HomeBlogBlog100-Gallon Propane Tank: Tankless Heater Runtime Guide

100-Gallon Propane Tank: Tankless Heater Runtime Guide

How long will a 100 gallon propane tank last for a tankless water heater?

A 100-gallon propane tank doesn’t hold 100 gallons of usable fuel. For safety, it’s typically filled to about 80%, so you usually have roughly 80 gallons of propane available. How long that lasts with a propane tankless water heater depends on how much hot water you use, the temperature rise needed, and the heater’s efficiency.

A practical estimate for most homes

Many propane tankless water heaters burn somewhere around 50,000–200,000 BTU per hour depending on how much hot water is flowing. Propane contains about 91,500 BTU per gallon. That means an 80-gallon usable fill provides about 7.3 million BTU of energy (80 x 91,500).

If your tankless heater averaged 100,000 BTU per hour while running, a full 100-gallon tank (at 80% fill) could provide about 73 hours of burner run time (7,320,000 ÷ 100,000). Because a tankless unit only fires when you use hot water, those hours are spread out over days or weeks—often several weeks for moderate usage, or longer for light usage.

What changes the runtime the most

Hot water demand: Long showers, multiple bathrooms running, or frequent laundry/dishwasher use will shorten tank life.

Cold incoming water: In colder climates or winter months, the heater works harder to raise water temperature, increasing propane consumption.

Flow rate and set temperature: Higher flow and higher setpoints require more BTUs.

Heater size and efficiency: Larger units can burn more propane when fully loaded; better efficiency can reduce fuel used for the same hot water delivered.

For a deeper look at sizing, venting, and cost considerations that affect real-world propane use, see this guide to propane tankless water heaters.

FAQ

What size propane tank do I need for a tankless water heater?

Many households do well with a 100–250 gallon tank, but the right size depends on total propane appliances, peak hot-water demand, and how often you want refills. If you also heat your home with propane, a larger tank is usually needed.