After analyzing data from the first year of operation, and taking into account better information about my electric load, I determined that I could use the production from additional solar panels covering my remaining south-facing roof area. Eight panels will fit, and conveniently this reaches the maximum string power for my second story string.
The expansion ended up being much cheaper than I expected - under $1 per watt! It is eligible for the federal investment tax credit (ITC) but not for my state-level incentives because it does not include a new inverter. The addition of this cheap capacity brings down the overall system's cost per watt by over 15%.
|$3,300||Base price for 2400Wdc upgrade||$1.37/W|
|($990)||Federal tax incentive (ITC)||$0.41/W|
|$2,310||Actual upgrade cost||$0.96/W|
|$12,673||Original system cost||$2.22/W|
|$14,983||Total expanded system cost||$1.85/W|
These new panels will be at a slightly less optimal angle than my existing subarrays, being pointed more east, but will receive the least shading. I expect these effects to approximately cancel each other out, so the new panels will provide 2400/5700 as much electricity as my existing array, or slightly under 3MWh per year.
The electricity generated by these new panels should be worth roughly $530/year, for a payback period of between 4 and 5 years. The payback period of the full system should drop from about 10-11 years to 8-9 years as a result of this expansion.