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We are installing a tankless water heater that requires *3 50 amp 2 pole breakers. We are running new wire from the main breaker to a sub panel that will have those breakers in it and from there to the tankless water heater. Does the sub panel have to be 200 amps as well to support the new breakers?

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    "200 amps as well" as what? Is the breaker feeding this from the main 200A - then yes, the subpanel has to be 200A (or larger). You've only mentioned 150A in breakers, so I can't see where you pulled 200A from here. Please edit to clarify.
    – Ecnerwal
    Commented Mar 8, 2022 at 21:16
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    If your home was using more than 50A of the 200A service before you decided to add 150A of tankless electric, you might have bigger problems, like needing upgrade to 400A service to the house in order to support the whole load.
    – Ecnerwal
    Commented Mar 8, 2022 at 22:53
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    How many KW is the heater, and can you post photos of your main panel please?
    – ThreePhaseEel
    Commented Mar 9, 2022 at 4:39
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    Don't! electric tankless are not good. so much power draw and limited flow. IMHO, stick with a modern tank type water heater, they are super well insulated and gives you a reserve of hot water. You may even consider a heat pump water heater. I firmly believe electrical resistance tankless will be a disaster when electricity rates go thru the roof.
    – George Anderson
    Commented Mar 9, 2022 at 5:17
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    Heat pump water heaters are way more efficient how can I say that? All the waste heat needed to creat instant hot water compared to a higher efficiency rate, our utility had specials that they paid the cost for a basic 40 gallon you were on the hook for installation. Our power bill dropped. If on a metered service and you use any hot water get ready for your power bill to go through the roof with a tankless, heat pump was a bit slower than gas recovery but not that much different than a regular electric tanked water heater.
    – Ed Beal
    Commented Mar 9, 2022 at 5:24

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You need to do a Load Calculation for the entire house (without and with) the new heater. That will determine if you have headroom for the heater. You don't get to use ad-hoc methods as a substitute, such as a promise to use large appliances carefully. (at least not yet -- emerging tech such as "smart panels" will make very difficult provisioning possible.)

I applaud your choice of a 150A heater because the biggest mistake in tankless is trying to scrimp and going too small. However, this obviously creates a huge demand on your service.

It's possible for an all-gas house to have 150A to spare in a 200A panel - but other than that, it will probably take "smart panel" type tech to be able to provision that heater.

You might consider smaller heaters closer to the point of use; this is how the British do it, with their "electric showers" in the 40-60A class. (note these also slow down the flow to 1-1.5 GPM, which makes the smaller size practicable).

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