Tankless water heaters are an increasingly popular upgrade for Aiken homeowners who want continuous hot water without the energy waste of keeping 40 or 50 gallons of water heated around the clock. The promise of never running out of hot water during back-to-back showers, reduced energy bills, and a compact unit that frees up floor space is genuinely appealing.
What many homeowners do not realize until they start researching is that electric tankless water heaters have enormous electrical requirements. These are among the most power-hungry appliances you can install in a home, and in many cases, switching from a conventional tank water heater to an electric tankless unit requires significant electrical upgrades. Understanding these requirements upfront helps you make an informed decision and avoid costly surprises.
Why Electric Tankless Water Heaters Need So Much Power
A conventional electric tank water heater uses one or two heating elements, typically rated at 4,500 watts each, to slowly heat and maintain a stored volume of water. The elements cycle on and off as needed to maintain temperature. At any given moment, the maximum draw is 4,500 to 9,000 watts.
An electric tankless water heater has no stored water. It must heat water instantly as it flows through the unit. To heat cold water to a comfortable shower temperature (typically 105 to 110 degrees Fahrenheit) at a usable flow rate (2 to 5 gallons per minute), the unit needs to add 40 to 70 degrees of temperature rise in real time. This requires massive amounts of electrical energy delivered simultaneously.
A whole-home electric tankless water heater typically uses three to four heating elements rated at 7,000 to 11,000 watts each, for a total power demand of 24,000 to 36,000 watts (24 to 36 kW). To put that in perspective, a 28 kW tankless water heater draws more power at peak demand than most homes' entire remaining electrical load combined.
Circuit and Amperage Requirements
The electrical specifications for a whole-home electric tankless water heater are substantial. A typical unit requires three to four dedicated 240-volt circuits, each protected by a 40-amp or 50-amp double-pole circuit breaker. Some larger models require even more circuits.
For example, a popular whole-home model like the EcoSmart ECO 27 requires three dedicated 40-amp, 240-volt circuits with 8-gauge copper wire. The Rheem RTEX-24 requires three dedicated 40-amp circuits. The Stiebel Eltron Tempra 29 Plus requires two 50-amp circuits with 6-gauge wire. Each manufacturer specifies exact requirements that must be followed precisely.
These are not circuits that can be shared with other loads. Each circuit runs directly from the electrical panel to the water heater, with nothing else connected. The wire runs must be sized correctly for both the amperage and the distance from the panel to the water heater location to prevent voltage drop, which can cause the unit to underperform or shut down on error.
At the panel, these three or four new double-pole breakers consume six to eight breaker spaces. If your panel is already full or nearly full, adding these circuits may require a panel upgrade or the installation of a sub-panel.
Panel Capacity: The 150 to 200 Amp Question
This is where the electrical requirements of tankless water heaters become a potential deal-breaker for some homes. A whole-home electric tankless unit drawing 120 to 150 amps at peak demand represents a massive portion of your home's total electrical capacity.
Consider a home with a 200-amp main panel. The NEC requires that the calculated electrical load not exceed the panel's ampacity. If your home's existing loads (HVAC, cooking, lighting, general circuits, dryer, etc.) total 120 to 150 amps of calculated demand, adding a tankless water heater that demands another 120 to 150 amps pushes the total well beyond the 200-amp panel's capacity.
Many older Aiken homes still have 100-amp or 150-amp electrical services. For these homes, installing a whole-home electric tankless water heater almost certainly requires upgrading the electrical service to 200 amps, and in some cases to 300 or 400 amps. A service upgrade involves replacing the panel, the meter base, and potentially the service entrance cable from the utility transformer, plus coordination with the power company. This upgrade can add $2,000 to $4,500 to the project cost.
A load calculation performed by a licensed electrician is the definitive way to determine whether your existing panel can accommodate a tankless water heater. This calculation considers all of your home's electrical loads using NEC demand factors to determine whether the total falls within your panel's capacity.
Wire Sizing and Installation Requirements
The wire running from your electrical panel to the tankless water heater must be sized correctly for the amperage and the length of the run. For a 40-amp circuit, 8 AWG copper wire is standard for runs up to about 50 feet. For longer runs, you may need to upsize to 6 AWG to compensate for voltage drop. For 50-amp circuits, 6 AWG copper is standard.
Each circuit requires its own cable run from the panel to the water heater. If the water heater requires three circuits, that means three separate cable runs, each containing two hot conductors (red and black), a neutral (white, though it may not be used on 240V-only circuits), and a ground (green or bare). These cables are typically 6/2 or 8/3 NM-B (Romex) for interior runs, or individual conductors in conduit for exposed or exterior runs.
