Here is a pic of my warehouse roof. It is approximately 85 ft X 130 ft. with some AC units here and there.
Conservatively, how many panels could I fit on it? I also have Split 240V configuration for power. What grid-tie inverter would I need?
Back of the envelope:
10' space all around (fire code for access) = 65 x 90 = 5850sf minus stuff on roof, minus space around stuff on roof (shadows) = 5000sf
15Wpdc per sf = 75kWpdc (70kWpdc is conservative)
six 10kW inverters (they don't make a 240V 1phase 60kW inverter), or a 250W or 300W microinverter on each PV panel. Inverter would be sized at about 80% of dc rating of system, due to system losses before inverter output (heat, wiring, inverter inefficiency, soiling)
Limitations:
1. Depending on the version of NEC in effect where you are located (and where the inverter breaker is located in the panel) you can only put 20% or 16% of your service entrance main busbar rating on your main panel, in terms of inverter output amps. (If your main breaker/fuse is rated smaller than the busbar, then you can put more).
Example, if you have 400A main breaker/busbar, you can only put 64A (pre-2014 NEC) or 80A (2014 NEC) inverter on that panel, meaning 15kW or 19kW inverter.
In that case, you would put only 19kWpdc or 22kWpdc on the roof. You could only use the 19kW inverter if you had space at the end of the panel busbar for the inverter breaker.
Exception: You can put up to 320A of inverter on a 400A panel if you do what's called a "line side tap", meaning intertying the inverter between the main disconnect and the meter. This is not always possible. It depends on the configuration of your existing equipment, or would require new or extra equipment to achieve. A 320A inverter would be 77kW, bigger than you would need.
2. Unless your utility gives you something for production over and above your annual use, you would want to size the system to not exceed in one year what you use. Example: If your last 12 electric bills sum to 50,000kWh, you would not want to put a system with more than 50,000kWh annual output on the roof. Depending on where you live, that is something between 25kWpdc and 35kWpdc of solar.
PS: A 70kWpdc system would support a 14Th/s "farm" 100%. Farm takes about 14kW to run, but solar PV has only about a 20% capacity factor, so you need five times as much as your max power draw to supply 100% of annual energy.
PPS: A 70kWpdc system would cost between 180,000 and 210,000 USD all in, depending on incentives in your area. More in Hawaii (shipping).
