Battle for Business

We had gone ahead and signed a contract with Akeena even though it was slightly more expensive. Sun Power, of course, wanted a chance to rebid. They said they could install a generator, but they weren't sure it would meet the noise ordinance or set back requirements. They offered to lower their price, but I didn't wan't to mess around with cancelling one contract and signing a new one unless there was something really wrong with Akeena.

As I had mentioned that Akeena fit more kW on fewer inverters, Sun Power brought up the concern that putting too many panels on an inverter could cause it to turn off on a bright breezy day just when power generating conditions are at their best. Indeed when I ran Sun Power's numbers, they made sure this could never happen.

Akeena was going to put 2 strings of 9 Sharp 165W panels on each SMA 2500U inverter. I figured 2 x 9 x 165W is 2970W DC. Taking 95% of that because the panels would be facing SSW at a 22 degree tilt instead of the optimal direction gives 2822W DC. Multiplying that by the inverter efficiency of 94.4% gives 2663W AC, which is 7% more than the 2500W that the inverter is rated for. I asked Akeena, wouldn't the inverter shut off? But no, there is always a safety margin. Going to page 27 of the SMA_SWR2500U.pdf that Sun Power sent me, recommended PV-generator power is "up to 3000Wp". Akeena's 2970W was below that.

When I pointed this out to Sun Power, they directed me to SMA's String Sizing page. After entering the SWR 2500U (240) inverter, the Sharp (Schott) NE-Q5E2U (165W) panel, choosing a minimum temperature of 41F, a maximum temperture of 104F, and clicking on "Get Sizes", it told me the 2 strings of 9 panels was a valid combination, but some derating is likely without adequate airflow. Derating is when the inverter reduces it's output power to protect itself from overheating. (SMA's derating FAQ)

What is adequate air flow? Apparently this refers to the inverter location. If you put it a shed or in direct sunlight, it could get too hot and the components would not be able to handle as much current. SMA does not define what "adequate airflow" means. My gut feel is that Akeena is right in saying there will not be a problem because the inverter would be put on the North side of the house where it gets no direct sunlight and is thus cooler, and it would not be in an enclosed space. If, after installation, we find a problem, we could add a fan that turns out when it gets hot. Apparently they did this for an installation in Gilroy in direct sunlight. Gilroy gets considerably warmer in the summer than it does here in Redwood City.

Carolyn Luce