My husband fixes hot tubs so I feel like I know something about them and I only recommend 5 brands. Hot springs including its sister brand calderra (basically the same tubs), jacuzzi and its sister brand sundance but only if you go for the upper crust because the lower end tubs are absolute shit, marquis, D1, and nordic.
I work on all hot tub brands and most outside of these 5 aren't worth the money, with hot springs being my personal favorite.
I only recommend the upper lines. If you get any of the low end, low cost tubs they're going to be shit. Anything from costco or home depot isn't even worth your time.
Out of all the brands I mentioned, a nordic tub will probably be the best middle of the line tub, the rest being pricier and higher quality.
Don't just factor in the intial cost when making your decision. Hot tubs cost money to run. Your electric bill will go up, by how much depends on how well the tub is insulated. Heating the tub will be the biggest cost. You don't experience new england winters in Texas, I'm sure, but last winter we had a guy call us up for an emergency winterization. His junk tub with absolutely no insulation cost him $200 per month to run, as opposed to the roughly $35-50 it took for a well insulated hot springs tub to run.
You can get a lot nicer premade tubs for a lot less than $15k, more like 4k - 6k.
This would include around 50 jets in different arrangements, ozone generator for sanitation, and an LED light system. Custom raises the price a bit, but it still would have had more features than what's shown in this. The only reason people typically do custom is when they're doing an in-ground installation. You're usually way better off going with a premade tub if this isn't a requirement.
You can get a top of the line Hot Springs for about $10k.
Hot springs is the Cadalac of tubs. At this price you would have things like under water speakers, a LED lit fountain, and a pop up TV. Plus the normal things like massaging jets and whatnot.
A tub is full of nooks and crannies, plumbing, insulation of various thicknesses. Ours would be impossible to calculate based on anything but actual consumption. Most tubs that I've seen aren't kept at temp.
During use is the largest concern. And even that amount of energy is very small compared to what can come off the surface whenever it's opened.
Fortunately you have a nice little metered heater that gives a specific output.
Mine has a pair of 4.4KW heaters (I run one or two based on ambient temperature, what's it's plugged into and if I'm paying the power bill or not). And then there is evaporation and steam losses.
So, what you want is a data logger that logs on time vs off time of your heater.
Figure out how much on time is required for top on or top off operation and use the energy input calculation.
Conduction through the Styrofoam - Assume the foam is 42° on one side and 7° on the other. This will give higher than actual loss, but the error is probably two orders of magnitude less than the next part.
Natural convection evaporation off the water surface. You'll lose heat from the water evaporating and you'll lose some through natural convection to the air.
For simplicity, assume that the tub is well mixed. Without doing any work myself I'm guessing the most significant heat loss will be from evaporation so focus on getting that right.
You should be able to find the relevant equations on Wikipedia and constants/properties from engineering toolbox (or wolfram alpha)
You'll want to measure the actual heater voltage under load and the amperage to get an exact draw number. Ratings are variable at best and real world voltage at the heater varies.
Don't forget to turn off any auto water adding system or it will mess up your results. Also make sure you aren't measuring pump draw, and the circulation pump is on low speed/air jets feed is off.
We had a super old tub, and it sprung a leak. We had to take some of the cabinet off, but we didn't have the stuff to fix it properly so it now just has an ugly sheet of plywood there!
I think we want to try to take that fake brick looking stuff and put it around it, not sure if that's a good idea though.
Don't bother. The system will last only a year or two before the ultraviolet output drops and it no longer sanitizes efficiently enough. Might as well stick with your regular water conditioning as a habit.
Replace it or rebuild right away. It's been 15 years, and by now various wound coils in the rotor and stator have fused and now unpredictably pull more current for less output than originally designed.
The motor will keep getting hotter, pull more current, and you'll start blowing fuses because circuit isn't expecting that much. After awhile, replacing fuses will get tiresome and dangerous and each trip might ruin some sensitive circuitry. If you have multiple pumps and a tight budget, replace the main one (main one is used for daily filtration, so presumably it's worn more.)
If you haven't replaced it with your heater, replace it now before it goes out and you start getting low water flow error messages. It's a $20ish part that'll save you some grief, and ensure your heater doesn't malfunction and burn out.
If you don't have them, see about building some neat ones. If you do have them, see about upgrading them.