Conventional wisdom says that if you put solar panels on your roof in the Northern Hemisphere, you should point them within 30 degrees of true south to generate the most energy in the course of a year. But a new study by Pecan Street Research Institute, an Austin, Texas-based research and development organization, suggests that most people may do better at covering their own electrical use and reducing stress on utility grids during summertime peak demand periods if they aim the panels westward instead. (See related quiz: What You Don’t Know About Solar Power.”)

Though the study has not been released publicly, Pecan Street provided a copy to National Geographic News. It analyzed 50 homes in the Austin area, looking not just at the amount of electricity generated by the panels, but at the impact on homeowners’ electricity use and the utility grid during periods of summer peak electrical demand, which generally occur between 3 p.m. and 7 p.m. (See related post: “Solar Researchers Dig Deeper Into Snow Issue.”)

The study found that west-facing solar systems reduced the amount of electricity that homeowners needed to draw from the grid during summer peak demand by 65 percent. Homes with the conventional true-south orientation, in comparison, achieved a 54 percent reduction.

Pecan Street chief executive and study co-author Brewster McCracken explained that west-oriented panels may be more effective for many homeowners because of their lifestyles.  West-oriented panels are more effective at capturing solar energy late in the afternoon, which also happens to be the time that most people get home from work and turn on their air conditioning in the summer, he said. So homeowners with panels facing in that direction are optimizing their solar power generation at precisely the time that their own usage surges, and when the utility company needs the most help coping with demand.

Additionally, McCracken said, using a west orientation for panels means that less electricity will go unused by the homeowners who generated it, and end up as surplus electricity being put back into the grid. That makes things easier for utility companies. “They’re not structured to handle high levels of electricity being sent back to the grid,” he said. “If they have to do that, it becomes a challenge to maintain electric reliability in an area.” (See related story: “New ‘Flexible’ Power Plants Sway to Keep Up With Renewables.”)

While solar panel installers have known that west-facing panels may be more effective for a while, utility companies still cling to the south-is-best rule-of-thumb, and refuse to give rebates to homeowners unless their panels are facing south, McCracken said.  He’s hoping that the study will prompt them to rethink their opposition to west-facing panels.

The Pecan Street study isn’t the first to show that west-facing panels may be more effective.  A California Public Utility Commission study published in 2011 also found that west-facing systems were “significantly better” at offsetting peak load that south-facing ones,  even though they had lower annual energy yields.

Solar Energy Industries Association spokesperson Samantha Page declined comment on the study.  A spokesman for Southern California Edison said that the utility continues to recommend a conventional south orientation for solar panels.

David Mooney, laboratory program manager for solar R&D portfolio at the National Renewable Energy Laboratory in Colorado, said that Pecan Street’s findings weren’t a surprise, but hoped that the more precise quantification would motivate both consumers and utilities to fine-tune their use of solar installations.

Mooney said that the benefits of tinkering with solar panel orientation would vary, depending upon the region, and that no single prescription necessarily fits all places.  “In Colorado, for example, we tend to have clouds rolling in off the mountains in the west in the summer, so west-facing arrays would produce the least electricity,” he said. “You would want to have a southern or eastern orientation here to get the most electricity, and that wouldn’t necessarily align with the utility company’s peak demand.”

Mooney also noted that the structure of the incentives provided by utility companies figures into how consumers chose to orient their panels. His own utility provider in Colorado, for example, offers incentives that reward optimum output over the year, rather than the amount of electricity generated during periods of peak demand. “I get no benefit from shaving their peak demand, so there’s no motivation to do it,” he said.

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Comments

  1. Tim
    Vegas
    June 19, 8:08 pm

    Whether your roof is facing west or south is fortunately becoming less of an issue nowadays.

    Much improved and cheaper mounting systems (like this one here: http://trasnowandsun.com/tra-snow-and-sun-develops-new-solar-mounting-design-for-east-west-facing-solar-mounting-project-in-las-vegas/).

    Previously the cost of installing solar panels in a different direction than the roofs sloop was prohibitively expensive, but not anymore. So, although the debate continues on South vs. West, at least homeowners have the option to choose now.

  2. Lorena Mitchell
    December 1, 2013, 11:15 am

    If you are OFF GRID face south 80 degree in snow climate as this will help with snow shed. Also Trackers are not a good Idea as they do give you 30% more power but Break down have low warranties and the cost of solar these days cheaper to add 30% more solar panels that have a 25year warranty vs 5 year average warranty for trackers. I will mention though that if you have space do a pole mount Seasonal tilt this will give you the most bang for your money.
    No disputing that a well placed solar system will always give more power but you have to see too how much is the cost vs the most optimal. I live off grid solar and love it. I have hundreds of off and on grid happy customers that took my advise. research is great we need to keep exploring the possibilities. What is needed the most is for people to understand this is not our planet that we are destroying we only rent it for the future generations people need to look at there consumption first. Solar can help with the rest. Have a solar day!

  3. Marc
    New York, NY
    November 22, 2013, 4:46 am

    Hello editors, National Science Foundation Graduate Research Fellow, Ph.D. Student at Columbia University and a former PV developer in NYC here. I would like to offer some clarifications with respect to your article.

    1. This research being trumpeted by Pecan Street as novel has literally been done hundreds of times over the past 20 years. Use google scholar or sciencedirect and the index terms “peak shaving”, “solar”, “PV”, etc. and you will quickly see that yes, the obvious fact that orienting your PV further west can help alleviate demand peaks in many cases as they are largely AC driven, which is heat-driven, whose peak lags the solar resource by 1-3 hours (it is heavily location-dependent.)

