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)
Resistance
Basis:
Watts stay the same with either parallel or series..
Volts x Amps = watts
Resistance (ohms) = volts divided by amps, V/I Resource: Formulas-for-Ohms-law.html
Math:
Example: 120 volts x 15 amps = 1800 watt => 120/15 = 8 ohms
Example: 60 volts x 30 amps = 1800 watt => 60/30 = 2 ohms
.... this shows that resistance goes down when when amps go up...
Theory:
During high solar conditions, low resistance (higher amps) harvests
more electric output.
During low solar conditions, higher resistance (lower amps) harvests
more electric output. Resource: How to optimize-solar-panel-array.html
Application:
A
string of 12 panels in series is best for low solar because it raises
resistance (low amps) ... so amps are lower in a string and volts are
higher. Note: transmission distances areincreased with high volts
because less amperage means less heat loss on the wire.
While12
panels in parallel are best for high solar because resistance is low
(high amps) ...so amps are higher in parallel and volts are
lower.
Choose
element rated for your output:
Volts and watts are printed on end of each element.
Ordinary elements work fine for AC or DC.
You
can connect 100-250 volt 30 amp solar installation directly to
residential water heater... except alterations are needed, otherwise
the thermostat will melt. Read about converting to high voltage DC
In
2017, most local hardware stores do not carry elements rated for low
volt and watt ratings, so you need DC element for those applications.
Single U and double U DC elements, 24 volt 500-900 watts.
+++++++++++++++++
Typically in residential water heating, small water heaters (20 gallon)
use 1000-1500 watt elements. Ordinary shower uses 10 gallons hot,
ordinary bath uses 20 gallons hot.
You can set thermostat to higher temperature to get more miles out of a
tank of hot. http://waterheatertimer.org/Increase-amount-of-hot-water.html
Larger water heaters use elements with more wattage in order to heat
the water.... if the tank is full of unheated water, it will cool the
water that is heated.
A 500 watt element would struggle to heat a 40 gallon tank
5.866689 Kwh to raise 40 gallons of water 60°F (starting at
60- and ending at120°F)
Typical shower might be 104°F
1000 watts for 1 hour is 1 Kwh
Your 500 watt element produces 1 Kwh for each 2 hours operation
So you would need 5.867 x 2 hours or 11.73 hours to heat a tank ... and
that is with ideal conditions, no consumption of hot during those
hours, full sunshine, no cooling of tank etc etc ... except the tank
would cool, and sun is not dependable each day.
.. or if you satisfied with less-than-thermostat-setting hot water, say
80°? http://waterheatertimer.org/Kwh-temp.html
If you have a double U element, the time would be less.
Conclusion:
the direct solar PV using low voltage DC elements plan would work best
with a smaller (20-30 gallon), well
insulated tank.... and that was the project I was planning at my house,
except hadn't gotten around to it yet.
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