Electric tankless sizing is climate-dependent in a way gas tankless isn\'t. The kW capacity must match the temperature rise (incoming water temperature to desired output temperature) at peak simultaneous flow. Get this wrong and you\'ll have tepid showers in winter.
Step 1 — determine your incoming water temperature
| Region | Winter incoming water | Summer incoming water |
|---|---|---|
| Florida, Hawaii, S. Texas, S. Arizona | 65-72°F | 75-82°F |
| S. California, S. Carolina, Georgia | 55-62°F | 70-78°F |
| N. California, mid-Atlantic, Tennessee | 50-55°F | 65-72°F |
| Pacific NW, Ohio Valley, mid-Atlantic | 45-52°F | 60-68°F |
| Upper Midwest, Mountain West, New England | 38-45°F | 55-62°F |
| Far north (Alaska, far Maine) | 33-40°F | 50-58°F |
Always size for WINTER, not summer. Your tankless must handle the coldest incoming water.
Step 2 — calculate required temperature rise
Target output: 120°F (energy-efficient, scald-safe, Legionella-suppressing).
- Sun Belt winter: 120°F - 70°F = 50°F rise
- Mid-Atlantic winter: 120°F - 50°F = 70°F rise
- Upper Midwest winter: 120°F - 42°F = 78°F rise
- Far north winter: 120°F - 37°F = 83°F rise
Step 3 — calculate peak simultaneous flow
Add up fixtures running at once worst-case:
| Fixture | Typical GPM |
|---|---|
| Standard shower (1.5-2.0 gpm head) | 1.5-2.0 |
| High-flow shower head (older 2.5 gpm) | 2.5 |
| Tub fill | 4-6 |
| Kitchen sink | 1.5 |
| Bathroom sink | 0.5-1.0 |
| Dishwasher | 1.5 |
| Clothes washer | 2-4 |
Example: 1 shower (2 gpm) + 1 bathroom sink (0.5) + 1 kitchen sink (1.5) = 4 gpm peak demand.
Step 4 — map to Tempra Plus capacity
| Model | kW | GPM at 45°F rise | GPM at 70°F rise | GPM at 78°F rise |
|---|---|---|---|---|
| Tempra 12 Plus | 14.4 | 2.6 | 1.7 | 1.5 |
| Tempra 15 Plus | 14.4 | 2.6 | 1.7 | 1.5 |
| Tempra 20 Plus | 19.2 | 3.5 | 2.3 | 2.0 |
| Tempra 24 Plus | 24.0 | 4.3 | 2.8 | 2.5 |
| Tempra 29 Plus | 28.8 | 5.2 | 3.4 | 3.0 |
| Tempra 36 Plus | 36.0 | 6.5 | 4.2 | 3.7 |
Find the row that supports your peak flow at your winter temperature rise.
Recommended sizing by climate + bathrooms
| Climate | 1 bathroom | 2 bathroom | 3+ bathroom |
|---|---|---|---|
| Sun Belt (50°F rise) | Tempra 12-15 | Tempra 20-24 | Tempra 24-29 |
| Mid-Atlantic (70°F rise) | Tempra 20 | Tempra 24-29 | Tempra 29-36 |
| Upper Midwest (78°F rise) | Tempra 24 | Tempra 29-36 | Tempra 36+ |
| Far north (83°F rise) | Tempra 24-29 | Tempra 36 | Tempra 36 (verify capacity) |
Flow Control safety net
Stiebel Tempra Plus units include Flow Control: if you exceed the unit\'s capacity, it auto-reduces total flow to maintain setpoint temperature. You\'ll feel slightly weaker water pressure, but you won\'t feel cold water in your shower. This is the key Stiebel advantage over US-market constant-power competitors.
Sizing for the Accelera heat pump (different — storage-based)
The Accelera 300 E is 80-gallon storage with heat pump recovery. FHR is 78 gallons in the first hour. Sizing rules:
- 2-3 person: capacity overkill, but Accelera is the only US size
- 3-5 person: ideal sizing
- 5-6 person: at the edge of capacity; consider supplemental storage or alternate brand
- 6+ person: not the right product; consider commercial heat pump
Bottom line
Tempra 29 Plus is the volume cold-climate pick. Tempra 24 Plus for moderate climates with 2 bathrooms. Tempra 12 or 15 Plus for warm climates and small homes. Always size for winter incoming water, not summer. When in doubt, size up — Flow Control handles capacity excess gracefully.