Quick Answer: Electric tankless water heaters draw far more amperage than most homeowners expect, because unlike gas units they use electricity for all of the heating, not just ignition. A small 18kW point-of-use unit like the EcoSmart ECO 11 draws about 33 amps on a single 40-amp double-pole breaker, while a flagship 36kW whole-home unit like the Rheem RTEX-36 draws up to 150 amps split across four separate 40-amp double-pole breakers on 8 AWG wire. The National Electrical Code requires the breaker to be sized at 125% of that draw, and Stiebel Eltron’s own installation guidance recommends a minimum 150-amp home electrical service for a 24kW unit and 300 amps for a 36kW unit — which is why many electric tankless installs also require a panel upgrade or dedicated subpanel, not just a new breaker.
Every brand roundup on this site compares GPM and BTU, but the number that actually decides whether an electric tankless heater is a straightforward DIY-adjacent swap or a full electrical-service project is amperage. A gas tankless unit only needs a standard 120V outlet for its control board; an electric tankless unit uses its 240V circuit to generate every bit of heat, which is why the amp draw scales so much faster with output.
Why electric tankless amp draw is so much higher than it looks
An electric tankless heater is close to 99% efficient at the point of use — almost none of the electrical energy is lost to venting or standby loss, unlike a gas unit. But that same efficiency means every watt of heating comes directly off the circuit, with no combustion doing part of the work. A 36kW unit is drawing 36,000 watts continuously while it’s running, and at 240V that works out to 150 amps — more than the entire main service in many older homes.
| Model | Rating | Amp draw (240V) | Breaker(s) | Wire gauge |
|---|---|---|---|---|
| EcoSmart ECO 11 | 18 kW | ~33 A | 1 × 40A DP | 1 × 8 AWG |
| EcoSmart ECO 18 | 24 kW | ~54 A | 1 × 60A DP | 1 × 6 AWG |
| EcoSmart ECO 27 | 27 kW | ~75 A | 2 × 40A DP | 2 × 8 AWG |
| EcoSmart ECO 36 | 36 kW | ~100 A | 3 × 40A DP | 3 × 8 AWG |
| Stiebel Eltron Tempra 24 Plus | 24 kW | ~100 A | 2 × 50A DP | 2 × 6 AWG |
| Stiebel Eltron Tempra 36 Plus | 36 kW | ~150 A | 3 × 50A DP | 3 × 6 AWG |
| Rheem RTEX-24 | 24 kW | ~100 A | 3 × 40A DP | 3 × 8 AWG |
| Rheem RTEX-36 | 36 kW | ~150 A | 4 × 40A DP | 4 × 8 AWG |
Figures pulled from published manufacturer installation manuals and spec sheets (EcoSmart, Stiebel Eltron, Rheem) as of August 2026. Always confirm against the exact manual for your specific model — manufacturers revise breaker counts between revisions, and local code can require different sizing.
Notice that two units rated at the same 24kW split the load differently: the EcoSmart ECO 18 lands it on a single 60-amp breaker, while the Rheem RTEX-24 spreads the same output across three 40-amp breakers. Neither approach is wrong — they’re just different internal element configurations — but it means you can’t reuse a breaker count from one brand’s 24kW model when installing a different brand’s 24kW model. See our best electric tankless water heater picks for the full spec sheet on each model before assuming the electrical requirements match.
The NEC’s 125% continuous-load rule
Electric tankless water heaters run continuously the entire time hot water is flowing, which the National Electrical Code classifies as a continuous load — anything expected to run 3 or more hours at a stretch. The code requires the breaker and conductor to be sized at 125% of the unit’s nameplate amp draw, not the raw draw itself, to prevent the circuit from overheating under sustained use. That’s why a unit rated at 100 amps typically lands on a 125-amp calculated load, rounded up to the next standard breaker size — most installers simplify this by using the manufacturer’s own pre-calculated breaker recommendation, which already bakes the 125% margin in.
Multi-circuit units complicate this further: each of the two to four double-pole breakers on a flagship unit is sized and wired as its own independent 125%-rated circuit, landing at its own terminal pair inside the heater. Skipping a circuit, undersizing one leg, or sharing a neutral between circuits is a common installation mistake that either trips breakers under full load or — worse — creates a fire risk that won’t show up until the unit is drawing its full rated amperage on a cold morning.
