Tankless Water Heater Sizing Guide: Gas & Electric

Tankless Water Heaters: How to Size the Right Unit for Your Home

The single most common mistake homeowners make when switching to tankless isn’t choosing the wrong brand — it’s choosing the wrong size. An undersized unit means fighting for hot water the moment a second shower or the dishwasher kicks on. An oversized unit means paying more upfront and, in the case of electric models, potentially triggering an expensive electrical panel upgrade you didn’t actually need. Getting the size right is what determines whether you love your tankless system or regret the switch.

If you’re a homeowner in Egg Harbor Township, Mullica Township, or Pine Hill weighing a tankless upgrade, here’s how sizing actually works, what real brands and models look like at each size tier, and what you should realistically expect to pay — for both gas and electric.

How Tankless Sizing Actually Works

Unlike a tank water heater, which is sized by storage capacity (gallons), a tankless unit is sized by flow rate — how many gallons per minute (GPM) it can heat continuously — combined with temperature rise, which is how many degrees the unit needs to raise the incoming water temperature to reach your desired output.

Temperature rise matters more in South Jersey than sizing guides written for warmer climates suggest. Groundwater here runs cold in the winter — often in the 40s°F — meaning a unit needs to achieve roughly a 60-70°F temperature rise to deliver a comfortable 110-120°F at the tap. A unit that performs beautifully in a warm-climate sizing example may deliver noticeably less usable flow once winter groundwater hits it here.

The basic method: add up the flow rate of every fixture likely to run at the same time during your household’s busiest moment, then size a unit that can hit that combined GPM at your winter temperature rise.

Typical fixture flow rates:

Shower: 2.0–2.5 GPM
Kitchen sink: 1.5–2.0 GPM
Bathroom sink: 1.0 GPM
Dishwasher: ~1.5 GPM
Washing machine (hot cycle): ~1.5–2.0 GPM

Two showers running at once, for example, already puts you at 4-5 GPM before anyone touches a faucet.

Sizing by Household — Quick Reference

Household	Typical Peak Demand	Gas Unit Size	Electric Unit Size
1–2 people, 1 bath	2.5–4 GPM	~150,000 BTU	18–24 kW
2–3 people, 2 baths	4–6 GPM	150,000–180,000 BTU	24–27 kW
Family of 4, 2.5 baths	5–7 GPM	180,000–199,000 BTU	27–36 kW
Larger home, 3+ baths	7–11 GPM	199,000 BTU (may need 2 units)	Electric often impractical at this tier

For larger South Jersey homes needing 9+ GPM in winter, electric tankless becomes a hard sell — even a top-tier 36 kW electric unit tops out around 7 GPM under cold-climate conditions, and simultaneous high-demand use (two showers plus a dishwasher) is where electric units in this climate genuinely struggle. Gas is almost always the more realistic whole-home option for larger South Jersey households; electric tends to make the most sense for smaller households, point-of-use applications, or homes without gas service at all.

Gas Tankless: Brands, Models, and Sizes

Gas tankless dominates the whole-home market, and for good reason — higher flow rates and better cold-climate performance than electric.

Brand	Model Tier	Approx. Flow Rate	Unit Price Range
Rinnai	RL75i	~7.5 GPM	$900–$1,400
Rinnai	RL94i	~9.4 GPM	$1,200–$1,700
Rinnai	RU199iN	Up to ~11 GPM (199,000 BTU)	$1,500–$1,995
Navien	NPE-180A2	~7.5 GPM	$1,075–$1,500
Navien	NPE-210A2	~8.6 GPM	$1,300–$1,900
Navien	NPE-240A2	~11.2 GPM	$1,800–$2,500
Rheem	Performance Platinum series	~7–9 GPM	$670–$1,500
Rheem	RTGH condensing series	~9–11 GPM	$1,200–$1,950
Bradford White	Infiniti series	~7–10 GPM	$1,000–$1,500
Noritz	NR series	9–11+ GPM (commercial-capable)	$1,300–$2,200

A note on brand differences: the equipment price gap between these brands is generally a few hundred dollars at most — the bigger cost driver is almost always your home’s specific venting and gas line situation, not which nameplate is on the unit. Rinnai and Navien dominate residential installs and both have strong service-network availability; Noritz is worth considering specifically if your household’s peak demand pushes toward the 11+ GPM range, since it’s built to handle higher-flow and multi-bathroom scenarios other brands sometimes require two units for.