The physical routing of three or four cable runs from the panel to the water heater location can be challenging, especially in finished homes where the panel is in the garage and the water heater is in a utility closet on the other side of the house. An experienced electrician can route cables through attic space, crawl space, or along exterior walls in conduit, but the routing should be planned carefully to minimize the length of each run and avoid obstacles.
A dedicated disconnect switch is also required near the water heater, within sight of the unit, so the heater can be de-energized for maintenance. This is a standard NEC requirement for permanently connected appliances.
Gas vs. Electric Tankless: An Electrical Comparison
Gas tankless water heaters (natural gas or propane) heat water using a gas burner rather than electric heating elements. From an electrical perspective, gas tankless units are dramatically less demanding. A gas tankless water heater typically requires only a standard 120-volt, 15-amp outlet for its electronic controls, ignition system, and exhaust fan. No special circuits, no panel upgrades, and no heavy-gauge wiring.
The trade-off is that gas tankless units require a gas line (often a larger-diameter line than your existing gas water heater uses, because tankless units have higher BTU demands), a category III or IV stainless steel vent pipe for exhaust, and adequate combustion air supply. They also require annual maintenance to clean the heat exchanger and check the burner assembly.
For Aiken homeowners with natural gas service, a gas tankless water heater is often the more practical choice from an electrical standpoint. The gas line and venting work is typically less expensive than the electrical upgrades required for an electric tankless unit. However, if your home does not have natural gas service, or if the water heater location does not have access to an exterior wall for venting, an electric unit may be the only option.
Propane tankless units are a third option for homes without natural gas but with an existing propane tank (common in rural parts of the Aiken area). The electrical requirements are the same as natural gas units, just a simple 120-volt outlet.
Point-of-Use vs. Whole-Home Units
Not every tankless water heater installation requires a whole-home unit with its associated heavy electrical demands. Point-of-use (POU) tankless water heaters are smaller units designed to serve a single fixture or a small group of fixtures.
A POU electric tankless unit for a single bathroom sink might require only a single 20-amp, 120-volt circuit and draw 3,500 to 4,000 watts. A POU unit for a shower might require a single 30-amp or 40-amp, 240-volt circuit. These are far more manageable electrical loads that most existing panels can accommodate without upgrades.
POU units are a practical solution for adding hot water to a remote bathroom, a workshop, an outdoor kitchen, or a guest house without running hot water supply lines from the main water heater. They are also a good option for boosting hot water delivery to a distant fixture where the hot water supply line from the main heater results in long wait times.
Some homeowners use a hybrid approach: keeping a conventional tank water heater for the primary hot water load and adding one or two POU tankless units for specific fixtures that need faster or dedicated hot water delivery.
Cost of Electrical Work for Tankless Installation
The electrical work for a whole-home electric tankless water heater installation in the Aiken area typically includes the following costs.
Running three to four dedicated circuits from the panel to the water heater location: $800 to $1,800, depending on the distance and routing complexity. Breaker installation: $150 to $300 for three to four double-pole breakers. Disconnect switch installation: $100 to $200. If a panel upgrade is needed, that adds $1,800 to $3,500 for a 200-amp panel replacement, or $3,000 to $4,500 if the service entrance and meter base also need upgrading.
Total electrical costs can range from $1,100 to $2,300 if your existing panel has capacity, to $3,000 to $6,000 or more if a panel or service upgrade is required. These costs are in addition to the water heater unit itself ($500 to $1,200) and any plumbing work needed to connect the unit to your water supply lines.
Before You Buy: Steps to Take
Before purchasing an electric tankless water heater, take these steps to avoid expensive surprises. First, determine your incoming water temperature. Aiken's groundwater temperature averages about 65 to 68 degrees Fahrenheit, meaning the unit needs to raise the temperature approximately 40 to 45 degrees to reach a comfortable 110-degree output. This temperature rise determines the unit size you need.
Second, calculate your peak hot water flow rate. If you want to run two showers and a dishwasher simultaneously (a total of about 5.5 GPM), you need a unit sized for that combined flow at your required temperature rise. Undersizing the unit means lukewarm water during peak demand.
Third, have an electrician perform a load calculation on your existing panel to determine whether it can accommodate the additional circuits without a panel upgrade. This assessment takes about an hour and gives you a clear answer about the electrical scope and cost before you commit to purchasing a unit.
Next Steps
Unity Power & Light provides electrical assessments, circuit installation, panel upgrades, and all electrical work associated with tankless water heater installations for homeowners in Aiken, SC and the surrounding CSRA. We work with your plumber to coordinate the electrical and plumbing scopes, ensuring everything is installed correctly, permitted, and inspected. Contact us for a load calculation and quote before you purchase your tankless unit.