Roof rack angle?

Re: Roof rack angle?

What is your load profile?

If you use AC during the summer and not too much power during winter, then flattening the array to ~(34 degrees - 15 degress=) ~20 degrees from horizontal.

In theory, ~34 degrees from horizontal will collect the most energy over a 1-year period.

Use PV Watts and "play" with the numbers to see what is best for your location/needs (PV Watts takes "average" weather into account). You can also play with facing array South East vs South vs South West--Morning fog vs afternoon thunderstorms, Cooler mornings (Vmp is higher, more power) vs afternoon A/C (use afternoon power to run AC instead of batteries)... Using 0.52 system eff for off grid vs 0.77 default:

"Station Identification"
"City:","Little_Rock"
"State:","Arkansas"
"Lat (deg N):", 34.73
"Long (deg W):", 92.23
"Elev (m): ", 81
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 2.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 34.8"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.78, 237, 17.54
2, 4.38, 244, 18.06
3, 5.19, 312, 23.09
4, 5.67, 318, 23.53
5, 5.89, 331, 24.49
6, 5.81, 309, 22.87
7, 5.93, 320, 23.68
8, 6.08, 332, 24.57
9, 5.41, 291, 21.53
10, 5.16, 299, 22.13
11, 3.69, 213, 15.76
12, 3.56, 218, 16.13
"Year", 5.05, 3422, 253.23

"Station Identification"
"City:","Little_Rock"
"State:","Arkansas"
"Lat (deg N):", 34.73
"Long (deg W):", 92.23
"Elev (m): ", 81
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 2.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 20.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.33, 207, 15.32
2, 4.04, 224, 16.58
3, 5.04, 303, 22.42
4, 5.80, 326, 24.12
5, 6.26, 355, 26.27
6, 6.31, 338, 25.01
7, 6.36, 346, 25.60
8, 6.29, 344, 25.46
9, 5.34, 287, 21.24
10, 4.81, 278, 20.57
11, 3.32, 189, 13.99
12, 3.10, 186, 13.76
"Year", 5.01, 3383, 250.34

"Station Identification"
"City:","Little_Rock"
"State:","Arkansas"
"Lat (deg N):", 34.73
"Long (deg W):", 92.23
"Elev (m): ", 81
"PV System Specifications"
"DC Rating:"," 4.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 2.1 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 50.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.02, 253, 18.72
2, 4.48, 250, 18.50
3, 5.06, 303, 22.42
4, 5.23, 292, 21.61
5, 5.20, 291, 21.53
6, 5.01, 263, 19.46
7, 5.18, 275, 20.35
8, 5.54, 300, 22.20
9, 5.19, 278, 20.57
10, 5.23, 303, 22.42
11, 3.87, 224, 16.58
12, 3.82, 236, 17.46
"Year", 4.82, 3268, 241.83

Don't get too wrapped up in the calculations--Real life will interfere and limit your accuracy to around 10% (year to year average variations). If you think you will need more power (don't plan on using 100% of predicted power daily, perhaps ~66% to 75% daily power, plus optional wash day/more A/C when plenty of sun, cut back in bad weather, run genset).

You can generate lots of data doing this. But it will give you a good feeling about how much you can affect your harvest with changes to your array installation.

-Bill