Shower Water Usage Calculator: How much water does a shower in an RV use?

Four minutes of running water at 1.3 gal/min (5 L/min) is 5.3 gal (20 L) – a 26 gal (100 L) tank thus allows five showers, and nothing else. And a 2.6 gal (10 L) heater at 140 °F (60 °C) mixed delivers only a good 4.9 gal (18 L) of warm water, i.e. just under four minutes. This calculator shows what showering costs in water, hot water and energy.

Water per shower

5.3 gal

11 gal per day at 2 showers

Showers per tank (showers only): 4.9
Warm showering with one heater fill: 3 h 44 min
Energy per heater heat-up: 0.56 kWh; ≙ about 2 oz of propane

The water heater is only enough for 3.7 minutes of warm showering – after that it turns cold or you wait for the reheat time (about 20–30 min). Turn the water off while soaping.

Show calculation

1Water per shower: 4 min × 1.3 gal/min = 5.3 gal
2Per day: 5.3 gal × 2 = 10.6 gal
3Showers per tank: 26 gal ÷ 5.3 gal = 4.9
4Hot water mixed: 3 gal × (140 − 54) ÷ (100 − 54) = 4.9 gal → 3.7 min warm
5Heating energy: 3 gal × 2.45 Wh/(gal·°F) × 86 °F = 0.56 kWh

How it's calculated

Water per shower = water running time [min] × flow rate [gal/min]
Showers per tank = tank volume ÷ water per shower

Hot water from the water heater

The 140 °F (60 °C) water from the heater is mixed with cold water to shower temperature – so the heater's content is enough for more water than its volume:

Mixed water = heater volume × (T_heater − T_cold) ÷ (T_shower − T_cold)

10 L × (60 − 12) ÷ (38 − 12) = 18.5 L (4.9 gal) → at 5 L/min (1.3 gal/min) 3.7 minutes

Heating energy [kWh] = liters × 1.163 Wh/(L·K) × temperature rise ÷ 1000

10 L (2.6 gal) from 54 to 140 °F (12 to 60 °C): 0.56 kWh ≙ about 50 g (1.8 oz) of propane or 0.56 kWh of electricity

Worked example

Two people each shower three minutes a day with a low-flow head (0.8 gal/min); 26 gal (100 L) tank, 2.6 gal (10 L) water heater at 140 °F, cold water 59 °F.

Inputs

Water running time per shower: 3 min
Shower flow rate: 0.79 gal/min
Showers per day (all persons): 2 ×
Fresh water tank: 26.4 gal
Water heater volume: 2.64 gal
Water heater temperature: 140 °F
Shower temperature: 100 °F
Cold water temperature: 59 °F

Result

2.4 gal

Water per shower

Showers per tank (showers only): 11.1
Warm showering with one heater fill: 6 h 31 min
Energy per heater heat-up: 0.52 kWh

Calculation

Water per shower: 3 min × 0.8 gal/min = 2.4 gal
Per day: 2.4 gal × 2 = 4.8 gal
Showers per tank: 26 gal ÷ 2.4 gal = 11.1
Hot water mixed: 3 gal × (140 − 59) ÷ (100 − 59) = 5.2 gal → 6.5 min warm
Heating energy: 3 gal × 2.45 Wh/(gal·°F) × 81 °F = 0.52 kWh

The variables explained

Water running time (min): Effective time with the water running – with a shut-off valve often only 2–3 minutes.
Flow rate (gal/min): Depends on shower head, pump and line pressure. Measure: 10 seconds into a measuring cup, result × 6.
Water heater volume and temperature: Truma Combi/water heater: 10 L (2.6 gal), usually 140 °F (60 °C) (eco: 104 °F); typical US RV water heaters: 6 or 10 gal.
Cold water temperature: Tank water – much colder in winter, then the heater is enough for fewer gallons.

Common mistakes

Equating heater volume with shower amount: thanks to mixing, 10 L of heater water is enough for about 18 L of shower water.
Eco mode (104 °F / 40 °C) in winter: then there's hardly any mixing water left – the shower turns cold after two minutes.
Forgetting the gray tank: 5 gal (20 L) of shower water ends up entirely in the gray tank.

Assumptions and limits

Ideal mixing without heat losses in the lines.
Specific heat capacity of water 4.186 kJ/(kg·K).
Propane equivalent at 12.87 kWh/kg and 90% efficiency.