US EIA publishes PV output tilt data

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A clear majority of PV installed in the US state of California this year has been on fixed mount systems with east-west orientation more popular with commercial users, according to a new report from the US government Energy Information Administration (EIA).

The US EIA has published a study into the orientation, tilt and tracking of PV systems, using California’s installations so far this year as its reference point. It showed that 66% of systems over 10kW were on fixed mount systems, while an even more overwhelming majority, 99%, of systems under 10kW were fixed mount.

The EIA, the USA’s primary federal government authority on energy statistics and authority, used data taken from the California Solar Initiative (CSI), the state incentive programme which provides upfront funding for solar to customers of California’s three investor owned utilities. EIA looked at distributed, non-utility owned PV, as logged in CSI’s California Solar Statistics Database and assessed their expected output by running a simulation using the National Renewable Energy Laboratory’s PV Watts resource calculator.

The majority of EIA’s reported conclusions from the data are likely to already be well known to solar industry professionals. Additionally the value of the data, having been run through a simulation rather than being taken from a real-world data set, could be questioned. Additionally, the use of California statistics to draw conclusions for the national industry could be a questionable tactic. Major differences exist in US solar from region to region and state to state, as well as more nuanced ones.

Among the conclusions are that east-west orientation of PV can help balance time of use charges for commercial customers, while at residential level the more traditional south-facing PV system is likely to be favoured, due to net metering schemes commonly in place in the US.

EIA explains that since most systems under 10kW are for residential users on net metering, they are financially rewarded for their PV electricity per kilowatt sold back to the grid, whereas many commercial systems over that size are owned by businesses and organisations which pay time of use charges on their electric bills. In this latter instance, PV panels that are oriented toward the west will provide higher output in the late afternoon as the sun sets than south-facing, which meets the usual pattern of electricity demand peaking in the late afternoon or early evening.

This finding on east-west rooftops matches recent industry thinking on the subject, with US residential installer and leasing company SolarCity having launched ZS Peak, a mounting product specifically aimed at the east-west rooftop market. German mounting systems specialist Renusol, which just before summer this year was taken over by US company RBI Solar, is another that has recently been focusing on systems for east-west rooftops.

The EIA claimed that solar trackers are “still relatively rare”. Larger systems with time of use charges are again more likely to use these than smaller systems without. The EIA makes no reference to the cost of installing trackers and the need to balance this against the increased output and reduction of seasonality that tracking systems can provide to some extent. It did find that 30% of CSI’s installed systems this year over 10kW used single-axis trackers, while only 4% used dual-axis tracking or a combination of trackers and fixed mounting. The remainder of those systems, 66% did not use tracking. Of those systems that did use tracking, around two-thirds are tilted at between 11 degrees and 30 degrees to the sun.

The EIA also reported some fairly obvious conclusions on tilt, presumably for the benefit of those not as familiar with the subject as readers of PV Tech are likely to be. It said installers will choose the appropriate angle of tilt for a PV system to optimise overall or seasonal performance, tilted further south in the northern hemisphere, and tilted higher the further away from the equator the system is. Tilting also affects shading and visual impact of a solar system, while residential PV owners might be limited in their choices by the shape of their roofs.

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