For "$70" ou can make something that would heat up some water but I doubt you'd get far into a proper domestic hot water system. A more reasonable price for a complete SHW system is $2 to 6 thousand. One example
But the rust on your tank is most likely cosmetic. A typical solar hot water storage tank is much like a regular hot water heater but without the heating elements (electric) or gas burner and heat exchanger (natural gas). And the typocally have two more connections for water going to / from the solar panels or heat exchanger.
The "tank" is typically a steal tank, coated in the inside with something like porcelain. Then covered with insulation and that is covered with a thin sheet metal - this is the white skin you can see when you look at it. This can rust off entirely and will not affect the tank (although if it's getting wet enough to do that you do have some other issues going on).
Like most all hot water heaters there is an "Anode Rod" which is there to slow the rust process. You should replace this rod (it works by rusting first - once it is rusted the rest of the tank will rust more quickly).
Now - why didn't you have any heat? Several reasons come to mind - not all apply to all systems:
- You're system has a clock (usually to turn of or "drain down" at night) and the clock is wrong because of a power failure or something.
- It was too cold where you live yesterday - if it is below freezing some systems will not operate (this prevents bursting pipes on the roof).
- Blown fuse or circut breaker - although the heat comes from the sun most systems use some electricity to pump the water to / from the roof.
- It has been cloudy were you live for a few days - even the best solar systems can only store heat for a couple of days.
- You solar panels are dirty - probably not the cause since this happens slowly, not over night but still wash them.
- There is some kind of "thermal loop" - this happens when the tank is lower than the panels (this is typical) and the hot water from the tank rises into the panels at night - cools off and falls back into the tank. There is almost always a valve to prevent this but sometimes it can stick open.
- Your pump has failed. They do wear out.
So I suggest you find a plumber who specializes in solar hot water systems
(there might be a name on your system) and have them come and examine your system. You have two particular questions for them:
1 - why isn't the solar hot water hot
2 - why didn't the backup system (whatever it is) turn on.
In my house I put a switch in the backup system so it will not come on during vacations when I turn the whole thing off. Might (hopefully) be something simple like that.
But it would be good to find a plumber who likes to work on your solar system now because if you need one in a hurry (and ALL water heaters fail eventually) you'll have one.