Rheem Marathon Lifetime 40-Gallon Electric Water Heater

The Rheem Marathon Lifetime 40-Gallon Electric Water Heater (MR40245) is the 40-gallon sibling of the celebrated Marathon 50G — same seamless polybutylene tank, same lifetime warranty against leakage, sized for 2–3 person households. 52 GPH first-hour delivery, 0.93 UEF, dual 4,500W elements. The right pick for long-horizon 2–3 person households without heat-pump install space.

Headline specifications

• Capacity: 40 gallons
• Fuel: Electric (dual 4,500W elements)
• Tank: Polybutylene seamless non-metallic
• First Hour Delivery: 52 GPH
• Recovery: 21 GPH at 90°F rise
• Energy Factor (UEF): 0.93
• Warranty: Lifetime non-prorated against tank leakage; 6-year on parts
• Anode rod: None required (non-metallic tank)
• Dimensions: 51.5" H × 23.75" diameter

Who this model is for

The Marathon 40G is the right pick for:

• 2–3 person long-horizon households planning 15+ year ownership
• Hard-water or well-water homes where conventional tanks fail prematurely
• Off-grid and rural installs where emergency replacement is expensive
• Risk-averse buyers who want to eliminate the water-heater-failure event

Lifecycle math: $1,199 Marathon vs $449 Performance 40G. Performance 40G fails year 8–12 and needs $450 replacement + $500 install = $950. Over 16 years, Marathon avoids 1–2 replacement cycles. For households staying 10+ years, Marathon's premium pays back.

Where it beats the alternatives

Vs Marathon 50G ($1,299): $100 less for 40G. Right pick when 52 GPH FHD is enough for your demand.

Vs Performance 40G Electric ($449): Marathon costs $750 more upfront but pays back on the 15-year math.

Vs Westinghouse Stainless Lifetime 50G Electric ($1,499): Westinghouse is the stainless-steel alternative with similar lifetime warranty. Marathon's polybutylene tank is theoretically more chemistry-inert; Westinghouse is narrower footprint.

Where it falls short

23.75" diameter is wider than steel-tank 40G alternatives (20"). Verify install space.

0.93 UEF — heat-pump alternatives at 3.4+ are 3.5× more efficient. Marathon's value proposition is tank longevity, not operating efficiency.

$750+ upfront premium vs budget electric tanks. Doesn't pay back for buyers selling within 5 years.

Install considerations

Standard electric tank install — 30-amp 240V circuit, no gas/venting. Verify 23.75" clearance.

Install cost: $250–$500 (lighter than steel tanks, easier one-person handling).

Maintenance

• No anode rod replacement (none exists)
• Annual flush still recommended
• Element replacement at year 8–12 typical
• T&P valve test annually

Bottom line

The Rheem Marathon 40-Gallon Lifetime Electric is the install-and-forget pick for 2–3 person households with long ownership horizons. Lifetime warranty against the corrosion failure that ends every other tank's life. For larger households, step up to the 50G Marathon. For households with heat-pump install space, the ProTerra Hybrid wins on operating cost. Click through to Amazon or a Rheem dealer for live pricing.