Fuel type · the honest version
Gas or Electric Tankless: What Your House Can Actually Carry
An electric whole-house unit is a panel decision before it is a plumbing decision.
An electric whole-house unit is a panel decision before it is a plumbing decision.
Quick answer: If the house has natural gas, a gas tankless unit is almost always the right tankless unit. A whole-house electric unit at 27 kW draws 113 amps and needs three dedicated 40-amp double-pole breakers — six slots — and the manufacturers recommend 200-amp service to carry it. On a 100-amp Charleston panel that is not tight, it exceeds the entire service. Electric tankless is genuinely right for a point-of-use application, an addition or a garage apartment; for a whole house with no gas, a heat pump water heater usually beats it. Call (843) 718-8177.
This is the question that most often produces a quote the homeowner did not expect, because the expensive part of an electric tankless installation is not plumbing at all.
The numbers below are the manufacturers’ own, off their published specification sheets. They are not opinions and they are not hard to check.
EcoSmart’s own electrical requirements sheet for the ECO 27, verbatim: “200 Amp Household Service Recommended … 113 Amp Max Draw … 3 x 40 Amp Double Pole Breakers Required … you must have enough room in your breaker panel to dedicate three 40 Amp double pole breakers solely to the unit.” (EcoSmart ECO 27 electrical requirements).
Read that again. One hundred and thirteen amps, for the water heater, before the air conditioning, the range, the dryer or anything else in the house.
Upgrading a panel from 100 to 200 amps runs $1,300 to $1,600, and a new 200-amp panel $1,800 to $2,500. (HomeGuide — cost to replace an electrical panel). Add a dedicated circuit at $250 to $900 (HomeGuide — tankless water heater installation cost), and the electrical work can exceed the water heater.
Stiebel Eltron publishes a service-size table rather than a single figure, and it is the clearest guide in the category: 100 amps for their smallest units, 150 for the Tempra 24, 200 for the Tempra 29, and 300 amps for the Tempra 36.
| Unit | What it draws | Service | Source |
|---|---|---|---|
| EcoSmart ECO 27 — 27 kW | 113 A, on three 40 A double-pole breakers and three runs of 8 AWG | 200 A recommended | EcoSmart ECO 27 electrical requirements · 2026 |
| Rheem RTEX-27 — 27 kW | 113 A, on three 40 A double-pole breakers and three runs of 8 AWG | 200 A recommended | Rheem RTEX electric tankless specification sheet · TKE-101 rev. 9, 06/24 |
| Stiebel Eltron Tempra 24 Plus — 24 kW | two runs at 50 A, on two 50 A double-pole breakers | 150 A recommended | Stiebel Eltron Tempra series technical specifications · rev. 6/2025 |
| Stiebel Eltron Tempra 29 Plus — 28.8 kW | three runs at 40 A, on three 40 A double-pole breakers | 200 A recommended | Stiebel Eltron Tempra series technical specifications · rev. 6/2025 |
⚠️ Every one of those manufacturers says recommended, not required. Say it their way. Rheem puts the consequence plainly for anyone swapping an electric tank: “When converting from an electric tank to an electric tankless water heater, household electric service panel upgrades are often required.”.
An electric tankless unit is rated in kilowatts, and kilowatts buy a temperature rise. How much water you get depends entirely on how cold the water arriving at the house is, and that changes with the season.
| Incoming water | Flow at 105°F | What that runs |
|---|---|---|
| 37°F | 2.7 gpm | One shower, and nothing else |
| 47°F | 3.2 gpm | One shower |
| 57°F | 3.8 gpm | A shower and a basin |
| 67°F | 4.9 gpm | Two showers, carefully |
| 77°F | 6.6 gpm | Two showers |
Manufacturer figures for the Rheem RTEX-27 and EcoSmart ECO 27, calculated to a 105°F set point. Source: Rheem RTEX electric tankless specification sheet. ⚠️ We will not publish a Charleston winter inlet temperature, because no utility or agency publishes one. Charleston Water System publishes a 69.8°F annual average for finished water at the plant, which is an annual average of treated water and not a winter design figure — using it to size a unit would flatter the result. Ask for a measured inlet temperature in January.
A gas unit trades the electrical problem for a fuel one. About 199,900 BTU/h against 40,000 BTU/h for the 40-gallon tank it replaces, about five times the gas demand of the tank it replaces (A.O. Smith ProLine atmospheric vent specification sheet).