When you need a panel upgrade or subpanel
| Unit | Manufacturer's minimum home service |
|---|---|
| Stiebel Eltron Tempra 24 Plus (24 kW) | 150 amp |
| Stiebel Eltron Tempra 29 Plus (28.8 kW) | 200 amp |
| Stiebel Eltron Tempra 36 Plus (36 kW) | 300 amp |
Many homes built before the 1990s still run 100-amp or 125-amp main service — enough for lighting, HVAC, and standard appliances, but nowhere near enough headroom to add a flagship electric tankless unit on top. Stiebel Eltron’s own installation guidance is explicit about this: a 36kW Tempra 36 Plus assumes at least 300 amps of total home service, nearly double what many houses have. If your panel is already close to its labeled capacity from existing loads (central AC, an EV charger, an electric range), a licensed electrician needs to run a full load calculation, not just check for open breaker slots.
Even when the main service has enough headroom, most panels don’t have two to four open double-pole slots sitting unused — which is why many installs add a small subpanel dedicated to the water heater rather than trying to free up slots in an already-full main panel. Some local codes also require a disconnect within sight of the unit, which a subpanel satisfies automatically if it’s mounted near the heater.
6 AWG THHN Copper Building Wire (Per Foot / Spool)
- 6 AWG copper is the wire gauge specified for the EcoSmart ECO 18 and Stiebel Eltron Tempra Plus double-pole circuits listed above — confirm your specific model's manual before purchasing, since some 27–36kW units call for 8 AWG split across more circuits instead.
- THHN/THWN-2 insulation is rated for both dry and wet locations, which matters if any part of the run passes through an unconditioned space like a garage or crawlspace.
- Buy per the exact run length needed for each circuit — this is licensed-electrician work on anything beyond a single-circuit point-of-use unit, given the double-pole breaker count involved.
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Double-Pole 40/50/60A Circuit Breaker (Panel-Matched)
- Breaker compatibility is panel-brand-specific — a double-pole breaker rated correctly in amps still won't fit or won't be code-legal in a panel it isn't classified for, so match the panel manufacturer before ordering.
- Multi-circuit units (24kW and up) need two to four of these, each landing its own conductor pair — buy the exact count your model's manual specifies, not one per unit.
- Breaker installation inside a live panel is licensed-electrician work in most jurisdictions and is required for the installation to pass inspection.
120/240V Load Center Subpanel (8–12 Circuit)
- A dedicated subpanel near the water heater location solves both the open-slot problem and, in many jurisdictions, the code requirement for a disconnect within sight of the unit.
- Sized well beyond a single tankless heater's circuit count so it has room for the required number of double-pole breakers without crowding a shared panel with unrelated circuits.
- Subpanel feed sizing, grounding, and bonding must still be calculated by a licensed electrician against your specific unit's total amp draw — this is not a DIY panel swap.
How to choose
- Match kW to your climate and fixture count first, then check the amp draw. Our tankless water heater sizing guide covers matching GPM/kW to your household before the electrical question comes up — a smaller point-of-use unit at 18kW draws roughly a third of a flagship 36kW unit’s amperage.
- Never assume two brands’ same-kW units split the load the same way. The table above shows a 24kW EcoSmart on one 60-amp breaker versus a 24kW Rheem on three 40-amp breakers — pull the exact manual for your model.
- Check your panel’s open double-pole slots and total labeled capacity before buying. A flagship 36kW unit assumes 300 amps of home service per Stiebel Eltron’s own guidance — many older homes need a panel upgrade or subpanel to clear that.
- Budget the electrical work as part of the install cost, not an add-on. Our tankless water heater cost guide breaks down where panel upgrades and subpanels typically land in total install pricing.
- If gas service already runs to your home, compare the total install cost against gas tankless. See our electric vs. gas tankless water heater guide — gas sidesteps the panel-capacity question entirely but adds venting and gas-line sizing instead.
- Pool and spa heating needs its own dedicated circuit and its own product line. A purpose-built unit like EcoSmart’s Smart POOL 18 needs its own 75-amp, 240V circuit separate from any domestic tankless heater — see our guide on using a tankless water heater to heat a pool for why a domestic unit can’t do double duty here.
- Always use a licensed electrician for the actual wiring. Multi-circuit electric tankless installs involve several double-pole breakers landing at one appliance, which most home inspectors specifically check for correct circuit count and wire gauge.
The bottom line
Electric tankless water heaters trade gas-line and venting work for amperage, and the amperage adds up fast: a small 18kW unit draws about 33 amps on one breaker, while a flagship 36kW whole-home unit can draw up to 150 amps split across three or four separate double-pole breakers. The National Electrical Code’s 125% continuous-load rule sets the breaker and wire size from that draw, and manufacturers like Stiebel Eltron publish minimum home-service recommendations — 150 amps for a 24kW unit, 300 amps for a 36kW unit — that many older homes simply don’t have without a panel upgrade or dedicated subpanel. Get your model’s exact breaker count, wire gauge, and minimum service size from its own installation manual before you buy, and have a licensed electrician confirm your panel has the headroom before the heater arrives.