A question that shows up constantly in home improvement forums: “My house only has 100 amp service, can I still install a tankless water heater?” The honest answer is more nuanced than a simple yes or no, and it depends on math most homeowners have never had to do before.
Rheem recommends 200-amp household electrical service for the RTEX-24, though whether you strictly need it depends on your home’s total existing electrical load, not just this one appliance. A licensed electrician can perform a load calculation to determine whether your specific home, even with less than 200-amp service, has enough spare capacity to safely add the RTEX-24’s roughly 100-amp draw.
Why Rheem Recommends 200 Amp Service
The RTEX-24 draws up to 100 amps at 240V when running at full capacity, split across three 40-amp double-pole circuits. That’s a substantial percentage of a typical home’s total electrical capacity, which is why Rheem’s own product listing specifically states “200 AMP Household service recommended” alongside the breaker requirements.
For context, a 100-amp panel, common in older homes, has very little room left for a 100-amp appliance once you account for existing circuits (lighting, outlets, HVAC, kitchen appliances, and so on). A 200-amp panel gives meaningfully more headroom to add a significant new load like this without exceeding your home’s total safe capacity.
What If You Only Have 100 or 150 Amp Service?
This doesn’t automatically rule out the RTEX-24, but it does mean you need a proper electrical load calculation before proceeding, not a guess. A licensed electrician calculates your home’s total existing electrical demand (all your circuits combined, using standard formulas from the National Electrical Code) and determines how much spare capacity actually exists.
If the math doesn’t work, you have a few options: upgrade your electrical service entirely (a project that involves your utility company and is more expensive than simply adding breakers), install a smaller RTEX model that draws less power, or in some cases, split hot water demand across multiple smaller units serving different zones of your home instead of one large whole-house unit.
The Real Cost of a Service Upgrade
Upgrading from 100-amp to 200-amp service is a significantly bigger project than adding a sub-panel, since it typically involves your utility company, a new meter base, and often permit and inspection requirements. This is a different scope of work entirely from the electrical work described by RTEX-24 reviewers who already had 200-amp service and just needed panel space or a sub-panel.
If you’re already facing a service upgrade for other reasons (a home addition, an EV charger, upgrading old aluminum wiring), it may make sense to size that upgrade generously enough to accommodate a future tankless water heater at the same time, rather than doing the work twice.
How to Find Out What You Actually Have
Your main breaker panel typically has the total amperage rating printed directly on the main breaker switch or somewhere on the panel door, commonly 100, 150, or 200 amps. If you can’t find or confirm this, a licensed electrician can verify it quickly during an initial consultation, well before you commit to purchasing a tankless unit.
A Practical Path Forward
If you already have confirmed 200-amp service with available panel capacity, you’re in the straightforward case most positive RTEX-24 reviews describe. If you have less than 200-amp service, don’t assume you’re automatically excluded, get a proper load calculation from a licensed electrician first. The cost of that consultation is small compared to discovering mid-installation that your panel can’t support the unit.
For the full picture of what electrical work this unit requires beyond just the amperage question, see our complete electrical requirements guide.
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater - Check Current Price on Amazon
The Bottom Line
Rheem recommends 200-amp service for the RTEX-24, but homes with less than that aren’t automatically excluded, a proper electrical load calculation from a licensed electrician is the only reliable way to know whether your specific home has enough spare capacity, and if not, what your realistic options are.
Frequently Asked Questions
Do I need 200 amp service for the Rheem RTEX-24? Rheem recommends 200-amp household service, though homes with less may still qualify depending on their total existing electrical load, confirmed through a professional load calculation.
Can I install a tankless water heater with 100 amp service? Possibly, but it requires a proper electrical load calculation to confirm you have enough spare capacity after accounting for all your home’s existing circuits. Many 100-amp homes will not have sufficient headroom.
How much power does the Rheem RTEX-24 actually draw? Up to 100 amps at 240V when running at full capacity across its three 40-amp double-pole circuits.
What is an electrical load calculation? It’s a standardized process, based on National Electrical Code formulas, that a licensed electrician uses to determine your home’s total electrical demand and how much spare capacity remains for a new appliance.
How much does it cost to upgrade from 100 amp to 200 amp service? This varies significantly by region and utility company requirements, and is a larger project than adding a sub-panel, since it typically involves your utility company and a new meter base.
What if my electrician says I don’t have enough capacity? Options typically include upgrading your electrical service, choosing a smaller RTEX model that draws less power, or splitting hot water demand across multiple smaller units.
How do I know what amperage service my home has? Check your main breaker panel, the total amperage rating is usually printed on the main breaker switch or somewhere on the panel itself. A licensed electrician can confirm this quickly if you’re unsure.
Is a sub-panel the same as upgrading my service? No. A sub-panel adds additional breaker capacity within your existing service rating, while a service upgrade increases your home’s total electrical capacity from the utility connection itself, a bigger project.
Should I upgrade my service if I’m already having other electrical work done? If you’re already planning other significant electrical work, it can make sense to size any upgrade generously enough to accommodate a future tankless water heater, avoiding the need to do similar work twice.
Is it dangerous to add a tankless water heater without confirming capacity first? Yes, exceeding your panel’s safe capacity can create real fire and safety risks, which is exactly why a professional load calculation before installation is strongly recommended rather than optional.
References
- Rheem RTEX-24 official electrical requirements and 200-amp service recommendation, Amazon.com manufacturer content, accessed July 2026: https://www.amazon.com/dp/B01MS9DVEE
- NFPA general residential electrical service sizing guidance, accessed July 2026: https://www.nfpa.org/News-Blogs-and-Articles/Blogs/2022/01/06/Considerations-for-Single-Family-Residential-Electrical-Services-Based-on-the-2020-NEC