Water Heater Options for ADUs

Small building, small closet, big decision. Water heating in an ADU is a three-way race between efficiency, space, and cost.

Heat-pump water heater (HPWH) — the code favorite. A tank unit that extracts heat from surrounding air; it uses a fraction of the electricity of a standard electric tank, and California's Title 24 loves it — specifying one often makes the rest of your energy compliance easy. Trade-offs: it's a tank (needs a closet, garage corner, or exterior enclosure), it wants some air volume around it, and it makes a little noise, so don't put it against the bedroom wall. Cost: $2K–$4K installed. The default choice for all-electric ADUs.

Electric tankless — the space saver with a catch. Shoebox-sized, endless hot water. The catch is electrical: whole-home electric tankless units draw enormous instantaneous power, which can blow up your load calculation and even force a panel upgrade by itself. Fine as point-of-use units for a small studio; risky as the main heater for a full ADU. Check with the load calc before falling in love.

Gas tankless — great if gas is already there. Compact, efficient, endless hot water, modest electrical draw. Only sensible when your ADU has gas anyway (lesson 7.6); running a gas line just for water heating rarely pens out. Cost: $2.5K–$4.5K installed plus venting.

Standard tanks? A basic electric resistance tank is cheap upfront and painful in Title 24 compliance and monthly bills. It's the answer only when budget rules everything else.

The practical guidance: all-electric ADU → heat-pump water heater, placed away from bedrooms, decided during design so the closet and condensate drain exist. Gas-equipped conversion → gas tankless. Studio with a tight panel → discuss point-of-use with the electrician holding the load calc.