Two costs in every shower
A hot shower costs money twice: once for the water and sewer charge, and once for the energy that heats the water. In most homes, the energy is the larger part.
- Water used = shower length × flow rate
- Energy = water mass × 4.186 kJ/kg·°C × temperature rise ÷ water heater efficiency
- Cost per shower = energy × energy price + water used × water price
The temperature rise is the gap between the shower temperature (about 104 °F) and the cold water coming into the house, which ranges from about 40 °F in northern winters to 75 °F in southern summers.
Worked example
An 8-minute shower with a 2.0 gpm showerhead, an electric water heater, electricity at 17 cents per kWh, water and sewer at $12 per 1,000 gallons, and cold water at 55 °F.
- Water: 8 × 2.0 = 16 gallons (61 liters).
- Temperature rise: 104 − 55 = 49 °F, or 27 °C.
- Energy: 61 × 4.186 × 27 ÷ 3,600 ÷ 0.90 = 2.13 kWh.
- Cost: 2.13 × $0.17 + 16 × $0.012 = $0.36 + $0.19 = $0.55.
- For two showers a day, that is about $405 a year.
Water heater types
| Water heater | Efficiency used | Effect on cost |
|---|---|---|
| Electric resistance | 90% | Most expensive per shower at typical rates |
| Gas | 80% | Usually cheaper, since gas costs less per kWh |
| Heat pump | 250% | Uses about a third of the electricity of a resistance heater |
Enter the price of the energy your heater uses. Natural gas at $1.40 per therm works out to about 4.8 cents per kWh.
How to cut the cost
- Install a low-flow showerhead. Going from 2.0 to 1.5 gpm saves 25% of both water and energy, about $101 a year in this example. Look for the WaterSense label.
- Take shorter showers. Each minute cut from both daily showers saves about $51 a year here.
- Lower the water heater setting to 120 °F, which cuts standby losses and scalding risk.
- Insulate hot water pipes so less heat is lost on the way to the shower.
Shower or bath?
A typical bathtub holds 30 to 50 gallons of hot water. That equals a 15 to 25 minute shower at 2.0 gpm, so a short shower almost always costs less.