Tankless vs Tank Cost & Payback
A tankless costs more to buy and less to run. This tool weighs the two: it prices the same delivered heat at each UEF, takes the difference as your annual saving, and divides the upfront premium by it to give the honest payback — often a slow one.
1 Enter your numbers
Tankless costs less to run and lasts longer, but the upfront and install premium mean the payback is often 15–20 years — about 18.3 years at these numbers. The honest math, not a sales pitch: enter your own gas rate, UEFs and upfront difference.
The tankless pitch is “endless hot water and lower bills.” Both are true — but the bills alone rarely justify the switch, because the upfront and install premium is large and the annual saving is modest. The only honest way to decide on money is a payback: how many years of fuel savings it takes to earn back the extra you paid.
This tool holds the water demand and temperature rise identical for both units, so the only difference is UEF and standby loss. That isolates the running-cost gap cleanly — then your real upfront difference sets the payback horizon. Longer life (a tankless often lasts ~18–20 years vs ~8–12 for a tank) is a separate reason to choose one; enter the numbers and see both sides.
Formula
Same delivered heat, two efficiencies, then a payback:
- ΔT = output_temp − inlet_temp; annual_BTU = daily_gal × 365 × 8.33 × ΔT
- tank cost = (annual_BTU ÷ 100,000 ÷ tank_UEF) × $/therm
- tankless cost = (annual_BTU ÷ 100,000 ÷ tankless_UEF) × $/therm
- annual saving = tank cost − tankless cost
- payback = upfront_difference ÷ annual saving
Worked example
Gas tank vs gas tankless. 64 gal/day, ΔT 70 °F, $1.20/therm:
- tank (UEF 0.62): 219.7 therms × $1.20 = $264/yr
- tankless (UEF 0.90): 151.3 therms × $1.20 = $182/yr
- annual saving = 264 − 182 = $82/yr
- payback (on a $1,500 premium) = 1,500 ÷ 82 = 18.3 years
That is longer than many tanks last — so if the case for tankless is purely operating cost, the math is honest but unflattering. Endless flow, longer life and the space savings are the real reasons people still switch.
Count the whole install gap, not just the unit
Measure this first: the true upfront difference — a tankless conversion adds stainless venting and often a bigger gas line or electrical circuit, so the install gap is usually wider than the unit-price gap alone (see the tankless installation tool).
Common mistakes: comparing a tankless’s UEF to a tank’s without pricing the same water; ignoring that higher daily use shortens the payback (more gallons = more saving per year); and treating payback as the whole story — a tankless’s longer life and endless flow have value the dollar figure doesn’t capture.
Reference table
Typical Uniform Energy Factor (UEF) by type — a labeled planning snapshot. UEF is delivered hot-water energy ÷ energy consumed, so a higher number means a cheaper year. Confirm your unit’s figure on its yellow EnergyGuide label; a heat pump exceeds 1.0 because it moves heat rather than making it.
| Type | Typical UEF |
|---|---|
| Gas storage tank | 0.62 |
| Electric storage tank | 0.92 |
| Gas tankless | 0.90 |
| Electric tankless | 0.98 |
| Heat-pump / hybrid | 3.50 |
| Condensing gas | 0.90 |
| Propane tank | 0.62 |
Frequently asked questions
Is a tankless water heater worth it?
On running cost alone, often not quickly — the payback is frequently 15–20 years because the annual saving is modest against a large upfront and install premium. It becomes worth it when you also value endless hot water, the longer lifespan (~18–20 years) and the reclaimed floor space. Enter your own numbers to see your payback.
How is the payback calculated?
Payback = the extra you pay upfront ÷ the money you save each year. We price the identical delivered heat at each unit’s UEF, take the difference as the annual saving, and divide your upfront premium by it. A bigger household (more gallons) saves more per year and pays back faster.
Does a tankless really use less energy?
Yes — a gas tankless has a higher UEF (~0.90 vs ~0.62 for a standard gas tank) and no standby loss, so it burns less fuel for the same hot water. The saving is real but usually a few tens of dollars a year, not hundreds, which is why the payback is long.
What if I compare electric tank to electric tankless?
The framework is the same, but electric tankless units draw very heavy amperage and rarely make financial sense whole-house; for electricity the big efficiency win is a heat pump, not a tankless. Use the heat-pump savings tool for that comparison.