With a gas heater, you won't need that.
The things are ridiculously powerful - 200,000 BTU/hr. It's difficult to imagine you needing that much hot water.
Let's crunch some numbers.
There's a lovely unit called the "BTU". Most heaters say they're rated in BTU, but actually the heaters mean "BTUs per hour".
A BTU will raise 1 pound of water 1 degree F. So let's think:
For math's sake, let's think about a flow rate of 1 gallon per minute. (A shower is 1.25 to 2.5 GPM). That is the same as 60 gallons per hour. Now, a gallon of water weighs 8.3 pounds so 1 GPM is 500 pounds of water per hour.
Now you want a rise in temperature from 39F to 110F. That is 71F rise. So... 500 pounds of water per hour x 71 F rise = 35,000 BTU per hour.
35,000 BTU is also around 10,000 watts if we're using electricity, or 42 amps.
Remember, this is all for 1 GPM.
If you don't know how many gallons per minute your shower or faucet uses, well, YOU decide that by how wide you open the faucets! But if you want "typical", set the faucets to your usual flow and get a 1-gallon bucket and a stopwatch.
Anyway, that probably won't be necessary for you. Since 1 GPM takes 35,000 BTU/hr, a 200,000 BTU/hr gas tankless is going to deliver nearly 6 GPM.
If you were dealing with an electric heater, then it's pretty simple, you need 10,000 watts per GPM that you want. So if you feel a need for 4 GPM you'll need a 40,000 watt tankless heater. "That's a WHOLE LOT" yes it is. That's why with electric, we think real hard about reducing the need for flow by changing to low-flow showerheads, or placing the electric tankless right at the point of use (e.g. if 10,000 watts for an electric shower works for the British, it ought to work for a bathroom with a 1 GPM low-flow head). A side effect of "multiple tankless at the point of use" is very fast hot water without need for wasteful recirculation systems.