise restrictions.

Automatic Transfer Switch And Power Management

An automatic transfer switch simplifies moving from shore power to generator power, reducing the risk of backfeeding. Power management systems can sequence loads to avoid overload by prioritizing essential circuits.

Noise, Fuel Type, And Efficiency Considerations

Noise levels matter in campgrounds and national parks. Look for generators with low decibel ratings and eco modes that reduce RPM under light loads for better fuel economy.

Most RV generators run on gasoline, though some models accommodate propane or dual fuel. Propane can be convenient for RVers with onboard propane tanks, though it may reduce run time slightly due to lower energy density.

Battery, Inverter, And Shore Power Integration

Some RVs use inverters and battery banks to run AC appliances from DC power. A well-sized inverter paired with batteries can run a furnace briefly and reduce generator use, but batteries require charging and careful management during extended cold snaps.

When shore power is available, the furnace should rely on it. Ensure the generator and inverter systems are configured to prevent simultaneous backfeeds and that transfer switches are present.

Common Mistakes To Avoid

• Underestimating starting surge: Motor startup can be significantly higher than running wattage.
• Ignoring combined loads: Multiple appliances running simultaneously can exceed generator capacity.
• Choosing only on peak wattage: Continuous running capacity and fuel efficiency are equally important.
• Overlooking THD: High THD can damage sensitive electronics and affect furnace controls.

Calculating An Example: Step-By-Step

Step 1: Identify furnace running amps and voltage from the nameplate. Step 2: Convert to watts: Watts = Volts × Amps. Step 3: Multiply running watts by 2–3 for motor starting surge. Step 4: Add running watts of other appliances expected to run simultaneously. Step 5: Add margin for safety, typically 20–25% above the calculated peak.

Example: Furnace 120V × 4A = 480W running. Starting surge estimate 480W × 3 = 1,440W. Add refrigerator 600W starting surge 1,200W and running 200W. Combined peak ~2,640W. Add 25% margin → ~3,300W. Recommended generator: 3,500W inverter generator.

Manufacturer Recommendations And Safety Tips

Follow furnace and generator manufacturer guidelines for wiring, grounding, and ventilation. Never run a generator in an enclosed area; carbon monoxide risks are significant. Maintain proper clearance and ensure exhaust is directed away from living spaces.

Use approved shore power cords, transfer switches, and surge protectors. Install a carbon monoxide detector and a propane detector in the RV for added safety during generator operation.

Maintenance And Best Practices

Regular generator maintenance extends life and reliability. Follow recommended oil change intervals, check air filters, and run the generator periodically to prevent fuel system issues. Use fuel stabilizers if storing fuel for long periods.

Test the furnace and generator together before travel to confirm startup under load. Keep battery banks charged and inspect connections for corrosion or looseness.

When To Consult A Professional

Consult a certified RV technician or electrician for complex installations, power management upgrades, or if the RV’s electrical system includes custom components. Professional assessment ensures safe wiring, proper transfer switch installation, and adequate ventilation for generators.

Key Takeaways

Choosing the right generator to run an RV furnace involves matching running and starting wattage, accounting for other concurrent loads, and providing a margin of safety. Most RV owners find 2,000–3,500 watt inverter generators suitable for common furnaces, while hydronic systems and multiple appliances often require 3,500–5,500+ watt options. Prioritize low THD, proper transfer switching, and regular maintenance for reliable cold-weather travel.