Your RV's Length Tells You Almost Nothing About Which Appliance to Buy
A 28-foot travel trailer has roughly 200 square feet of usable floor space. That number, not the rig's listed length, is what drives BTU calculations. Manufacturer-listed RV length includes the hitch, bumper, and tongue, making it unreliable for sizing any appliance.
Before you buy a new AC unit, furnace, or any major RV appliance, verify three compatibility pillars: BTU output, amperage draw, and cutout dimensions. Get any one of these wrong and you're looking at short-cycling, tripped breakers, wiring damage, or a unit that physically won't fit your rig.
Brand loyalty is the wrong starting point. Specs are the only reliable guide. Each pillar is covered below so you can buy with confidence.
BTU Output: How to Size Cooling and Heating for Your Actual Living Space
Start with interior square footage, not rig length. The baseline rule for RV cooling is approximately 60 to 80 BTUs per square foot, with 70 BTU/sq ft as a solid middle ground. That 28-foot travel trailer with 200 square feet of floor space needs 11,000 to 13,500 BTU. A 40-foot rig with slide-outs may need 22,000+ BTU spread across two units.
Each large slide-out adds 50 to 80 square feet of living area. Always calculate with slide-outs fully extended. Using the retracted footprint will leave you undersized and sweating through July.
Five Factors Beyond Square Footage
Raw square footage is just the starting point. Five additional variables shape your actual cooling and heating needs:
- Insulation quality: Pre-2015 rigs with single-pane windows and thin walls may need a full BTU tier higher than newer, better-insulated models.
- Climate zone: Desert camping in Arizona demands more cooling capacity than weekends in the Pacific Northwest.
- Occupancy load: Each additional person generates body heat that your AC has to counteract.
- Roof color: A dark-colored RV roof absorbs up to 40% more heat than a white one. Factor this in or you'll undersize your cooling.
- Ducted vs. non-ducted system: Ducted systems distribute air more evenly but can lose efficiency through ductwork; non-ducted units cool the immediate area more aggressively.
Furnace BTU Sizing
Heating follows similar logic but with different numbers. Warmer climates call for roughly 25 to 35 BTU per square foot, while colder climates push that to 45 to 60 BTU per square foot. Common propane furnace sizes run from 20,000 to 40,000 BTU.
Oversized furnaces short-cycle just like oversized ACs, rapidly flipping on and off, reducing comfort and accelerating wear. A 30,000 BTU furnace running continuously burns approximately 0.328 gallons of propane per hour (calculated from the standard 91,500 BTU per gallon). Match the BTU rating as closely to your original unit as possible, and resist the urge to size up unnecessarily.
Amperage Draw: The Compatibility Factor That Can Trip Your Breaker or Fry Your Wiring
The most overlooked spec in RV appliance shopping is the difference between running amps and startup surge amps.
A 13,500 BTU AC draws 10 to 14 running amps at 120V. Sounds manageable. But at startup, that same unit surges to 40 to 50+ amps, often enough to trip a breaker instantly. A 15,000 BTU AC draws 13 to 16 running amps and up to 32 amps at startup, roughly 2 to 3 more running amps than the smaller unit.
Know Your Service Ceiling
Your RV's electrical service type sets a hard compatibility ceiling. 30-amp service caps at 3,600 watts (30A × 120V). 50-amp service provides up to 12,000 watts across two 120V legs. There's no negotiating with these limits.
Here's a real-world scenario that catches people every summer: running a 15,000 BTU AC (about 13 amps), a microwave (about 12 amps), and a hair dryer (about 12 amps) simultaneously on 30-amp service pulls roughly 37 amps total. That's 7 amps over your limit, and the result is a tripped breaker at best or overheated wiring at worst.
Soft Starters and Off-Grid Considerations
A soft starter can reduce startup surge by up to 70 to 75%, making it possible to run your AC on a smaller generator. For boondockers, this is a critical upgrade.
Off-grid battery sizing is equally important. A 13,500 BTU AC running at 13 amps draws approximately 110 to 130 amps DC from a 12V battery bank. Standard lead-acid batteries drain in about 15 minutes under that load. You need at least 400 Ah of lithium battery capacity to run a 13,500 BTU AC for 3 to 4 hours off-grid.
Don't overlook your furnace's 12V DC draw. For boondockers running on battery power through cold nights, it's a meaningful line item in your energy budget. In extreme heat above 100°F, expect your AC's amp draw to increase 10 to 15% above factory specs. Build that margin into your generator and battery sizing.
Cutout Dimensions and Physical Fit: The Spec Most Buyers Skip Until It's Too Late
Almost all rooftop RV AC units, from 11,000 to 15,000 BTU, fit a standard 14" × 14" roof opening. Upgrading your BTU capacity rarely means cutting a new hole.
The nuance matters, though. Some newer units require a 14.25" × 14.25" cutout. If your existing opening is larger than 14.37", you'll need gaskets to achieve a proper seal and prevent leaks.
Height Above the Roofline
Standard rooftop AC units add 12 to 14 inches of height above the roofline. Low-profile units reduce that to 8 to 10 inches. If your rig sits near the common 13'6" clearance limit for bridges and overpasses, those extra inches are the difference between a smooth trip and a very expensive mistake.
Furnace Replacement Dimensions
Newer replacement furnace models may be slightly deeper than the unit you're pulling out. A trim ring or bottom discharge kit can correct the fit without cutting into your rig's structure. Always measure your existing cavity before ordering.
Ducted vs. Non-Ducted and the R32 Refrigerant Transition
Switching AC brands can involve thermostat wiring harness differences and air distribution box placement changes. Verify compatibility before buying, not after the old unit is already on the ground.
The biggest compatibility break in recent years: all new RV AC systems since 2025 use R32 refrigerant due to EPA regulations. Systems from 2024 and earlier use R410A. Parts between the two refrigerant types are not interchangeable. If you're replacing a pre-2025 unit, this distinction determines whether you can swap components or need a complete system change.
Your Pre-Purchase Compatibility Checklist: 3 Numbers to Confirm Before You Buy
Before you add anything to your cart, confirm these three specs:
- BTU: Measure interior square footage with slide-outs extended. Factor in your climate zone, roof color, and insulation age. Apply the 70 BTU/sq ft baseline and adjust up or down based on conditions.
- Amperage: Confirm your service type (30A or 50A). Calculate total simultaneous load across all appliances. Check both startup surge and running amps. Determine if you need a soft starter or generator upgrade.
- Dimensions: Measure your existing roof cutout. Check height clearance above the roofline. Verify refrigerant type (R32 for 2025+ units vs. R410A for older systems). Confirm duct routing and wiring harness compatibility if switching brands.
Matching specs, not brand names, is what guarantees a successful installation and long-term reliability. Tools like Fogatti's RV Water Heater Calculator exist specifically to help you spec-match before you spend a dollar.
Spec-Match First, Then Choose Your Brand
Brand reputation matters, but only after BTU, amps, and dimensions are confirmed to match your rig. As a trusted OEM supplier for manufacturers like Thor Industries, Forest River, and Coachmen, we know firsthand that compatibility standards are non-negotiable at the factory level. The same standard applies to every aftermarket purchase you make.
The RV market is growing fast. Roughly 11.2 million U.S. households own an RV, and Q1 2025 shipments rose 13.9% over the prior year. More buyers are making these decisions right now than ever before.
Use manufacturer spec sheets, RV sizing calculators, and phone technical support before purchasing. The right appliance is not the most popular one. It's the one engineered to fit your rig, your power system, and your travel lifestyle.
