Need help with PV watts #

Re: Need help with PV watts #

0.77 for panel + charge controller derating
0.80 for flooded cell battery derating
0.90 for AGM battery derating

So, for your system, with flooded cell:

• 0.77*0.80 = 0.62

Close enough for a first cut.

As an example using PV Watts, for Detroit MI, 1,000 watts of panels (if you have 240 watts just simply multiply the numbers by 0.24 for that size array), fixed array titled to latitude:

"Station Identification"
"City:","Detroit"
"State:","Michigan"
"Lat (deg N):", 42.42
"Long (deg W):", 83.02
"Elev (m): ", 191
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.620"
"AC Rating:"," 0.6 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 42.2"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.3 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.90, 58, 4.81
2, 3.59, 64, 5.31
3, 4.13, 78, 6.47
4, 4.84, 87, 7.22
5, 5.52, 98, 8.13
6, 5.58, 93, 7.72
7, 5.42, 91, 7.55
8, 5.48, 95, 7.88
9, 5.18, 89, 7.39
10, 3.96, 73, 6.06
11, 2.59, 46, 3.82
12, 2.15, 41, 3.40
"Year", 4.28, 913, 75.78

You would collect ~41-98 kWHR per month:

• 41,000 watt*hours * 1/30 days per month = 1,367 WH per day in December (long term average)

-Bill

Re: Need help with PV watts #

Except Dan needs full solar for December--so the reason for more panels is the winter requirements (little sun, long nights).

Do you want the the lighting to be on from dusk + X hours, or Dusk to Dawn, or programmable by time/day?

You may be correct--your best bet is a small solar panel + a MorningStar Sunlight solar lighting controller 10-amp 12 volt (or other voltage/current that meets your needs). The one above from our host's site says a minimum of 1 watt solar panel to activate the timers.

I believe you will still need to put some sort of (not too big) solar panel on the Sunlight controller so it knows when to turn the lighting on and off.

The optimum current rating for a MPPT type charge controller would be:

• 450 watts * 1/14.4 volts * 0.77 = ~24 amps

So a 30 amp charge controller (12 volt bank, or 15 amp for a 24 volt bank) would seem to be a good choice.

The Morning Star 15 amp MPPT controller would be nice for a 24 volt battery system. The Rogue 30 amp MPPT would be nice for either 12 or 24 volt.

Using a generic voltage drop calculator, Say we want 30 amps at 17.5 volts and around 1-3% voltage drop. And you want 50 foot one way run:

• 17.5 volts * 1% = 0.175 volt drop rough min.
• 17.5 volts * 3% = 0.525 volt drop max.
• 50 feet of 1 awg copper cable and 30 amps will give you 0.45 volt drop

Obviously, that is very a expensive solution. But, since we are using MPPT type charge controllers here--We can use a higher voltage array. So lets use 2 panels in series for 35 volt drop:

• 35volts * 3% = 1.05 volt drop max.
• 50 feet of 8 awg copper cable and 15 amps will give you 1.14 volt drop

So, doubling the voltage and 1/2 the current allowed us to use 8 awg wire for the 50 foot run. That is probably the maximum input voltage for the Rogue. The Morning Star 15 amp MPPT with 75 VDC maximum input might be able to take three panels in parallel (and reduce wire gauge again--but you if stick with 12 volt battery bank, you would still need 2x MS 15amp MPPT controllers).

However, you can look at a Morningstar TriStar 45 amp MPPT solar charge controller (this is not cheap--showing you how to "play the game" with Vmp and controller changes and how it affects your wiring options).

You can run this at 150 VDC maximum input voltage which works out to a maximum Vmp of ~100 VDC. Or 5x 17.5 Vmp panels in series (Vmp=5x17.5=87.5volts).

• 24 amps / 5 series panels (instead of 5 parallel panels) = 4.8 amps nominal
• 87.5 volts * 3% = 2.625 volt drop max.
• 50 feet of 16 awg copper cable and 4.8 amps will give you 2.34 volt drop

So, there are lots of options that can reduce the wiring costs, but at the increased expense of using higher end MPPT charge controllers.

You also have to look at how to series/parallel the array (with XXX watt, YY Vmp panels) to get something that meets all of your needs and cost target.

Also, perhaps in the summer you can use the excess power to run a small water feature/fish pond aerator/water pump.

When looking for solar panels, 100+ watt panels tend to be less expensive--So whatever array/panel mix you end up with, take a look at the $$$ per Watt and make sure that a smaller array (made with smaller panels) does not ending up costing you more money.

Also, something else to think about--24 volt LED strings... This allows you to use smaller (and cheaper) charge controllers (15 amp @ 24 volt will work just fine). And, if you have to string your landscape lighting long distances--running them at 24 VDC (or even 48 VDC) will reduce your voltage drop/wire gauges for wiring them up too.

Lots of options here--Sorry to be so confusing, but I hope this helps you narrow down the choices that are correct for your application.

-Bill