Tankless water heater sizing Last reviewed June 30, 2026
Electric Tankless Water Heater Sizing Guide
By the WhatSize editorial team 4 min read Standards-reviewed
Electric tankless formula: kW = GPM ×8.33 ×ΔT × 3412. For 7 GPM at 65°F rise = ~11.1 kW (modest) but for whole-house 7 GPM at 75°F rise × 14.5 kW minimum. Standard sizes: 11, 13.5, 18, 24, 27, 36 kW.
Why electric tankless sizing is different from gas
Electric tankless water heaters use the same ENERGY STAR UEF test (DOE 10 CFR 430 Subpart B Appendix E) as gas units. The formula: kW = (GPM ×8.33 ×ΔT) × 3412.
Worked example: 7 GPM at 65°F rise (moderate climate): (7 ×8.33 ×65) × 3412 = 11.1 kW. For 7 GPM at 75°F rise (cold climate): (7 ×8.33 ×75) × 3412 = 12.8 kW. Standard sizes: 11, 13.5, 18, 24, 27, 36 kW.
Top electric tankless models (manufacturer spec analysis, June 2026):
- Camplux TEW27: 27 kW, ~6.5 GPM at 75°F rise, $539 retail
- EcoSmart ECO 27: 27 kW, 6.5 GPM at 75°F rise, $500–$700 retail
- Stiebel Eltron Tempra 29 Plus: ~7 GPM at 75°F rise, $1,400–$1,700 retail
- Bosch Greentherm 9000 T9800: 8.4 GPM at 35°F rise, $2,000–$2,500 retail
- Rheem RTEX-27: 27 kW, 7 GPM at 75°F rise, $600–$900 retail
Cold-climate derate: Per Fixr 2025, electric tankless loses ~10–15% per 10°F inlet drop. A 27 kW unit delivers ~6.5 GPM at 75°F rise (cold) but ~8 GPM at 50°F rise (warm).
Electrical requirements: 24+ kW units require 200A service + 2–3 ×40A 240V breakers. Most US homes have 200A; older 100A homes need panel upgrade ($1,500–$3,000).
Top picks for this size
These Amazon-available picks meet the calculated spec. For region-specific or licensed contractor-installed options, consult a local professional. See our Disclaimers.
When electric tankless makes sense
- Point-of-use (1 fixture): 11–18 kW (Camplux TEW11 $239, TEW14 $287).
- 1–2 person whole-house: 18–24 kW (EcoSmart ECO 18, Rheem RTEX-18).
- 3–4 person whole-house: 24–36 kW (Stiebel Eltron Tempra 29 Plus, Bosch Greentherm 9000).
- Avoid: Electric for 5+ person households — gas dominates on GPM capacity and operating cost.
Frequently asked questions
What size electric tankless water heater do I need?
For point-of-use (1 fixture): 11–18 kW. For 1–2 person household whole-house: 18–24 kW. For 3+ person whole-house: gas tankless is more practical than electric. Electric requires 200A service and high-amp breakers. Real top units: <a href="https://camplux.com/collections/tankless-water-heater" class="text-brand-800 hover-underline">Camplux TEW27</a> (27 kW, $539); EcoSmart ECO 27 (27 kW, 6.5 GPM at 75°F rise).
What electrical service do I need for electric tankless?
Electric tankless (24+ kW) requires 200A service minimum and 2–3 ×40A 240V breakers. If your home has 100A service, you need a panel upgrade ($1,500–$3,000) before installing electric tankless.
How many GPM does an 18 kW electric tankless deliver?
An 18 kW electric tankless delivers about 4.4 GPM at 65°F rise (per kW formula). At lower ΔT (40°F rise, warm climate), it delivers ~7 GPM. Cold climate (45°F inlet, 75°F rise) drops to ~3.8 GPM — derate 14% per 10°F.
Is electric tankless cheaper to run than gas?
No. Electric tankless typically costs $600/year to operate; gas tankless $250/year. Electric is cheaper upfront ($500–$1,500 vs $1,500–$3,500). Over the unit's lifetime, gas saves money on operating cost. Per Fixr 2025, electric operating cost is 2.4×higher than gas per BTU.
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