All four major manufacturers specify a 3/4 inch gas connection at the unit, and all of them size the supply for the whole house: “The gas supply line shall be gas tight, sized, and so installed as to provide a supply of gas sufficient to meet the maximum demand of the heater and all other gas consuming appliances at the location without loss of pressure.” (Rinnai RU199i specification).
If this water heater is replacing an existing water heater, it is important to verify the capacity of the gas system. Check the capacity of the meter/regulator, and verify the pipe lengths and sizes. — Rheem’s instruction to installers (Rheem tankless use & care manual with installation instructions).
⚠️ Half-inch gas pipe is not automatically disqualified. Rheem permits it where system pressure is 8.0 inches water column or greater; Navien permits it subject to the sizing tables and to local approval. Anyone who tells you the pipe size without doing the calculation has not done the calculation.
A small point-of-use unit under a distant sink is an excellent, cheap fix for a two-minute wait for hot water. It is a different product from a whole-house unit and it needs one ordinary circuit.
Small hot water demand, no gas run available, and often a new sub-panel going in anyway. This is where electric tankless genuinely shines.
If the load calculation works, an electric whole-house unit is quiet, compact and has no vent, no combustion air and no condensate. The load calculation is the whole question, and it should be done in writing before anyone orders equipment.
An all-electric house on 100-amp service with a full panel. The unit costs less than a gas one and the installation costs more, and the result is a heater that cannot deliver its rated flow on a January morning. Price a heat pump water heater instead.
A 27 kW whole-house unit draws 113 amps and requires three dedicated 40-amp double-pole breakers with three runs of 8 AWG wire. EcoSmart and Rheem both publish exactly those figures for their 27 kW units.
Smaller units are lighter but still substantial — Stiebel Eltron’s Tempra 24 takes two 50-amp double-pole breakers. All three manufacturers say a service size is recommended rather than required, but a 113-amp draw on a 100-amp service exceeds the entire service, which is a question of arithmetic rather than of opinion.
A whole-house one, almost certainly not. Manufacturers recommend 200-amp service for their 27 to 29 kW units, and Stiebel Eltron publishes 100-amp service as the minimum for its smallest models only.
The options are a service upgrade, which runs $1,300 to $1,600 to go from 100 to 200 amps, a smaller unit that will not deliver full flow in winter, or a different appliance. In an all-electric Charleston house the third of those is usually a heat pump water heater, which is also the only water heater in this state with meaningful rebate money attached.
Gas, if there is gas at the house. It delivers far higher flow for a given size, it does not touch the electrical panel, and it is what the whole-house market is built around.
Gas brings its own work: the supply line is sized for the whole house at around 199,900 BTU per hour, the vent is new because the old tank flue cannot be reused, and a condensing unit needs a condensate drain. Electric brings no vent, no combustion air and no condensate — and a panel problem instead.
Yes. The controls, the ignition, the fan and the freeze protection are all electric, so a gas tankless unit does not produce hot water during a power cut.
That matters more here than it sounds. Freeze protection on these units is electric, and Charleston averages 24.4 freezing nights a year with individual years reaching fifty-one. A unit mounted outside or in an unheated garage during a winter outage has no freeze protection running, which is a good reason to have isolation valves fitted so it can be drained.
It depends entirely on how cold the incoming water is. A 27 kW unit delivers 2.7 gallons a minute at a 37°F inlet and 6.6 at 77°F, calculated to a 105°F set point — that is the manufacturer’s own table.
So the same unit runs two showers comfortably in August and struggles with one in January. Anyone sizing an electric unit for you should be using a measured winter inlet temperature. Charleston Water System publishes a 69.8°F annual average for finished water at the treatment plant, but that is an annual average of treated water and it is not a winter design figure — using it would flatter the result.
Charleston, Mount Pleasant, North Charleston, Summerville, Goose Creek, James Island, West Ashley, Daniel Island, Johns Island, Hanahan, Ladson, Moncks Corner, Isle of Palms, Folly Beach, Park West, Rock Hill, Fort Mill, Tega Cay, Lake Wylie, Clover, York, Indian Land — across the Charleston Lowcountry and York County, South Carolina.
Southeastern Renovation contracts the job as the general contractor. The plumbing and gas work is performed by licensed plumbing subcontractors who pull their own trade permits.
Part of Southeastern Renovation's plumbing division. Charleston's water chemistry is covered in more depth by the water treatment division.