Redirecting solar panel energy or branching

Re: Redirecting solar panel energy or branching

Transfer switch can work--But the bigger issue is the array configuration.

Typically, MPPT battery chargers have Voc of 150-200 volts or so maximum. Most GT inverters have a minimum Vmp of 200+ volts and a maximum of 500-600 VDC.

You not only have to transfer the +/- connections, you have to reconfigure the array.

There are options... Schneider/Xantrex has a new 200-600 VDC MPPT Solar Charge controller.

Or, you can transfer your GT inverter output to a TSW inverter's output and back drive it to recharge the battery bank. There are a few more issues--but it can be done with some hardware.

So--A bit more detail on your needs would be helpful.

-Bill

Re: Redirecting solar panel energy or branching

Brian,

For a small system--Grid Tied (at least "legal" grid tie) is not cost effective. Out here, you would be charged ~$300-$$900 or so for a building permit, plus a 10-20 page contract from the utility+you need to meet their insurance requirement. Just does not make sense for a few hundred watts of Grid Tied solar.

So--a small system. Assume 12 volt battery bank, 30 amp Rogue MPPT charge controller, and 0.77 system derating. The maximum (roughly cost effective solar array) that you can use would be:

• 30 amp controller * 29 volts charging * 1/0.77 system derating = 1,130 Watt Solar Array

The above is just a rough rule of thumb. Above that array size, you start loosing a fair amount of charging power during the middle of the day as the controller limits to ~30 amps maximum (it is perfectly safe to limit current--just a "waste" of money).

The rule of thumb for the battery bank would be ~5% to 13% charge rate for good battery life (you can go to 25% rate of charge--but it is pretty expensive for solar panels):

• 200 AH * 14.5 volts charging * 1/0.77 derating * 0.05 rate of charge = 188 Watt minimum array
• 200 AH * 14.5 volts charging * 1/0.77 derating * 0.10 rate of charge = 377 Watt "good sized" array
• 200 AH * 14.5 volts charging * 1/0.77 derating * 0.05 rate of charge = 490 Watt maximum "cost effective" array

The amount of usage from the battery bank at night... Assume 50% maximum discharge, 400 watt load and 85% efficient inverter:

• 200 AH * 12 volts * 0.50 max discharge * 0.85 inverter eff * 1/400 watt load = 3 hours (actually a bit less than 3 hours)

Assuming you have a minimum of 4 hours of sun per day, 0.52 system derating, and 377 watts of solar panels with a 400 watt load (charge during day, watch at night):

• 377 watts of panels * 4 hours of sun minimum * 0.52 system derating * 1/400 watt load = 1.96 hours of use per "sunny day"

Those are how the numbers look (using my "made up" numbers for array size) and such... You can substitute your "real numbers" and get things more accurate for your needs.

-Bill

Using PV Watts for San Diego, we see you will get around 4.6 to 6.x hours of sun per day over the year:

"Station Identification"
"City:","San_Diego"
"State:","California"
"Lat (deg N):", 32.73
"Long (deg W):", 117.17
"Elev (m): ", 9
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 32.7"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:","12.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.83, 72, 9.00
2, 5.35, 72, 9.00
3, 5.91, 88, 11.00
4, 6.52, 93, 11.62
5, 6.05, 89, 11.12
6, 5.98, 84, 10.50
7, 6.26, 90, 11.25
8, 6.62, 94, 11.75
9, 6.02, 84, 10.50
10, 5.88, 86, 10.75
11, 5.16, 74, 9.25
12, 4.67, 69, 8.62
"Year", 5.77, 994, 124.25