Tankless Size Chart

Flow rate and temperature rise of small tankless water heaters for ADUs, cabins and off-grid buildings

Section 13.1

Sizing example: one-bath electric ADU in Wisconsin

A one-bedroom backyard ADU in Wisconsin, all electric, needs 3.0 GPM at a 68 F rise. Rinnai's map puts Wisconsin ground water at 37 to 42 F[1], and this example uses 37 F. Only the two 36 kW models deliver that flow.

Fixtures at the same time
WaterSense 2.0 GPM shower plus a 1.0 GPM bathroom sink, 3.0 GPM in total
Rise
105 F minus 37 F = 68 F
Heat needed
500.4 x 3.0 x 68 = about 102,000 BTU/h, or about 30 kW
Table 13.1 One shower plus one bathroom sink, 37 F water, 105 F outlet. Sources: notes 2 and 3.
ModelFlow at 68 F riseAgainst 3.0 GPMManufacturer requirement
Stiebel Eltron Tempra 36 Trend / Plus3.7 GPMDelivers 3.0 GPM3 x 50 A draw, breakers 3 x 50 A, 300 A main service
EcoSmart ECO 363.6 GPMDelivers 3.0 GPM150 A draw, breakers 4 x 40 A DP, 300 A main service
EcoSmart ECO 80.8 GPMShort by 2.2 GPM33 A draw, breakers 1 x 40 A DP
Stiebel Eltron Tempra 12 Trend / Plus1.2 GPMShort by 1.8 GPM50 A draw, breakers 1 x 50 A, 100 A main service
EcoSmart ECO 111.3 GPMShort by 1.7 GPM54 A draw, breakers 1 x 60 A DP
Stiebel Eltron Tempra 15 Trend / Plus1.5 GPMShort by 1.5 GPM2 x 30 A draw, breakers 2 x 30 A, 100 A main service
EcoSmart ECO 181.8 GPMShort by 1.2 GPM75 A draw, breakers 2 x 40 A DP, 150 A main service
Stiebel Eltron Tempra 20 Trend / Plus2.0 GPMShort by 1.0 GPM2 x 40 A draw, breakers 2 x 40 A, 125 A main service
Stiebel Eltron Tempra 24 Trend / Plus2.5 GPMShort by 0.5 GPM2 x 50 A draw, breakers 2 x 50 A, 150 A main service
EcoSmart ECO 242.4 GPMShort by 0.6 GPM100 A draw, breakers 3 x 40 A DP, 200 A main service
EcoSmart ECO 272.7 GPMShort by 0.3 GPM113 A draw, breakers 3 x 40 A DP, 200 A main service
Stiebel Eltron Tempra 29 Trend / Plus2.9 GPMShort by 0.1 GPM3 x 40 A draw, breakers 3 x 40 A, 200 A main service

13.1.1 Result

No unit under 36 kW delivers 3.0 GPM at a 68 F rise. A 36 kW unit draws 150 A on either manufacturer's sheet[2][3], more than many ADU subpanels carry. A 1.5 GPM showerhead with a 0.5 GPM sink aerator lowers the required flow to 2.0 GPM. The 24 kW models deliver that. The Tempra table lists 2.52 GPM at a 66 F rise for the Tempra 24[2]. A separate point-of-use unit at the sink is the other option.

Whether the main service and the ADU feeder can carry any of these loads is for a licensed electrician to determine.

Notes and sources

  1. Rinnai, non-condensing tankless brochure with United States Ground Water Temperature Map and V53De table, 06/2022 (PDF hosted by Woodstove Pro)
  2. Stiebel Eltron, Tempra table of flow rates at temperature rises (technical bulletin PDF)
  3. EcoSmart, electric tankless sizing sheet with zone table, 105 F set point (PDF)