My 40 gal electric HW heater was dying (18 years old - time to go, even for a Sears Kenmore). I had EIGHT 220 watt PV panels left from an installation (at my office), and I wanted to use them to heat water (much more efficient use than sitting in my shed for a year! Right? ). 220 x 8 is only 1760 watts. I found a new "Brand X" 50 Gal Electric HW heater locally for $315 (no delivery - had to fetch it - UGH!). Was 1760 watts enough to heat my tank to 130 or preferably even higher (say 170F), in 4 hours of full sun??? That was the MAIN question - and would the thermostats work right? (they ARE mechanical and heat is the key, so it should be OK).
I get a little over 4 hours of sun in winter where I wanted to put the panels - almost 5 hrs in summer (flat on porch roof - can't be seen except by Google Sats or planes from the air). They say that normal tank recovery to 140F was 1.25 hours at 3300 watts from 68 degree cold water (it seems too long a time - I didn't do the calculations, but I will - eventually) - takes longer for colder water - luckily I'm in Fla. Cold water temp is fairly stable 65 to 70. A 50 gal tank will last my small family (3 of us) 3 to 4 days (I HOPE), but only if I set it 165 or 170 and use a cold water mixer also called an "infuser" I think (it mixes cold with hot on the output and keeps water from being dangerously hot in the shower, or for kids at a faucet, etc.). The infuser is a trick I learned some years ago with a smaller 30 gal tank from my brother-in-law (thanks RICK!).
I had just enough 8 AWG wire - 10 AWG would have worked, but I had 8 so that's OK - another volt or two not dropped on the way in, I guess. After 7 days and 4 problems (all not directly related to the project itself, but a PAIN nonetheless), I got it installed (the tank was last).

And the answer turns out to be YES - and NO.
Yes it works. But on the NO side,
The CONS:

1. I am only getting 221 VDC at 7.4 amps - should be 230 or so under full sun (29VDC at max output x 8 panels in series). What happened to my 10 volts or so? Not that big a deal, I suppose. The coils are so low a resistance they are MAXING out the panels as I thought they would. Top coil comes on first, and an hour and a half to 2 hours later, CLICK - the lower one is on.
   
2. It does not always get hot enough - not even a full hour of clouds half-way through the heating cycle messes it up (pretty badly) - lots of warm water! (Note to self: Wife says "warm" is 100% FAILURE. Ooooo-Kay. HOT it must become... somehow ).
   
3. I only get two days of hot water after 1 full day of sun (4.5 to 5 hours), instead of a minimum of 3 as I want (and need), maybe because I only have it set to 135-140. 170 was WAY too hot since the infuser does not come on right away and with the first burst of hot water you can get scalded pretty badly if you are not careful (thankfully - he have no small kids). I need a faster acting infuser - this one is some type of bi-metal switch that mounts around/against the hot-out-bound copper pipe right out of the tank and it reacts too slow (looks like a WW-II or maybe a WW-I surplus thing). I need an Electric sensor/switch of some kind, or maybe adjustable-pressure based unit of some kind. I am searching for one. Something MUCH faster, anyway.
   
4. If they were not spare panels - cost might be be prohibitive. We paid 68 cents/watt (per whole palette of 25) - 220 x 8 was $1760 plus $315+tax for the 50 gal "el-cheapo" electric tank, $45 for small breaker box and DC beaker for PV panel supply). No mounting costs - they just lay flat on the porch roof with one bolt as a tie down (protected from wind, great drainage, slightly tilted up toward west - better for the afternoon sun, too).
   
5. Payback right now is 3.5 years + possible alimony for life if not made HOT-HOT-HOT (about 20% to 22% of electric bill - $550/year). Too long to suit me (but not too bad, really - panels will surely outlast the new tank unless we get a bad hail storm (very rare down here, BUT...I still worry about it). I PRAY a new infuser and going up to 170F will give me the extra day I want (and make my WIFE happy... it HAS to... no choice in the matter.)
   
   NOW for the PROS:
   
6. Super EASY to install. Bozo the clown could do it, except for "sweating" the copper pipes for the new tank (torch soldering). Adding them to an existing tank installation would be a NO brain-er.
   
7. No (leaky) pipes to run - no heat exchanger, no "Fred & Ethel Glycol" (old fluid mechanics joke) or other fluid heat carrying medium, or not pushing the cold up, and hot water down from a direct concentrator system.
   
8. GREAT use for spare PV panels. Actually a good deal if you can get panels for less than about 65 cents/watt any time (without having to buy a train load of them).
   
9. We have always had AT LEAST a warm shower in 6 months of use - Never cold - (I do warm, OK - My WIFE does NOT - NEVER WILL - EVER - even if H**L freezes over... EVER, NEVER EVER. PERIOD OVER AND OUT! Got it?? I got it.
   And did I mention mt WIFE does not like WARM showers. NEVER, EVER? ok? ok.).
   
10. Also, make sure to check with your wife BEFORE you do this. To be on the safe side - that is. ALWAYS check with your wife! OK? GOT THAT? ok. Me too. I GOT that. And I hear about it often... even 6 months later... once a week at least...
    
    YIKES! I need that NEW, FASTER infuser - (anyone have one?). I wonder who gets the dog in a divorce?
    I KNOW for a fact that I will get the NEW hot water heating system. And I may even have to wear it out of the house - around my neck someday.
    If you try it out, GOOD LUCK!
    THE FINAL ANALYSIS:
    SUN POWER! PV direct for hot water - It works!!!
    (If warm showers once a month, or so, are OK for you --AND YOUR WIFE... that is).