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How much water is heated using PV solar water heater

>
100 watt solar panel
Free shipping
Back view                      Front view
Example Renogy mono-crystalline panel

Buy from my affiliate links:
One 100 watt solar panel at Amazon $149
One 250 watt panel at Amazon $350
Two 150 watt panels at Amazon $300
Buy solar panels at Amazon
Branch wires
Mounting brackets

"90% or so of their original performance after 25 years.  They can easily last longer than that, slowly decreasing output as time goes on."
Solar panel bundle
250W Renogy mono-crystalline panel


Buy Bundles;
4 panels 250 watt at Amazon $1275
6 panels 250 watt at Amazon
10 panels 250 watt at Amazon
20 panels 250 watt at Amazon

Solar panel connectors
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Solar panel respurces

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)

All costs are energy
Invest in things that make energy!
Everything is on sale today... price only get higher... for wire, solar panels, fuel, aluminum

Solar calculations start here

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. 
Gene
See map with average ground water temperature
Water pipes that are located underground to prevent freezing... this is geothermal heat preventing the pipes from freezing

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 to 120
Dave
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)
120-240 100 amp Kwh meter
B00GMZRXE8

EKM 120-480 Volt
5000 amp AC
single or 3-phase
B00APOQK4G
B008B15IR0 best link?
module
B005BHRRDK

120 Volt 50 amp Kwh meter
B00APOHSCY

Z-wave module
B00FKJBUX2

RC Watt meter
B001B6N2WK
Gene
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 the Load consumes power.
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

Stand-by loss for PV 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?
solar clothes dryer
solar panel array
Solar resistance charts
Solar resistance charts
Procedure: Array was put into various configurations and then the DC amps and volts under element load were measured.

Calculations:
Vm = Volts Measured
Am = Amps Measured
Wc = Watts Calculated = Vm * Am
Wpv = Watts PV name plate rating of the array
EF = Efficiency Percentage = Wc / Wpv

PV panels in single series string.

1 pv panel  Vm=19.41 * Am=1.0,  19.41Wc / 64Wpv     = 30.3% EF

2 pv panels Vm=34.75 * Am=2.5, 86.87Wc/ 128Wpv     = 67.8% EF

3 pv panels Vm=44.30 * Am=3.9,  172.77Wc / 192Wpv = 89.9% EF

4 pv panels Vm=49.03 * Am=3.6,  176.50Wc / 256Wpv = 69.9% EF

5 pv panels Vm=52.62 * Am=3.9,  205.21Wc / 320Wpv = 64.1% EF

6 pv panels Vm=53.97 * Am=4.1,  221.27Wc / 384Wpv = 57.6% EF

6 pv panels (2 parallel strings, 3 panels per string)
                   Vm=53.55 * Am=4.2,   224.91Wc / 384Wpv = 58.5% EF


Conclusion: 

3 panels of US64 is the ideal number in single series string for 4500W element as per calculations:
Speaking of maxing the yield.  I'm thinking this thing blows the techluck away because there is no heat sink that wastes energy.  Just about all the energy generated by the panels goes directly into element.  Might be a small amount lost in the wire if its too small.  Just got to make sure PV Impedance is close to Element Resistance.
 
Lets take the 100W Renogy mono-crystalline panel you have an amazon link on your site as an example.


Optimum Operating Voltage (Vmp): 18.9V
Optimum Operating Amps (Imp): 100/18.9 = 5.29A
Optimum Impedance = V/A = 18.9V/5.29A = 3.57Ohms

4500W element 240V = 12.8Ohms

12.8 Ohms Element /  3.57 Ohms PV = 3.58 panels per string.  ... rats! Its not an even number.   But! It is half way in between!

Easy to make it work.   7 panels per string with 2 strings, 14 panels * 100 watts = 1400 watt array.

7 * 18.9Vmp = 132.3Vmp array
2 * 5.29Amp = 10.58Amp array

Divide V by A and you get 132.3Vmp/10.58Amp = 12.49 Ohms which is close to 12.8Ohms of the element.

#-------------------------------------------------------------------------------
Other way to make work: Use different size element.

5500W element 240V = 10.47 Ohms

10.47 Ohms Element / 3.57 Ohms PV = 2.933 Panels.  We have a winner. That's very close to 3

So its easy to make a small 300W system with 3 panels:

3 panels per string with 1 string.  3 panels * 100 watts = 300 watts.

3.57 Ohms PV * 3 = 10.71 Ohms that almost matches the 5500 watt element Ohms of 10.47.

or make one 4 times as big!

