Home PV System Design

Home PV System Design

Step #1 Evaluate Home Energy Needs (based on 2007 & 2008 PG&E bill) Averaged annual electricity usage is around 4,500kWh Step #2 Determine System Requirements (Grid-tied, Fixed tilt) 4500kWh per year / 365days per year / 5.4 h/day = 2.28kW Estimated system size (AC watts) is 2.28kW, don t want to oversize it Step #3 Site Survey - House is facing south - Neighbor s tree is at southwest corner - Rooftop is the only suitable location for the PV system - Multiple planes and levels of roof - Roof pitch is 5:12 - Roof type is composition shingle (~1yr old) - There are skylights and vent pipes on the roof surface

Step #4 Shading Analysis Rooftop layout from Google Earth:

Use Solar Pathfinder shading analysis tool to analyze 12 zones on the roof. A summary report is in the next page.

Zone 2 & 3 Zone 4 Does Zones 2 & 3 alone have enough space? The plane is about 250 sq ft. However, there s one skylight there. The shape of the plane will also limit the useable space Based on the Solar Pathfinder Shading Analysis report, top 2 planes are: - Zones 2 & 3 (on the same plane south facing, avg. shading effect 91.2%) - Zone 4 (east facing plane, shading effect 79.95%) Step #5 Select PV System Location PV System Design and Sizing

Step #6 Select PV modules - Considered modules with STC > 200 Watts only - Dimension of the module is critical - Chose Sanyo HIP-205BA19 (205W HIT Power 205 Hybrid Amorphous/ Mono-Crystalline) solar module over the others, because of it s smaller in size (dimension: 51.9 x34.6 x1.8 ) - CEC PTC rating for Sanyo HIP-205BA19 is 191.9Watts - How many modules do we need? DC to AC derate factor came out to be 0.751 (assuming inverter s efficiency is 95%, shading factor is 91.2%) AC Watts/derate factor/module s PTC = 2280/0.751/191.9 = 15.8 - Need a total of 15 or 16 modules (use 15, so we don t oversize it). Zone 2 & 3 can fit 10 modules only. Need to use zone 4 also. Step #7 Select inverter - With modules on two roof planes (with different orientation too). We will need either two inverters or use Enphase micro-inverters (efficiency 95%). - Chose to use Enphase micro-inverters M200-32-240-S01/2, not only because it will decrease system power reduction due to single-module shading, but also because it s more cost effective ($3,000 vs >$4,000) too.

Step #8 Layout of the PV modules - 10 modules to put on Zones 2&3 (south facing plane) and another 5 to put on the Zone 4 (east facing). - PV modules to be installed in parallel with the roof surface, with 6 inch mounting space between the modules and roof surface. - One AC branch circuit junction box for each roof plane.

Step #9 Estimate System Performance with PV Watts http://rredc.nrel.gov/solar/calculators/pvwatts/version2 Zones 2&3: - DC Rating = 205w x 10 / 1000 = 2.05kW - Shading factor = 91.2% - Overall DC to AC derate factor = 0.751 - Array Tilt (degrees) = 22.62 (since roof pitch is 5:12) - Array Azimuth (degrees) = 180 (south facing roof plane) - Cost of Electricity (cents/kwh) = 11.904 - AC Rating = 1.54kW Zone 4: - DC Rating = 205w x 5 / 1000 = 1.025kW - Shading factor = 79.95% - DC to AC derate factor = 0.658 - Array Tilt (degrees) = 22.62 (since roof pitch is 5:12) - Array Azimuth (degrees) = 90 (east facing roof plane) - Cost of Electricity (cents/kwh) = 11.904 - AC Rating = 0.67kW Total AC Rating = 1.54kW + 0.67kW = 2.21kW (short of 3% only)

Step #10 Evaluate the cost & payback - Calculate California Rebate (CSI EPBB Calculator - http://www.csi-epbb.com) Rebate for the system on zones 2&3: $2,624 Rebate for the system on zones 4: $1,026 Total Rebate = $2,624 + $1,026 = $3,650 - Calculate total system cost (excluding labor cost): The cost of a Sanyo HIP-205BA19 module is ~ $974 The cost of a Enphase micro-inverter M200-32-240-S01/2 is ~ $222 The cost of permit, meter and other parts are estimated to be ~ $3,000 Total Cost of parts and permit = ($974+$222)*15+$3,000 = $20,940 - Calculate Federal tax credit: ($20,940 - $3,650) * 30% = $5,187 - Net cost after rebate & tax credit = $20,940 - $3,650 - $5,187 = $12,103 - Simple Payback - with monthly savings about $44 The payback period = $12,103 / $44 / 12 = ~ 22.9 years

Conclusion: - Shading has the most impact for this PV system design and sizing (Use of Enphase micro-inverters add a lot of flexibility to the system design) - Payback in ~22.9 years may seem long, however, considering the environmental benefits, this is still a viable option -There are still ways to reduce my electricity usage, so a smaller system may be sufficient - What-If I only install the system on zones 2&3 (south facing roof plane). With the use of micro-inverters, it is possible to add the part of the system on zone 4 later on. - AC Rating = 1.54kW - Total Cost = ($974+$222)*10+$2,000 = $13,960 - CA Rebate = $2,624 - Federal Tax Credit = ($13,960-$2,624) x 30% = $3,401 - Net Cost = $13,960 - $2,624 - $3,401 = $7,935 Payback = 7,935 / 44 / 12 = ~15 years