By Matt
We positioned one of the long faces of the house approximately due south so that we could capture more sun on the solar panels we plan on installing on our roof (also for passive solar gain, but that’s another post). Its a rightsized house, so it is not going to have an expansive roof, but it is a simple roof (no dormers, etc) where most of the south facing roof will be available for solar panels. The amount of energy we can get from the whole array will of course depend on how many panels we can fit on the roof, the potential wattage of each solar panel and the amount of sun we expect to get in southeastern Minnesota.
The roof is 56.7 feet long, including the 2 foot rake overhangs on the east and west side. The roof has an 8/12 pitch, the width of the house is approximately 18 feet and there are 3 foot eves on the north and south sides. Doing the math this means the width of the roof is 15 feet. So we have approximately 850 square feet of roof to mount solar panels. Then there are three constricting factors for the number of panels which can be mounted.
(1) The dimensions of the solar panels. Solar Connection, our solar panel supplier and installer strongly suggested Solarworld Sunmodule XL as a good balance of quality and low cost. These panels have a taller dimension (78.5′ x 39.4′) than most other standard solar panels (65 inches by 39 inches).
(2) We want to mount panels in portrait mode which therefore limits the number of rows of panels such that the extra height of the Solarworld panels helps in using more of the roof surface. Both with the Solarworld Panel and standard solar panels we could only fit two rows.
(3) The building restrictions which require two access paths from the eve to the ridge for firefighter access. Two three foot paths are required.
Placing the panels side by side in two rows of 15 panels for a total of 30 panels with firefighter access on the east and north ends (as shown in photo above).
The amount of energy we can expect on these panels depends on their wattage (potential output) and the amount of sunshine we can expect in southeastern Minnesota.
Typical solar panels range from 200 to 350 watts which is influenced by how efficient each array in the panel coverts solar energy into electricity and the dimensions of the panel. Since we are somewhat restricted in the amount of space we are choosing a panel with a higher wattage per panel so we can maximize output. Other homeowners may choose a panel with less wattage if they have more space and/or lower energy requirements as they are cheaper than the high wattage panels. The bid for our solar panels uses Solarworlds Sunmodule XL at 350 watts per panel.
To calculate the size of our array we multiply the wattage of the solar panels by the number of solar panels (350 watts x 30 panels) resulting in a 10.5 kW array. How much electricity can we expect from a 10.5 kW array? There are a number of web based calculators to do such a calculation. Here we will just plug in the numbers and use NREL data.
So a 10.5 kW array is the starting point. The sun actually has to be shining of course, so day length and orientation in the sky matters, so one has to look at a solar map to determine how much solar energy a particular location receives. Rochester MN on a yearly average receives 4 to 4.5 kWh/m2/day so to calculate the output we multiple our array size (10.5 kW) by the amount of solar energy Rochester receives (we will be conservative and say 4 kWh/m2/day) and then multiply that by the number of days (365). So our solar power array has the potential to produce 15,120 kWh per year (10.5 x 4 x 365). However, there are typically various losses due to temperature, wiring and inverter efficiency which means you only can harvest about 80% of the panel output so now its down to 12,096 kWh.
Bottom line, we can fit thirty panels and if those panels are 350 watt panels they will hopefully produce approximately 12,100 kWh of electricity. Now we just have to see if this will be enough electricity to meet all of our energy needs.
UPDATE – Our solar contractor indicated that firefighter access strips can be as small as 1 foot wide so technically we could add an additional column of panels to up the total to 32. In addition we would probably want to center the array vertically on the roof as Minnesota regulation specifies the solar panels can not exceed the height of the ridge line.