Stiebel Eltron Tempra 29 Plus: A Cost Controller’s Verdict on the 28 kW Rating (and How to Install It Right)

If you're looking at the Stiebel Eltron Tempra 29 Plus, you've already seen the marketing: "28 kW of on-demand hot water." I'm going to tell you the one number you actually need to know, and it's not 28. It's the real-world draw, which is closer to 24 kW under typical residential 240V service, and why that difference makes this unit a solid buy for some installations but a potential pain point for others. I've managed procurement for a mid-sized electrical contracting firm for over 6 years, handling about $180,000 in annual spending on everything from wiring to water heaters. We've installed about 45 of these units (both Tempra and other brands) in that time, and I've tracked every service call and warranty claim.

My Experience: The 28 kW Number Isn't a Lie, But It's Not Your Number

Let's cut through the spec sheet fog. The Tempra 29 Plus is rated at 28 kW. That's the maximum power draw, and it's calculated at the unit's design voltage, which is typically 240V. Here's what I've seen on our installs:

The actual power draw we measure on our Fluke meters at the sub-panel is almost never 28 kW. It's usually 26-27 kW on a good 240V line. But here's the kicker: in quite a few homes, especially older ones with a 200A main service, the voltage coming in is closer to 234-238V. At that voltage, your actual draw drops by about 3-5%. We've logged units pulling only 24.5 kW on a hot day with neighborhood load. That's still a lot of power, but it's 12% less than the sticker claims.

Does this matter? Yes, and I'll get to when it does. But first, the headline: For a standard 2-3 bathroom home in a moderate climate, this is still one of the most reliable tankless units we've installed, provided you have the electrical service to support it.

The Real Cost of the Electrical Upgrade (The Part They Don't Tell You)

Most people compare the $800-900 unit price against a gas heater. They don't budget for the electrical work. We've done installs where the client's existing 100A or 125A panel was maxed out. The Tempra 29 Plus requires a dedicated 120A breaker (two 60A breakers in a 240V split-phase setup, or one 120A if you have a big main).

I said "120A" and one client heard "it's just a breaker." Result: they approved the quote for the water heater but not the $2,800 panel upgrade. Discovered this when our electrician arrived and the main panel had literally no space. That decision—not getting a full electrical load calc before buying—cost them a $150 service call fee and a week of delay while waiting for the panel upgrade to be scheduled. In our experience, about 1 in 3 installations of the Tempra 29 Plus requires a panel upgrade. That can add anywhere from $1,500 to $4,000 to the total project cost.

How to Install a Stiebel Eltron Water Heater (The Checklist I Wish I'd Had)

After installing around 45 of these, our crew has a pretty standard procedure. If you're doing this yourself or managing the install, here's the process we follow, broken down into the steps that actually cause problems. The official manual is fine, but it doesn't warn you about the real-world pitfalls.

The 'Red Flag' Before You Start the Install

Step 1: The Load Calculation. This is non-negotiable. Do not skip this. We use a simple calculation: Total amp load of the house at peak times (A/C, oven, dryer, pool pump) + 120A (the Tempra 29 at full draw). If that number is anywhere close to your main breaker rating (say, 190A on a 200A panel), you're asking for a trip. We've seen houses where the AC alone pulls 60A, and adding the Tempra on a hot day will drop your main breaker. We had a job last summer where we had to install a load-shedding relay for the water heater because the AC was pulling 72A. That added $350 to the job.

The Actual Wiring (Don't Guess on the Wire Size)

Step 2: The Right Wire. You need 3 AWG copper wire for a 120A circuit. This is thick, heavy wire. Do not use aluminum unless you know exactly what you're doing—we've seen corrosion issues. We also run a dedicated ground wire (6 AWG minimum, usually 4 AWG to match local code). This run from the panel to the heater can be expensive. A 50-foot run of 3 AWG copper, plus conduit, will cost you about $2.50-$3.00 per foot just for materials.

The Mounting and Connections (The 'Gotcha' Moments)

Step 3: Mounting. The unit is heavy—about 32 lbs. It needs to be mounted on a wall that can support it. We use a 3/4-inch plywood backing for secure mounting, especially in garages. Here's a common mistake I've seen: mounting it too close to the ceiling. The unit needs about 12 inches of clearance above it for the electrical connections and to allow airflow for the internal electronics. If you mount it flush with the ceiling, you're going to have a bad time when you try to connect the 3 AWG wire in a tight space.

Step 4: Water Connections. It uses 3/4-inch NPT connections. We always install isolation valves (ball valves) on both the hot and cold sides. This is not optional. If the unit ever fails, you need to be able to isolate it without shutting off the whole house's water. We also use a water hammer arrestor because the internal flow sensor can cause a slight hammer in some systems.

The Final Check (What the Manual Doesn't Tell You)

Step 5: The Startup Sequence. This is where we've seen installers fail. The unit has a complex startup procedure that involves purging air and verifying the flow sensor is working. One crew I knew just connected it and turned on the power. The internal resistance heating elements fired with trapped air inside, creating a steam pocket that damaged a heating element. They had to replace a $350 part. Always follow the manual's start-up procedure to the letter: open the hot water fixture first, let it flow for 2 minutes to purge air, then turn on the 120A breaker.

Why I'd Still Buy This Unit (And When I Wouldn't)

From a cost-control perspective, my view is that the Tempra 29 Plus is a good product if your electrical infrastructure is ready for it. Here's my honest opinion:

I recommend it for: New construction or major home renovations where you're already putting in a 200A+ electrical panel. If the electrical work is already paid for, the unit itself is reliable, the flow rate is solid (up to 6.5 GPM in mild climates), and the 'lifetime warranty' from Stiebel Eltron is actually decent. We've had to claim on it twice because of user error (one installer used the wrong wire, another installed it upside down—seriously), and Stiebel processed both without much pushback.

I'd avoid it for: Retrofitting into an older home with a 100A panel or a 150A panel that's already loaded. The total cost of the system (unit + panel upgrade + wire + labor) is often more expensive than installing a traditional 50-gallon tank water heater with a 4.5 kW heating element. That's the 'value over price' calculation: a $900 unit can become a $4,500 total investment. And in that case, a $600 tank water heater with a $200 install is a better financial decision for most homeowners, even accounting for the slightly higher electric bill.

The Bottom Line (With a Grain of Salt)

I'm not 100% sure this is the right unit for every home, but I'm confident it's the right unit for my typical install: a newer home with 200A service and a homeowner who wants unlimited hot water. The real-world 24-26 kW draw is more than enough for most families. Don't get hung up on the 28 kW number.

Take this with a grain of salt: I've seen some installers quote the Tempra 29 Plus as a retrofit in a 100A panel, promising to install a smaller 'load-shedding' relay. It's not a terrible solution, but it means the heater might not work when the AC is running, which defeats the purpose for some people. Also, one thing I've learned the hard way: the Stiebel Eltron Tempra series is not as tolerant of hard water as some other brands. If you're on well water with high mineral content, you might want to look at a different unit or budget for a water softener, because scale buildup will kill the flow sensor over time.