contractor wants the roof slope changed

Re: contractor wants the roof slope changed

What will your net metering plan be like... Summer afternoons, I get $0.30 per kWH. Winter, I get "paid" about $0.10 per kWH--So, having an array with better summer numbers is actually a much better deal for me.

It also depends on what your generation profile will look like... If you use lots of summer afternoon power (store, shop, A/C, etc.), then you want to make sure that your Time of Use billing is not killed by high summer loads. Lots of winter production will not make up for excessive summer billing/usage.

Using PV Watts for Seattle WA, fixed array at 25 degrees and 47 degrees, and 10kw array. 47.5 degree first:

"Station Identification"
"City:","Seattle"
"State:","Washington"
"Lat (deg N):", 47.45
"Long (deg W):", 122.30
"Elev (m): ", 122
"PV System Specifications"
"DC Rating:"," 10.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 7.7 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 47.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 6.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 1.54, 333, 21.31
2, 2.50, 503, 32.19
3, 3.71, 837, 53.57
4, 4.37, 957, 61.25
5, 5.31, 1177, 75.33
6, 5.52, 1166, 74.62
7, 5.88, 1271, 81.34
8, 5.17, 1120, 71.68
9, 4.98, 1048, 67.07
10, 3.00, 657, 42.05
11, 1.76, 370, 23.68
12, 1.26, 256, 16.38
"Year", 3.76, 9697, 620.61

And 25 degrees:

"Station Identification"
"City:","Seattle"
"State:","Washington"
"Lat (deg N):", 47.45
"Long (deg W):", 122.30
"Elev (m): ", 122
"PV System Specifications"
"DC Rating:"," 10.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 7.7 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 25.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 6.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 1.33, 278, 17.79
2, 2.24, 449, 28.74
3, 3.55, 803, 51.39
4, 4.50, 993, 63.55
5, 5.74, 1287, 82.37
6, 6.14, 1312, 83.97
7, 6.43, 1400, 89.60
8, 5.41, 1179, 75.46
9, 4.77, 1007, 64.45
10, 2.73, 597, 38.21
11, 1.56, 321, 20.54
12, 1.08, 211, 13.50
"Year", 3.80, 9838, 629.63

Make this a pure dollars and cents calculation...

Note that on a year over year basis, the two system performance results are about the same. PV Watts is acutual ~20 year measurements--So marine layer and weather definitely are taken into account.

And remember that winter does not supply much power anyway... 20% increase (of a small amount) in winter is not a big deal.

-Bill

Re: contractor wants the roof slope changed

Same as above, but 11.8 degrees:

"Station Identification"
"City:","Seattle"
"State:","Washington"
"Lat (deg N):", 47.45
"Long (deg W):", 122.30
"Elev (m): ", 122
"PV System Specifications"
"DC Rating:"," 10.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 7.7 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 11.8"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 6.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 1.13, 219, 14.02
2, 1.97, 385, 24.64
3, 3.25, 736, 47.10
4, 4.34, 960, 61.44
5, 5.70, 1284, 82.18
6, 6.20, 1330, 85.12
7, 6.42, 1401, 89.66
8, 5.27, 1152, 73.73
9, 4.36, 920, 58.88
10, 2.42, 520, 33.28
11, 1.36, 268, 17.15
12, 0.92, 164, 10.50
"Year", 3.62, 9339, 597.70

Does not affect summer peak much, but you do lose a good amount in winter...

But a ~100 kWH per month is about $10-$20 per month (December)... But overall, you are only losing ~500 kWH per year over the over the other settings. Somewhere around $50 to $100 per year at current power prices. Is the "extra work" worth it to you financially?

Again, your billing options may make a difference in your interpretations too (in California, we are forced to use Time of Use billing. This means that "small systems" may actually increase costs as the summer usage can be billed at 3x winter rates--and if there is not enough panel to reduce summer usage--the high TOU rates for summer can increase billing costs in those months.

In California, our complex/Time of Use/Tiered Rate plans make understanding solar PV power production very difficult to accurately model the interaction.

-Bill

Re: contractor wants the roof slope changed

Keeping the panels at ~25 degrees will optimize your summer output (and overall output a bit too). And keep the panels a bit low and possibly less visual impact (and less chance of catching storm winds--if a problem in your area).

-Bill