Tankless water heater sizing Last reviewed June 30, 2026
Propane Tankless Water Heater Sizing Guide
By the WhatSize editorial team 4 min read Standards-reviewed
Propane tankless BTU sizing is identical to natural gas. Use BTU = GPM ×500 ×ΔT. Propane delivers 2,516 BTU/ft3 vs NG 1,030 BTU/ft3 (2.4×volume). 1 gallon propane = 91,500 BTU gross / 73,000 BTU delivered at 80% efficiency.
Why propane tankless makes sense for rural homes
Propane tankless water heaters work identically to natural gas units in terms of BTU sizing formula and standard sizes. The main differences:
Fuel content: 1 gallon propane = 91,500 BTU gross (higher heating value at 60°F); at 80% efficiency, ~73,000 BTU delivered per gallon. Propane delivers 2,516 BTU/ft3 vs natural gas 1,030 BTU/ft3 — so propane uses 2.4×the volume for the same BTU output. Per U.S. Energy Information Administration reference data.
Standard propane tankless sizes (same as NG): 130k / 150k / 180k / 199k / 250k BTU. Top propane-specific models: ENERGY STAR lists Bradford White RTGR150X1 (150k BTU, 4.3 GPM propane); Rinnai RU199iP (199k BTU propane, ~11 GPM at 35°F rise); Noritz NRCR111DV (199k BTU dual-fuel NG/LP, 5.5 GPM at 67°F rise, UEF 0.98).
Propane consumption math: A 199,000 BTU Noritz NCC199 at 95% condensing efficiency: 199,000 × (91,500 ×0.95) = ~2.29 gal/hr propane at full fire. A 5.28 GPM Camplux (170k BTU) at 80% efficiency: 170,000 × 73,000 = ~2.33 gal/hr propane. Monthly consumption for typical 4-person household: 30–50 gallons propane for water heating alone.
Tank sizing: A 250-gallon propane tank holds ~915,000 BTU at 80% efficiency = enough for ~6 hours of continuous max-fire use. Most homes use 100–500 gallon propane tanks for whole-house propane tankless.
Top picks for this size
These Amazon-available picks meet the calculated spec. For region-specific or licensed contractor-installed options, consult a local professional. See our Disclaimers.
When to choose propane vs natural gas
- Rural home without NG line: Propane is the only gas option. Standard propane tankless installs are functionally identical to NG.
- Off-grid / cabin: Propane tankless with tank storage. Fogatti InstaShower 7 (42k BTU) is the budget RV pick at ~$300.
- Conversion from NG to propane: Most units have manufacturer conversion kits ($50–$150) + plumber installation.
- Operating cost comparison: Propane typically costs $2.50/gallon vs NG $1.20/therm — propane is more expensive per BTU delivered.
Frequently asked questions
How much propane does a tankless water heater use?
Per EIA reference data, a 199k BTU propane tankless uses ~2.29 gal/hr propane at full fire (95% efficiency). For a 4-person family of 4, monthly propane consumption for water heating alone is 30–50 gallons (Fixr 2025 estimates $350/year for propane tankless operating cost).
Is propane tankless cheaper than electric?
Yes, but with different BTU-per-volume math. A propane tankless at 199k BTU and 95% efficiency uses 199,000 × (91,500 ×0.95) = 2.29 gal/hr propane at full fire. An NG tankless uses 199,000 × (1,030 ×0.95) = 203 ft3/hr natural gas. Same heating output, different volumes.
Can a propane tankless run on natural gas?
Yes. Most propane tankless units can be converted to natural gas with a manufacturer conversion kit ($50–$150) and a plumber's installation. Some units are factory-configured for one fuel only — check the spec sheet before purchasing. Both Rinnai and Noritz offer factory-dual-fuel models (NG/LP switchable).
What size propane tank for tankless water heater?
For a 199k BTU propane tankless, a 250–500 gallon propane tank is typical. Smaller tanks (100–250 gal) require more frequent refills. For off-grid homes with propane tankless only, 500+ gallon tanks are common. A standard 20 lb BBQ tank (4.7 gallons, 344,000 BTU usable at 80%) runs a 100k BTU tankless for ~3.4 hours of continuous max-fire use — useful for emergency RV use only.
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