Solar thermal can be sized as two separate systems to heat the pool and spa. Pretty straightforward.

How to make it work? Move out of CA!

The best way to have a spa in this situation (and really all situations) is some sort of covered/insulated way and then just keep it hot all the time. That is the only way solar or a heat pump would work too.

If you do wind up using electric resistance heat, 240V is 4 times faster than 110V - all else being equal. You can do some math to see what this would cost. A kwh will raise 410 gallons a degree. So raising a 410 gallon spa from an ambient 55 to a comfortable 105 is 50 kwh assuming no further losses and 100% efficiency. So $25 for many peak CA rates. There are 11 kwh heaters available so that would take about 5 hours. The losses probably add 20% to cost and time.

It's the amps you provide that determine how fast you can heat the water, and for that size pool you need a lot of amps. Amps are a measure of heating power, and I did some rough calculations to get a handle on this. Here are my assumptions:

Pool size: 15' X 45' X 4' (average depth) = 2700 cubic feet = 168,500 lbs

Temperature of water supply, 55 F; desired temperature rise 45 F, so heated pool is at 100 F

It takes 1 Btu to heat 1 lb of water by one degree F, so to heat all the water in the pool by 45 F would take:

Total Btu = 168,500 X 1 X 45 = 7.58 million Btu = 2200 kwhr

I see that the average cost of residential electricity in San Francisco is $0.223/kwhr, so the first time you heat the pool to 100F, the cost will be about = $(2200 X 0.223) = $490. It will also take quite a bit of energy to heat the concrete walls of the pool, and you will likely be in a much higher electricity rate class because you are using so much energy, so I am guessing that the first time you heat it, the cost will be something like $600-700.

As far as how fast it can be heated, I looked at electric pool heaters and found a suitable one that cost $2500 and whose specs were 45 kw with a 125 amp connection at your breaker box. A 125 amp breaker is a large amount given that typical residential breaker boxes for whole houses are typically in the 100-200 amp range, so you might have to upgrade or put in a subpanel. So, getting to heating speed, if you have to supply 2200 kwhr to heat the pool the first time, this powerful heater would require: 2200 kwhr/45 kw = 49 hours. If you were going to heat it subsequently from, say, 85 F to 100 F, it would probably take about 1/3 of that, or about 16 hours. For that reason, many people keep the pool heater on continuously through a thermostat so that it doesn't require waiting for many hours to use the pool when you decide you want to (but of course that uses up a fair amount of energy).

Now, admittedly, once the pool is to 100 F, as long as you use a well-insulated pool cover you won't be heating from 55 F, but simply heating from some temperature between groundwater temperature and the desired temperature of 100 F. You will get heat loss through the pool cover, through the concrete itself (which is at ground temperature), and additionally through the air each time the pool is uncovered. Of course, if you are using it and heating it during the colder months, there would be a significant energy cost.

I don't have a pool, but I do live in the Pacific NW in a climate very similar to San Francisco's and my neighbor has a pool (slightly smaller than yours) with a connected spa. He was able to heat it with natural gas (after paying to have a larger sized meter installed). But even with the cheaper cost of gas, his first month bill to keep the pool at a comfortable temperature in June came to over $2000. After that shock, he decided to install a solar heating system that substantially cut down on the cost of heating the pool: still not cheap, but more bearable. And the way he had the pool and spa plumbed allowed him to maintain two different temperatures. So you might want to consider a hybrid system where you can hopefully get most of your pool heat from solar and a smaller electric heater to keep the temperature up when the solar panels alone can't. And I do know that solar works in San Francisco because the panels on my daughter's house there generate enough energy that she sells some of it back to the utility.

P.S.: I agree with David Cary's calculations, but I will point out that a 410 gallon spa is only about 55 cubic feet; a 6‘ X 6' spa that is, say, 3' deep would be twice the volume, and twice the cost.

Yeah - I used 410 to just give an idea, figuring that it was easy to multiple from there.

Not sure anyone heats a pool to 100 degrees. I find 80 pretty balmy. Now a spa - 105 is a good temp.

$2000 a month for a non essential energy use is fairly shocking. For essential it would still be shocking.

Here in warmer NC, people describe $100-$200 to get a pool going in April. Then mostly coasting the rest of the time with maybe a $50-$100 off season party. I don't have heat - may install solar at some point but otherwise I am way too stingy with energy use to pay to heat a pool.

David-

I agree that heating a whole pool to 100 F would be pretty warm. On the lower end, I’m estimating that initially heating it to only 85 F would still cost about 3/4 of what I calculated. And the continuing costs to maintain it at that temperature would be similarly lower. I lived both in NC and in San Francisco, and they are definitely different climates. SF summers can be intermittently windy, foggy, and even nippy.

Thank you all for your helpful inputs.

Some clarifications on the original post :

1 - We will be going the solar route to power the house and hoping to be energy independent for the house at least. It would be great if we can have the pool use the electricity generated by our solar panels but we will have to see if the extra solar install is justified for the limited use of the pool that we would have during the winter months.

2 - Primary use of the pool will be in he hotter months but we like the flexibility of having electric heat pump versus solar heater

3- The 100F that I mentioned in my original post is for the spa only. The pool temperature will be 80/85 as some of the responders have suggested on the thread

4- We will rely on zoning to partition the spa versus the pool heating and from the responses on this thread it sounds like 240V/200A should give us reasonable usability of the spa even in the cooler months.

5- Reducing the pool size is very much an option that is on the table. The numbers posted by @kudzu9 and @David Cary are definitely going to be very helpful in determining a workable combination

Don't waste your time with electric heat. It will take too long too bring to temperature.

Consider propane. You probably have a propane dealer in the area that can set up a tank on site or you could bury it. Not sure of CA's regulations on that. Tank sizes will be based on the BTU of the heater.

I have many clients in New York state that do not have natural gas. It's either oil or propane. We commonly bury 500 gallon tanks on new construction. Smaller, multiple tanks be set at ground level, but depending on heater usage, may have to be filled more often.

A gas fired pool heater, whether it be natural gas or propane, will heat your pool or spa considerably faster than any electric heater.

LP pool heaters are common place and widely available.