Electric Tankless: Brands, Models, and Sizes

Electric tankless makes the most sense for smaller households, homes without gas service, or as a point-of-use solution for a single fixture rather than whole-home coverage.

Point-of-use (single fixture) models:

Brand	Model	Output	Unit Price Range
Rheem	RTEX-08	8 kW	$180–$250
Rheem	RTEX-06T	6.5 kW	$150–$220
Rheem	RTEX-13	13 kW	$250–$300
EcoSmart	ECO 11	13 kW	$200–$280
Stiebel Eltron	Tempra 12 Plus	12 kW	$350–$450
Bosch	Tronic 3000T	7.2 kW	$200–$280

Whole-home models:

Brand	Model	Output	Approx. Flow Rate	Unit Price Range
Rheem	RTEX-24	24 kW	~4.1 GPM (moderate climate)	$400–$550
Rheem	RTEX-36	36 kW	~6–7 GPM	$500–$700
EcoSmart	ECO 27	27 kW	~4–5 GPM	$400–$500
Stiebel Eltron	Tempra 29 Plus	29 kW	~5 GPM	$500–$650
Stiebel Eltron	Tempra 36 Plus	36 kW	~7 GPM	$650–$850

The electrical panel is the real constraint, not the unit itself. A 36 kW whole-home electric unit typically needs roughly 150 dedicated amps at 240V — more available capacity than most panels installed before 2000 have to spare. If your panel is older or already near capacity, budget for a panel upgrade as a real possibility, not an edge case, before committing to whole-home electric tankless.

Installed Cost: Unit + Labor

This is where the real budgeting happens, since the unit itself is typically only 30-50% of the total installed cost.

Gas tankless — typical installed totals:

Basic swap (existing gas line and venting adequate, 1-2 bath home): $1,800–$3,200
Moderate complexity (some venting or gas line work, 2-3 bath home): $3,000–$4,800
Larger home, extended venting, gas line upgrade: $5,500–$8,000

Labor alone typically runs $600–$2,500, with installs taking roughly 6-14 hours depending on venting complexity and whether the gas line needs to be upsized. A 199,000 BTU unit often requires a minimum 3/4-inch gas line for short runs, and a full 1-inch line on longer runs to maintain proper pressure — this is one of the most common reasons a gas tankless quote comes in higher than expected.

Electric tankless — typical installed totals:

No panel upgrade needed: $800–$1,500
With a panel upgrade: $1,500–$5,500+, since the panel upgrade itself commonly runs $1,500–$4,000 on top of the unit and base labor

Electric installs are generally faster and less invasive than gas — no venting to run — but the panel question needs to be answered before the total cost is really knowable.

A Note on 2026 Incentives

The federal 25C tax credit that used to offset qualifying tankless installs (up to $600 for high-efficiency gas units) expired for installs after December 31, 2025. For 2026 installs, the remaining incentive pathways are state and utility rebates and the income-qualified IRA HEAR program (point-of-sale rebates up to $1,750 for qualifying electric water heaters) — worth asking about during your quote, since availability and amounts vary by program and household income.

The Pain Point: Undersizing Is an Expensive Mistake to Discover Later

Here’s what makes sizing worth getting right the first time: an undersized tankless unit doesn’t fail outright — it just quietly disappoints every single day, delivering lukewarm water the moment two people need hot water at once. Many homeowners live with this for months, assuming it’s just how tankless works, before learning the unit was simply undersized for their household’s actual peak demand. Correcting that after the fact means paying for a second, larger unit — essentially paying for the sizing mistake twice. A proper load calculation before installation, accounting for South Jersey’s winter groundwater temperatures specifically, is a small step that prevents a genuinely expensive one.

Let Professor Gatsby’s Size It Right the First Time

Sizing a tankless water heater correctly means accounting for your specific household’s peak demand, your home’s existing gas line or electrical capacity, and South Jersey’s winter groundwater conditions — not just picking a popular model off a list. Our licensed technicians serve homeowners throughout Egg Harbor Township, Mullica Township, Glenloch, and the surrounding South Jersey area, and we’ll calculate your home’s actual load before recommending a brand, model, or fuel type.

Ready to go tankless the right way? Contact Professor Gatsby’s or book an appointment online for an accurate sizing assessment and a straightforward installed quote.