6 panels per string with 2 strings.   12 panels * 100 watts = 1200 watts.

or

9 panels per string with 3 strings.  27 panels * 100 watts = 2700 watts.

or 

12 panels per string with 4 strings. 48 panels * 100 wats = 4800 watts.


Realistically for a large system I would buy the cheaper 250 watt sized panels instead of the 100W since they are cheaper per watt.  I just wanted to use an example with a product you linked to on your site.

The term string refers to a number of PV panels in series.  + from one panel feeds into - on the next.

 Single panel:   -PV+

 String of panels:    -PV+ -> -PV+ -> -PV+

 The term array refers to one or more strings in parallel.   The + and -  ends of multiple strings are connected to each other.  A PV combiner box simplifies wiring and adds over current protection using DC breakers.

7 panels per string with 2 strings, 14 panels looks like this:

-PV+ -> -PV+ -> -PV+ -> -PV+ -> -PV -> -PV+ -> -PV+
|                                                                                    |
-PV+ -> -PV+ -> -PV+ -> -PV+ -> -PV -> -PV+ -> -PV+
There are lots of different types of PV systems. Type 2 and 3 are most common.

1) Off-Grid DC Loads.

Sun -> PV Array -> Charge controller -> Battery Bank -> DC Loads

2) Off-Grid AC Loads (this is what my system is, Outback VFX).

Sun -> PV Array -> Charge controller -> Battery Bank -> Inverter -> AC Loads

3) Grid Tie Battery-Less

Sun -> PV Array -> GridTie Inverter -> AC Mains/Grid

4) Grid Tie with Battery Backup (example Outback GFX)

Sun -> PV Array -> Charge controller -> Battery Bank -> Grid-Tie Inverter -> AC Mains/Grid and AC Loads
                                                                                                                    
#------------------------------------------------

Our DC Water heater system is:

Type 5)  Direct Power DC Loads, Battery-Less

Sun -> PV Array -> DC Relay -> Resistive Load

Techluck would also be a separate type in my opinion.

Type 6) Techluck, Battery-Less

Sun -> PV Array -> DC to AC Converter Box (non 60Hz) -> Resistive Load

The main advantage to type 5 is:

No electronics to fail
No Heat sink to waste power
No Batteries
No Inverter
No Lightning protection needed
No Charge controller

This eliminates much of the upfront equipment cost and also the maintenance cost / time of that equipment.   No moving parts for PV or electric water heater, they go well together.

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 insolation = Vmp / 1mp 30.1V / 8.32A = 3.617 Ohms (per panel)

#---------------------------------------------------------------------------------------------

Lets design a system with Element Wattage = 4500W
Element Resistance: 240 * 240 / 4500 = 12.8 Ohms

Ideal number of panels per single string assuming 1000W/m^2 solar insolation:

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.

#-----------------------------------------------------------------------------------------------

Now lets try with Element Wattage = 3800W
Element Resistance: 240 * 240 /3800 = 15.15 Ohms

Ideal number of panels per single string assuming 1000W/m^2 solar insolation:

15.15 Ohms /  3.617 Ohms = 4.18 panels, round down to 4 panels per string.

System C) 1000 watt system, 4 panels, 1 string, Ideal Resistance = 30.1 * 4 / 8.32 = 14.47 Ohms

System D) 4000 watt system, 16 panels, 8 panels per string, 2 strings, Ideal Resistance = (30.1 * 8) / (8.32 * 2) = 14.47 Ohms  

** Note, the PV array wattage is slightly higher than the 3800W power dissipation rating on the element so it may wear out quicker than normal. . 

#-----------------------------------------------------------------------------------------------

Now lets try with Element Wattage = 3000W
Element Resistance: 240 * 240 /3000 = 19.2 Ohms

Ideal number of panels per single string assuming 1000W/m^2 solar insolation:

19.2 Ohms / 3.617 Ohms = 5.308 panels, round down to 5 panels per string.

System E)  1250 watt system, 5 panels, 1 string, Ideal Resistance 30.1 * 5 / 8.32 = 18.08 Ohms

#-----------------------------------------------------------------------------------------------

Now lets try with Element Wattage = 2500W
Element Resistance: 240 * 240 /3000 = 23.04 Ohms

Ideal number of panels per single string assuming 1000W/m^2 solar insolation:

23.04 Ohms / 3.617 Ohms = 6.36 panels, round down to 6 panels per string.

System F)  1500 watt system, 6 panels, 1 string, Ideal Resistance 30.1 * 6 / 8.32 = 21.70 Ohms

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