Vaisala Analysis Highlights Importance of Grid for Solar + Storage Systems

Solar panels on a utility-scale solar project
Naomi Stringfield
Nov 5, 2015
Renewable Energy

Solar performance analysis shows solar + storage systems risk increased power outages even in sunny San Antonio, TX

In the past few years, as new storage technologies have been introduced and become more cost competitive, there has been debate about whether residential and commercial customers will still need the U.S. electricity grid in the future if they can power their homes and businesses with energy from on-site solar PV systems backed up by batteries.

Recent research conducted by Vaisala, which was recently covered by Greentech Media​​​, analyzing solar resource inter-annual variability across the U.S. suggests that even with storage, the grid will still continue to play a vital role in supplying reliable electricity.

Solar performance map

The study examines monthly solar resource variations during 2014-2015 at locations across the U.S. and places them into a long-term context. This reveals significant deviations from long-term average solar irradiance conditions, notably in Texas and the surrounding area where solar resources were 10% or more below average in the first two financial quarters of 2015.

Seeing this low anomaly, the Vaisala solar research team conducted an analysis of a hypothetical off-grid solar system located in San Antonio, TX. The home's annual energy load was based on a 2009 Texas residential energy consumption survey conducted by the U.S. E.I.A. (Energy Information Administration). The system was sized using the simulation tool PVsyst, with design criteria of 3 days of autonomy and zero outages during a typical meteorological year, the industry standard for assessing PV systems.

The resulting PV system consists of a 15.1 kWp array with a 96 V, 1480 Ah (142 kWh) battery bank and appropriate balance-of-system components. This is as large an array as can reasonably be installed on a typical home, and the battery bank is larger than what is currently being sold by manufacturers like Tesla and others for home use.

However, after analyzing a year of theoretical system operation from August 2014 through July 2015, factoring in actual weather conditions experienced during that time period, Vaisala's study shows that a prolonged outage would have occurred on March 3 to 5, 2015, lasting about one and a half days (37 hours) due to heavy fog conditions in the area.

Outage graph

In order to determine whether or not such outages would be common for this system, the Vaisala team extended its analysis to the full 1997-2015 period for which Vaisala's satellite solar resource data is available at this location. The upshot is that such outages would actually be fairly common for this system: on average, the system would experience over two days of outages per year, with the longest outage in December 2011 lasting more than five days (133 hours).

"The main U.S. electricity grid typically achieves outages on an average of 1 day per every 10 years. That kind of reliability is difficult to beat," said Gwendalyn Bender, Product Manager for Assessment Services at Vaisala. "This can be compared to about 23 days of outages per 10 years for the simulated off-grid system in San Antonio."

Ms. Bender added, "Given the results of this study, going off-grid completely is clearly not a simple matter. While the outages described above depend on system sizing, increasing system size means increasing costs as well wasted energy, not to mention possibly running out of roof space with good solar exposure. Already, the system described above wastes 43% of the energy that it could produce when battery banks are full and cannot absorb the output of the PV arrays."

"Instead of going off-grid, residential and commercial consumers can use the grid as a virtual battery along with their own, adding flexibility to the electricity system and thereby making it easier for others to connect renewable energy systems to it." ​​​

This was also illustrated in a recent report from the Rocky Mountain Institute looking at the economics of battery energy storage. The report highlights ways in which grid-connected batteries can be used to provide several services at the same time – for instance increasing the amount of PV power that is consumed on-site, while also shaving peak demand and shifting consumption to off-peak hours where electricity prices may be lower.  

Gwendalyn Bender

Gwendalyn Bender

Gwendalyn Bender
Solar Product and Production Manager

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