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How much standby loss with solar water heater
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1000 watt system: four 250 watt panels
3000 watt system: twelve 250 watt panels

Specifications of the 250 Watt RENOGY panels
Maximum Power: 250W
Optimum Operating Voltage (Vmp): 30.1V
Optimum Operating Current (Imp): 8.32A
Panel Ideal resistance at 1000W/m^2 solar isolation = Vmp / 1mp 30.1V / 8.32A = 3.617 Ohms (per panel)

Element Wattage = 4500W
Element Resistance: 240 * 240 / 4500 = 12.8 Ohms
Ideal number of panels per single string assuming 1000W/m^2 solar isolation:
12.8 Ohms / 3.617 Ohms = 3.538 panels, round down to 3 panels per string.

System A) 750 watt system, 3 panels, 1 string, Ideal Resistance 30.1 * 3 / 8.32 = 10.85 Ohms
System B) 3000 watt system, 12 panels, 6 panels per string,  2 strings, Ideal Resistance (30.1 * 6) / (8.32 * 2) = 10.85 Ohms.

Element size (resistance) needs to have an approximate match to the PV array (Vmp/Imp)

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Stand-by loss for solar hot water: No cost for standby?
Dave writes:
Standby-loss and PV hot water: 

My opinion on this is that when you have a PV system the biggest loss you have is not having enough water storage to hold all the energy you could harvest on a good solar day and use that energy to tide you through a few days of bad solar yield.   Lets call this effect "Opportunity loss"

Standby loss pales in comparison to lost opportunity of yield due to inadequate storage.  
Therefore: Standby-loss only counts with respect to hot water that is heated with expensive NG, Propane or grid electricity.

There is no use heating up water by burning dollar bills and then having it cool off (standby-loss) and having those dollars dissipate into the air).
Solar PV hot water is totally the opposite.

 Its another instance where solar is totally upside down from what you would have normally thought and requires people to think upside down .... good luck.

Large Hot water thermal storage devices used to be the most cost effective energy storage devices on the planet.

Solar PV has no standby loss ..... in a realistic sense.   Technically yes it does but looking at things as a whole system it is better to harvest the energy and have a portion of that energy lost into the air than to not harvest it at all.

Therefore the backward part is:
When dollars are turned into hot water, the smaller the tank the better because the lower the standby-loss.

When sun (free once the system in built) is turned into hot water, the larger the tank the better because the lower the opportunity-loss.
Gene writes;
This is hugely interesting point about solar water heating standby.
It costs nothing compared with carbon-burning...... hummmm

This thinking could revolutionize the monetary pro-and-cons over solar water heating.
This is a new idea to me,, and I will consider it across many applications.
It will make me rethink several webpages... because people should read this idea.

We should somehow factor the cost of PV system, since AC generation always talks about loss due to infrastructure costs.

I am also wondering how the standby idea will factor with solar glycol units that seem to be the manufacturer's favorite?
....if solar standby loss is unimportant, then maybe glycol inefficiency is not important either?
I would expect standby idea to be true for glycol and water thermal systems. 

There are three places that heat can be lost in a glycol (or even water) based system.

Piping (bad)
The Solar Collector (bad)
Into the Water Heater (good, because this is what you want.).

I would expect the piping to be the major loss.  Usually the collector is as close a possible to the exchange tank.   Some of my PV panels are over a hundred feet away.    PV off grid and grid tie systems have the PV voltage very high typically (over 80volts, some as high as 600VDC).  This helps with reducing transmission loss.    Some people run PV wire many hundreds of feet.  Maybe they are down low behind a hill or in some trees and they put the panels on the top of a hill or in a clearing.    You don't see that happening in a typical thermal collector system.  Its gotta be right next to the tank or its gonna lose heat big time.

 I agree its possible that ping loss is not very relevant unless the distance is extreme.  Collectors are cheap to build.  Its all the other parts that are needed to make it work where things get expensive and complicated.
Eventually need a Pro/Con page that compares Water Thermal, Glycol Thermal, Techluck and PV Direct.
How much water is heated using PV solar water heater
Dave writes:
My rule of thumb for solar is 5x to 6x the panel rating during a really sunny day. 
1250 watts of PV should make about (1250watts x 5) = 6.250 Kwh

My water is 60F on average.  55F in winter.  65F in summer
5.866689  Kwh to raise 40 gallons of water 60F (60-120F), so 6.250 Kwh will raise 40 gallons of water about 64F

But 5 to 6Kwh of hot water isn't enough for me. 
I burn more like 10 to 15Kwh of hot water per day.  Sometimes more if its a heavy laundry day or have to run two loads in the dishwasher.

