Question

Can solar work on a northeast-facing house?

Asked by Valerie, Zionsville, Pa.

We are about to do a large remodel to our existing small house and I have two questions. First, our house sits with the existing front at a northeast angle, so unless our addition is placed at an odd angle it won't have a long south axis. Is it still cost-effective to add solar roof panels, and if so, in which direction should they face? Also, should we open the southeast and southwest corner up with windows for passive solar heating, or is this only efficient on truly south-facing walls?

Answer

Mick Dalrymple

Answered by Mick Dalrymple

Scottsdale, AZ

Eco-Friendly Building Center

May 22, 2008

Your house sits in that grey area where, while solar photovoltaics are great, their effectiveness is going to be reduced a bit, burdening the investment model. In order to get you a concrete idea of the impact, I asked Nathan, our design director, to take a look at your situation. As usual, he came back with the following precise and thorough, stellar response:
  1. Great questions! To answer the first, we have to account for several variables – your home’s latitude, the roof’s orientation in relation to true south, the roof’s slope, the percentage of shade the roof receives throughout the day, the cost of electricity in your area, and the size and efficiency of the system you plan to install. So, I set about applying numbers to these variables.

    I found Zionsville, Pa., to be roughly 40.6 degrees north latitude, and the average electricity cost in Zionsville is 9.74 cents/kWh. I then assumed: the house is 45 degrees from true south; the roof’s angle matches the local latitude – 40.6 degrees (the recommended panel tilt for your area); the roof is 0% shaded; the system size is 3 kW; and the Derate Factor (system efficiency) is .77. Using a program called PVwatts v2 by the National Renewable Energy Laboratory (NREL), I found that the annual power production of a 45-degree southwest-facing solar PV array is roughly 93% of that produced by a true south-facing array. Likewise, a 45-degree southeast-facing array will produce roughly 92% of a south-facing array. While a 7-8% reduction in production may seem relatively minor, it does add up over time and will stretch the effective payback period for the solar PV system.

    However, if you plan to live in your house for a while, and you take advantage of all of the federal, state and utility incentives, you will enjoy renewable energy and insurance against rising utility rates for years to come. I would suggest speaking with a couple of your local solar installers to have them look at the existing house and the future plans. They will help you map out the size and cost of the system, the applicable incentives and rebates, and the expected payback period. They may also have a few tricks up their sleeves to help reduce the production losses.

  2. The answer to your second question is much simpler: Yes. In Arizona, where I’m located, we do everything in our power to avoid windows facing west and southwest. We are much more concerned with solar shading and passive cooling than we are with passive solar heating. In Pennsylvania, however, passive heating is a good thing for much of the year. Allowing direct sunlight into the home is the primary goal during the winter months, so it doesn’t matter which direction the windows face as long as the sun can get in. Having said that, you’ll still want to shade the windows during the warm summer months.

    Architectural options for shading the southeast and southwest facades are pretty much limited to roof overhangs and awnings. Another very effective, affordable solution is to strategically plant a couple of shady deciduous trees in your yard so they block the sun in the summer, but let it in during the winter after they lose their leaves. I hope these answers help. Good luck with the remodel and the new solar PV system.

You can see why we love having Nathan on our team. The only additional suggestions his answer inspired are: 1. Ask the solar installer if he can build an assymetrical rack mounting system that can skew the panels slightly more to true south. Aesthetics and the cost/benefit of the racking system will have to be factored into your decision. 2. If you decide to plant deciduous trees, make sure that they will not eventually grow to a size (or location) where they will create shade on any of the solar panels. Shade on any panel will seriously reduce the output of the entire row of panels (any panels connected in series with the shaded panel). Thanks for thinking solar!

Tagged In: passive solar, tree shade

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