RV Water Heater Calculator (Showers per Tank, Recovery Time, Propane vs Electric vs Tankless)

How it works

hot fraction = (shower T − cold T) ÷ (tank T − cold T) · usable hot ≈ 0.7 × tank; mixed gallons = usable ÷ hot fraction; minutes = gallons ÷ gpm · recovery min = gal × 8.34 × ΔT ÷ (BTU/h into water) × 60 (propane × 0.6 efficiency; electric 1,440 W ≈ 4,900 BTU/h) · tankless gpm = BTU/h × 0.82 ÷ (500 × temperature rise)

A small RV water heater goes further than its size suggests because shower water is a blend: at 130 °F in the tank and 55 °F from the cold line, a 104 °F shower is only two-thirds hot water, so six gallons of hot becomes about nine of shower water — six minutes at a typical head. The tank cannot be fully drained hot because cold water refills it as you draw, which is where the 70% usable figure comes from. Recovery depends on the heat source: the propane burner is two to three times faster than the 1,440 W electric element, and both together is quickest. Tankless heaters remove the limit but are governed by flow and temperature rise instead.

Worked example

A 6-gallon heater at 130 °F with 55 °F cold water and a 104 °F shower at 1.5 gpm: the shower is 65% hot water, 4.2 usable gallons of hot become 6.4 gallons of shower water — about 4.3 minutes, so not quite one 5-minute shower without pausing. On propane plus electric (≈ 10,200 BTU/h into the water) the tank recovers from cold in about 22 minutes and takes about 18 minutes to recover after a 5-minute shower; a 1.25 gpm head or a navy-shower habit stretches the tank to two people.

Frequently asked questions