Solar heating is not always predictable:
Take today for instance.  I will be lucky if I get 30Kwh of electricity for whole house, including water heater, as compared to the 66Kwh I harvested yesterday. 
For more detailed water heater calculation:
Wattage is a rating that does not measure heating power.
Wattage over time, or watt-hours or Kilowatt-hours (Kwh) is measure of heating power.
If you have 1250 watts for 1/2 hour, then you have 625 watt-hours or .625 Kwh.
If you have 1250 watts for 2 hours of the day, and 750 watts for 3 hours, then you have 4.75 Kwh... which should raise 40 gallons of water from 75
0.0002931 Kwh to raise 1 pound of water 1F
0.0024444 Kwh to raise 1 gallon of water 1F
0.146666  Kwh to raise 1 gallon of water 60F (60-120F)
5.866689  Kwh to raise 40 gallons of water 60F (60-120F)
How do you figure actual Kwh coming from solar PV array.
The actual output varies by rating of your solar array, age and efficiency of panels, and how much solar is available
1) You need electric meter to record the output as it varies per hour.
2) Or you need to measure the rise of temperature inside tank by turning off household power, letting tank cool completely... and then measure water temperature before and after solar heating takes place, using average temperature from TP valve and drain valve.

Compare: 4500 watt element under full 240 Volt AC output, heats about 20.7 gallons per hour... unknown rise in temperature See water heater recovery chart
Compare: 1500 watt element under full 120 Volt AC output, heats about 6.9 gallons per hour.... unknown rise in temperature See water heater recovery chart
What does this mean?
If maximum output is 120 volt at 1250 watt for 2 hours, then you would get about 11.5 gallons. Add 120 volt at 750 watt for 3 hours, 10.3 gallons
Add 11.5 gallons and 10.3 gallons for total 21.8 gallons fully heated on a good solar day
Yes, its very interesting that different systems have different ways of improving overall efficiency.   Reducing loss is something every system designer strives for.

  Getting rid of stand-by loss is part of the great appeal of tank-less water heaters.   They just have to be sized for peak instantaneous demand. 

With tank water heaters the user has to balance efficiency (stand-by loss with a larger tank) vs hot water availability (running out with a smaller tank). 

With solar, the concept of stand-by loss gets kicked out the window.  Opportunity loss takes precedence and eliminates the downside of stand-by loss.   Its is better to harvest it and lose it than to never harvest it at all.

Of course when designing a solar hot water system there are limits.  Bigger storage tanks, more storage tank all cost money and take up space.  It is possible to go too big.    An ideal size would be a system that holds about 4 days of hot water.  For me that would be two 105 gallon tanks with mixing valve instead of the single 105 gallon tank + mixer I have now.  But I only have room for one monster tank so 2 days of storage isn't ideal but it is very acceptable.

    Gene: I am also wondering how the standby idea will factor with solar glycol units that seem to be the manufacturer's favorite?

I think the effect is the same glycol and water based systems. 
There is one other aspect that comes into play with hot glycol systems.  Are heat exchangers more efficient at transferring energy to cold water or warm water?   The larger the tank the more often the system is transferring to colder water than with a smaller holding tank.

Heat-transference loss in glycol system has to be large.
The heat is gathered in a solar panel,
... then piped to the water heater through holes cut in roof,
... then circulated through a double-wall coil tube located inside water heater.

But if solar standby loss is unimportant, then maybe glycol inefficiency is not important either?

Glycol is good because it won't freeze.
But what about risk?
Is there a warning system if deadly anti-freeze glycol leaks into your water heater and water pipes?

And what about hurricanes along Gulf Coast or big storms elsewhere?
How vulnerable are the panels?
Can you get insurance?
Do they attract lightning?
Does home need lightning rod?

I remember the old solar recirculating systems
They circulated water directly through your regular water heater.
One of the salesmen told me they were telling buyers 3 year payback.
...then laughed and said the payback was more like 25 years.
And that was here in Houston with good summer solar. But cold winters and several days freezing.

Now the old water systems could never work efficiently.
Because you had to circulate tank-made hot water in winter to keep pipes from freezing.
Then resale of home in Houston required removal of system to meet market expectation.
The systems leaked at bad solder joints.
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