There are about 800-900 litre of water in a 1.8x1.8m hottub. Usually around a 1.5-3kW heating element running (usually) on electric power - and usually ONLY when tub is running (so add 100-800 watt for circulation pumps dependent on the age. So water needs to be heated to between 28 and 40 deg. c. Incoming water temperatures in EU are usually between 3 and 10 deg c - and sometimes in the summer 12-15 but I have rarely seen that high temperatures. There are gas heated hot tubs but they are few and far between and usually very expensive and has annual service requirement.
This will work but you will need to heat the water during the day when is sunny is not useful if you want to heat and use the hot tub at night
So you are talking about a 25 to 35°C delta for say 800 liters of water then that will require 800 x 35°C x 1.16 = 32.5kWh so fairly significant.
You will need a 4 to 5kW PV array working all day to do that. So while much cheaper than grid electricity it will only be so if you use this every day or at least in average once every two or three days but if hot tub has very occasional use say maybe once or twice a month then the investment in solar just for that is not economical and you better pay for the grid electricity.
At 20 cent/kWh say for grid price heating that hot tub will cost $6.5 in EU will probably be the same in euro or even more depending on country and grid electricity rate
But what I'm talking about is you making it into a full product You are not getting John/Jane Doe to put in your system as it is not "mum-friendly" - your current system is for "tinker" people who like to get their hands "dirty" and know a bit. So put together a product - find the solar cell's - find some heat elements that work and are light weight - so they can be moved when you move the cover. But make it into "mum-safe" product anyone can install - and I think you will have a good source of income.
Yes in order to use my system you need to know what you are doing. But also what I propose is a complete house energy source so is not only a large total investment but also requires a lot of work that need a certain level of knowledge and skill.
My two products the SBMS and DMPPT represent just the logic part (the controller) of the system and probably just 5% of the total cost. Since PV panels and all other stuff are large and heavy international shipping is not an option so there is plenty of space for local solar installers or business to open a business offering complete solutions based on my technology.
I think the potential is huge since as far as I know is the lowest cost house energy solution and even better for new houses where additional savings can be had by no connecting to utilities like electricity and natural gas.
The problem is that there is nothing like this as far as I'm aware so is hard to explain even to knowledgeable people what this DMPPT this is all about.
Educating people is a realy hard job