TLDR: Replace house's electric 100A resistance strip-heat "furnace" AND 40A its air conditioning with a 30A wide-range heat pump that does not require emergency heat. Enjoy lower electric bills all year. Install subpanel to allow three 40A breakers to be in the same panel.
An on-demand is a great idea.
And 27 kw is serious enough that your project isn't likely to end in frustration with tepid water, as will happen when they are carelessly undersized. Now keep in mind a few design aspects.
Flow is everything -- when you max out flow capacity, temperature will drop, and the cure is to reduce flow. You can do a lot of good right up front by carefully sizing your loads, e.g. a "low flow" shower head.
That long slug of cold water sitting in the hot water piping is still going to be a problem. All that water must be pushed out of the pipe by hot water: that is the delay you get in the far rooms. Locating a tankless centrally will do nothing to cure that - it will be just as bad, or worse if you use low-flow strategies e.g. low-flow faucets and shower heads.
One answer to this is distributed, smaller heaters. At extremes, the British have "electric showers" where the shower has a built-in on-demand heater right at the showerhead - of 8500-9500 watt capacity (40A), good for a little over 1 GPM, but instant hot water - no waiting. I don't know that I'd go that far, but certainly if your house has 2 or 3 "islands" of hot water use far apart, 2 or 3 smaller units may be worth considering (or even 1 per shower-equipped bathroom + 1 for kitchen/laundry).
You won't be able to go much over 27 kw, unless you "go nuclear" with a 400A service upgrade, in which case limit total water heaters to 48 kw.
That panel has some issues.
It is a "Split-bus" aka "Rule of Six" panel. That means it does not have a main breaker, those 6 large breakers are together the main breaker. That was a trick done in the 1960s-70s to avoid the then-very-high cost of 100A and 200A breakers. However it's a safety problem because those breakers together can draw well over panel rating, and nothing protects the panel. The safety came from the house's original load calculation done at build time, but that totally counts on the load calculation being re-done everytime anything is added to the panel. And real world, that does not happen.
This is not a Code violation because it was legal at the time it was installed... but it's something to take care of if it aligns with other plans. Separate from this, you need additional breaker spaces anyway. So....
You could fit a new 200A panel, make it the main panel, make the split-bus panel a subpanel of it (even supplied via "thru-lugs" removing need for another 200A breaker).
You could replace the meter pan with a 200A "Meter-main" which also includes some spaces for large breakers, which will provide the extra breaker spaces you'll need.
If you are forced to upgrade service to 400A, this will happen as a side-effect. (400A meter-mains provision two 200A breakers, once again because of the high cost of too-large breakers).
That "furnace" is in the way.
You have an all-electric neighborhood. There are "good" ways and "cheap" ways to provide heating in all-electric neighborhoods.
- Worst: electric strip/toaster heating. Cheap to install, murderous on the electric bill. Only makes sense where a region has a glut of unusable nuclear/hydro baseload and has low power pricing/tariffs to match.
- Compromise: Early or cheap electric heat pumps which work great, but the coldest temperatures are out of their working range. Therefore they need massive amounts of cheap electric strip heating, as "emergency heat". This emergency heat must be fully provisioned, i.e. calculated as part of the house's load calculation, even though it's rarely used.
- Best: Modern high-range heat pumps which work at any plausible temperature (in Texas, at least, yes including your -5F of late). These only need to be provisioned for their heat-pump draw.
What you have now is toaster heaters, and they take a huge chomp out of your available electrical service, to the point where there's no room for 27 KW of on-demand water heater. The only way you can have both toasters and on-demand is to upgrade your electrical service to "Class 320"/400A.
Every possible scenario will cost you a moose-load of money. The question is only how:
- Stand pat. Buy a tanked water heater, keep the toasters and old inefficient A/C, and pay outlandish electrical bills (ouch, every month, forever).
- Install 400A service (big ouch, once), install the on-demand (yay) and keep the outlandish electrical bills (ouch, every month, forever)
- Install an advanced full-range heat pump (ouch), and use its generous 100A provision for the on-demand heater (yay). Lower electrical bills even in A/C season due to the high SEER number of modern heat pumps... massively lower electric bills in the winter, and no more being afraid of the thermostat.
Choose your poison.