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Residential Grid-tie FAQ

* How much roof space do I need?
Rule of them: One hundred square feet of roof equals 1 kilowatt capacity equals 1200 kilowatt-hours per year. (Really it's nowhere near that simple, but it'll get you started). We do free site reviews in the Missoula area to tell you what you can expect from your specific site/situation.

* How big are the panels?
Panels in 60-cell or 72-cell configurations are currently the most common types of panel designed for residential grid-tied systems. They are approximately 40" wide by 66" or 78" tall. They can be rated at anywhere from 250 to 350 watts.

* Do they need to face due south?
Not necessarily. There are losses in production when they face a different direction, but there are tools that allow us to estimate the impact. On a roof slope of about 20 to 30 degrees, you may lose up to around 10-15% by facing east or west if there are no other shading issues. The steeper the roof angle the larger the losses. Similarly, the further off from due south the larger the impact.

* What is the optimum tilt angle?
There are many theories, but the general consensus is that the ideal tilt for our latitude is about 37 degrees. We tend to find that there is a balance between cost and efficiency that is best met by mounting panels flush to a given rooftop to minimize extra installation costs. An array that is mounted at 20 degrees versus 35 degrees may only lose 5 to 10 percent production overall - a value that does not justify the extra (potential) costs of tilting the panels up away from the rooftop.

* How much power do I need?
Let us figure that out. We look at your power bill, which shows how much power you've used monthly over the past year. We then use several different tools to give you an estimate of what an array on your rooftop would be expected to produce yearly. We consider the direction it would be facing, the slope of the roof it would be mounted to, the number of panels and any shading from trees, chimneys, roof gables, plumbing vents and surrounding hills or mountains. We can compare this expected output to your usage and tell you how much of your consumption you may be able to offset.

* How much will it cost?
Currently, here in Missoula, our installation price has been between $2.50 to $2.85 per installed watt before incentives. This is an average cost - special mounting hardware, complicated wiring or pole-mounted arrays will change this number drastically.

* What incentives are available?
The federal government offers a tax credit for 30% of the cost of a qualifying system. See federal tax form 5695 for more information. This credit is available in full through 2019, then decreases over the following years. The state of Montana offers a tax credit of up to $500 per taxpayer, up to $1000 per household. Most system qualify for the full state credit. See state tax form ENRG-B for more information. This credit does not currently have an expiration date. The website has a good summary of both these incentives.

* What is the payback/return-on-investment period?
It all depends on the ratio between the installation cost and the rate the utility will pay you (in the form of credits) for your power. Currently, on a straightforward installation here in Missoula after tax credits have been factored in and using a fixed price per kWh of $0.115 we are seeing simple payback periods of around 12 to 14 years. If you assume a moderate increase per year in utility company raises power prices by a penny per kilowatt-hour, it takes about a year off the payback period.

* How much money can I make?
Currently, none. Most utility companies in Montana will pay you retail "price" for the power you produce in the form of credits per kilowatt-hour and let you use up those credits over the year. They will not pay you for any extra power you produce beyond what you've used in a year.

* Do I have power when the grid is down?
Not with a simple grid-tied system. To have back-up power available you need batteries. The inverter technologies are very different and battery-based inverters are significantly more expensive. There are also additional costs associated with rewiring of emergency loads from a typical electrical costs associated with rewiring of emergency loads from a typical electrical panel. We don't recommend this type of system unless you have frequent and prolonged power outages.

* Can this be a DIY project?
Sometimes. Be aware of all NEC2014 code requirements. All installations must pass an electrical inspection. In the City of Missoula and Missoula County to obtain an electrical permit for solar, a structural engineer must approve the structure to hold the weight of the panels. This is not necessarily the case in other cities or with State electrical permits.

All information is estimated and should not be taken as absolute. Every power system is different in design and production, equipment prices fluctuate, utility company policies change and I am fallible. This guide is meant as an overview only - it is up to you - the homeowner and customer - to verify and all information before beginning an installation.