    2. In the united states, the vast majority of retail electricity customers do not pay a time-of-use rate, i.e. they do not pay more when demand is so high that it constrains supply and jacks up the price. Even if they did, the way most legislation for solar is set up around the country (and world) is a fixed remuneration rate based on a long-term power purchase agreement contract, a feed-in-tariff or a net-metering agreement. This is a fixed $/kWh rate that is unchanging, regardless of when electricity is pumped into the grid. Thus, the best they could do is offset their own demand at these peak price times (were they on that type of time-of-use contract) through self-consumption of their PV-generated electricity.

    Most PV system designers orient the array due-south and at a tilt-angle approximately equal to the site’s latitude. This is because over the course of a year, it will produce the most of any other orientation as it receives the most sunlight relative to all other orientations. As most customers with PV are remunerated based on a fixed contract linked to the amount of energy produced, and not what time it is produced at, this is the obvious choice.

    I’ll give an example. In NYC, the optimal orientation is 40.8 degrees tilt (the latitude), and due south. This will produce 1349 kWh/kW per year. At a net-metering rate of 14.5 c/kWh, that is equivalent to $195/kW/yr. If you tilted the array due west at 40.8 degrees, due west, you would produce only 1015 kWh/kW per year. At the same net-metering rate, this is equivalent to $147.18/kW/yr … nearly 25% less energy, nearly 25% less revenue and 3.3 year longer payback time @ 2$/W with no incentives compared to the due south optimal tilt case.

    Here’s another example, in Austin, TX, where this research institute performed their study. In Austin, the optimal orientation for PV is 30.3 degrees tilt, due south. This orientation will produce 1507kWh/kW per year. At a net metering rate of 9.7c/kWh (utility rates are lower in Austin than NYC), this is equivalent to $146/kW/yr . Were you to tilt your panels at the same tilt but due West, they would produce 1292 kWh/kW per year. At the 9.5c/kWh net-metering rate, this is only $125/kW/yr .. about 14.4% less energy, 14.4% less revenue and a 2.5 year longer payback time (given $2/W with no incentives) than the due south orientation.

    Here are some more westward orientations, their energy and cashflow produced (at different tilt angles) and their no-incentive payback time:
    Due West, 30.3 degree tilt, Austin: 1292 kWh/kW/yr | $125.32/kW/yr | payback w/o incentives: 15.96 yrs <–their chosen tilt
    Due West, 25 degree tilt, Austin: 1321 kWh/kW/yr | $128.14/kW/yr | payback w/o incentives: 15.61 yrs
    Due West, 20 degree tilt, Austin: 1343 kWh/kW/yr | $130.27/kW/yr | payback w/o incentives: 15.34 yrs
    Due West, 15 degree tilt, Austin: 1359 kWh/kW/yr | $131.82/kW/yr | payback w/o incentives: 15.17 yrs
    Due West, 10 degree tilt, Austin: 1369 kWh/kW/yr | $132.79/kW/yr | payback w/o incentives: 15.06 yrs
    Due West, 05 degree tilt, Austin: 1371 kWh/kW/yr | $132.99/kW/yr | payback w/o incentives: 15.04 yrs

    Compare it to Due South Tilt systems:
    Due South, 30.3 degree tilt, Austin: 1507 kWh/kW/yr | $146.18/kW/yr | payback w/o incentives: 13.68 yrs <–the best
    Due South, 25 degree tilt, Austin: 1507 kWh/kW/yr | $146.18/kW/yr | payback w/o incentives: 13.68 yrs
    Due South, 20 degree tilt, Austin: 1497 kWh/kW/yr | $145.21/kW/yr | payback w/o incentives: 13.77 yrs
    Due South, 15 degree tilt, Austin: 1478 kWh/kW/yr | $143.37/kW/yr | payback w/o incentives: 13.95 yrs
    Due South, 10 degree tilt, Austin: 1450 kWh/kW/yr | $140.65/kW/yr | payback w/o incentives: 14.22 yrs
    Due South, 05 degree tilt, Austin: 1413 kWh/kW/yr | $137.06/kW/yr | payback w/o incentives: 14.59 yrs

    Clearly, for Austin, Texas, any of the western-facing arrays will produce less than any south-facing orientation, no matter the tilt angle. The best (in terms of energy produced, and therefore expected cashflow and length of CapEx payback time) is clearly latitude tilt, due south. So I reiterate that in the vast majority of cases, where PV is remunerated at a fixed rate (PPA, FiT, or net-metering), Due south at the latitude is always best.

    Sources:
    * NREL NSRDB @ RReDC (National Renewable Energy Laboratory National Solar Radiation Database @ the Renewable Resource Data Center)
    * David F. Mennicucci, "Photovoltaic Array Simulation Models", Solar Cells, 18(1986) pp. 383-392.
    * Perez, R., Ineichen, R., Seals, J., Michalsky, J., & Stewart, R. (1990). Modeling Daylight Availability and Irradiance Components from Direct and Global Irradiance. Solar Energy , 44, 271-289.

  4. Daniel Phelan
    United States
    November 20, 2013, 7:32 pm

    It depends entirely on what you are doing with your photovoltaic power. If you are charging batteries then you most certainly want to face South. If you want the longest duration of nominal power generation, you want to face south. Facing West cuts the duration of nominal power generation by 30%. That is allot of missing production. If the value of your power changes over time and you are selling back to the grid at time of use rates, then it might be a good idea to face southwest, in order to produce power when it is most expensive.

    Unfortunately, most homeowners don’t have the luxury of deciding exactly what direction the module will face as most home owners are installing on their fixed roof tops. If anyone has the opportunity to pole mount their PV, they should most certainly consider a one or two axis tracker as you can extend the nominal power output by 25-35% depending on your latitude.