The gallons of hot water/minute is shown on each model. Ratings are the achievable temperature rise at various gallon flow. In Florida, potable water is much warmer than north states. Tankless units north work great, just need to be a higher capacity to handle a larger heat need. Look at a few manufacturer sites because they have great explanations of this most common question.
Examples of home units:
A small unit can heat 45deg supply water to 120deg at 2gpm (gal per minute). It can also heat 70deg water to 120deg at 5gpm.
A medium unit can heat 45deg supply water to 120deg at 4gpm (gal per minute). It can also heat 70deg water to 120deg at 8gpm.
A larger unit can heat 45deg supply water to 120deg at 9gpm (gal per minute). It can also heat 70deg water to 120deg at 15gpm.
How many BTU's - depends upon the size of unit bought. More BTUs needed for higher heat rates. It is multiple 10's of thousands.
Better than a 45 gallon hot water heater? - In my opinion, absolutely. A tankless is infinite hot water, at a constant temperature. Zero waste spending money to keep a tank heated. Once drawing 25 gallons, that tank is cooled off significantly. So your shower handle gets cranked up, drawing not-as-hot water faster. Soon tank is cold.
What would you recommend instead of a hot water tank? I replaced a tank with tankless years ago (before moving here) and it was fantastic. Never runs out of hot water, no matter how long it's needed, or how many things are running at the same time.
What model do you recommend? Pick a name brand. Noritz, Takagi, Rheem, Rennai. Look atonline somewhere like PlumberStock
Here the tankless needs a yearly flush with vinegar to clean out sediment. Many mfg also sell a valve assembly to match their units - get that too. Makes cleaning trivially easy to do.
Also, tanks build up sediment and they need to be cleaned out yearly too. Many people neglect doing that and the bottoms rust out far to early as a result. This is also called "de-scaling".
Some newest models are saying they might not need cleaning because of new designs. I have not followed that much, but check on that.
There is a myth that tankless take longer to get hot water than a tank. That is highly mis-stated. A tankless produces hot water in seconds. So does a tank. The "slower" concept is ONLY a result of where the tankless is located. If there is more pipe length then water needs to flow further. That causes a delay. The one I put in was placed much closer to everything than the tank was. It delivered hot water faster.
Some tankless units now have built in recirculation pumps, if you prefer to save a bit of water getting hot water thru the pipes.
Here they build using outside units. There are also inside units, and if put where the existing tank is located, it will outperform the tank (capacity, constant temperature), and deliver hot water at the exact same speed as a tank. If you put in an outside unit, you gain storage space where the tank used to be. Indoor units will need fresh air pipe, and exhaust pipe.
Your gas meter might need to be upgraded (simple swap) to a higher volume unit. Depends upon house age, and several other factors. Your meter could be fine as is. Worth the check before deciding.
One thing I would do if building new. I'd put small electric demand heaters in the kitchen and bath sinks. Near instant hot water where very low flow rates would equate to very low cost units for a nice performance treat. Bonus would be no hot water supply piping needed. Since a quick hand wash would use so little water it could be more efficient overall.
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