Putting Power on the Grid

The Solar Harvest Net Meter Utility Interactive 1 KWHr Peak PV System

In the spring of 2000 the crew here at the home of The Harvest made the decision to go utility intertie. It was not a simple call.  First, there was the cost (we are a non-profit org) and then there was the philosophical issues to mull through. What we finally came to was that a net metered utility interactive PV system made sense for us.  Below you'll find the story in pictures and words of how we came to that conclusion and what we had to do to get the power meter running backwards.

Why? Reasonable question.  It turns out that the stand alone system could only produce all the home's needed power around 300 days a year after the owners had a child. The decision came down to; invest more in PV panels and/or a larger wind turbine or convert the existing PV array to utility intertie.  The economics heavily favor the intertie solution.

How? Wanting to keep their independence from grid outages, the owners converted their existing stand-alone system into a stand-by system.  Two 50 watt panels were added to trickle charge the nominal 24 volt battery bank.  The home has 3 to 5 days stand-by capacity in this configuration.  The existing 960 peak watt hour PV array was rewired from 24 volts to 48 volts and connected to the new Advanced Energy GC-1000 utility intertie inverter.

Permits and Agreement with the Utility. A standard State of Montana electrical permit was required for the work.  This cost $45 (US). The agreement with MPC, Montana Power Company, is basically their standard net metering agreement.  MPC was receptive to, and did include, several minor revisions that mostly clarified the system owners rights.  The permit was obtained in a single visit to the State Electrical Inspector's office. The agreement took 7 weeks but this was due as much to the requested revisions as any other cause.

Add on-s. Two things added to the entire effort; 1) the array was relocated back about 100' from the original location. This had two benefits - a) it made all the cable runs considerably shorter, and b) it allowed for a better use of the land in the future. 2) the existing power drop to the home, unused before the change, was relocated from the house to the shop building so that MPC personnel would have unhindered access to the disconnect without having to enter the yard proper.

The story:

The array is aligned to solar south.  In this case it faces 162 degrees (180 - 18 (the difference between magnetic and true south for this location) = 162).

The original stand-alone array with the new mounting frame being assembled in the background. The array produces 960 watt hours peak in full direct sun.

The commercial grid is adjacent to this property making a utility intertie installation possible.  The close availability of the grid was a significant factor in the decision to go net meter.

The frame mount is set 24" deep in concrete. Posts are 5" X 7" pressure treated cedar. Non-wood mounts are usually recommended for longevity but cost constraints and a desire to eventually install a tracking rack made pressure treated wood post acceptable.

The system owner is mounting the mild steel frame to the support posts. All of the power tools used on this project were powered by the owner's stand-alone PV system for a pollution free install.

The stand-alone array half disassembled. This array was wired for nominal 24 volts. The new array is wired for nominal 48 volts. This requires four panels in series. Careful matching of panels for peak wattage is necessary to ensure maximum production.

The array is ultimately mounted on this mild steel double H frame.  The frame provides the structural rigidity needed in windy conditions. The H frame was locally produced at a cost of $110 (US).

Here you can see the panels being relocated and matched for output. The distance between the two sites is about 100'.

The new array is half complete in this picture.  Note the gravel pad beneath the array - it serves to minimize grass and weed growth as well as provide for minimal snow build-up directly in front of the array.

The Advanced Energy CG-1000 utility intertie inverter wired to the customer supplied umbilical. The owner pre-wired as much of the system as possible so the final installation would be fairly quick to perform.

The unit purchased included the optional string combiner/DC disconnect breaker/GFI circuit. The lightning surge suppressor you see here is on the AC side and required (as well as supplied) for warranty coverage.

MPC's net metering agreement requires a lockable disconnect between the intertie inverter and the connection to the service meter.  Here you see a simple two pole, hot and neutral, AC air conditioner switch being used to meet this requirement.  Cost was $28 (US).

MPC staff change out the electric meter.  This home is the first in the Helena area to install a net metered system. Local Sr. meter man Blaine Palmquist discusses this new technology with MPC's renewable engineer David Ryan.

The meter had to be changed because MPC uses ERT type meters.  These typically will run backwards but ADD production to a customer's bill as usage.  A special version of the ERT meter is needed to accurately record both usage and production.

The final meter installation consisted of a new ERT type meter, a lockable disconnect switch, and a data logger to record instantaneous and long-term production of the system.

The GC-1000 installed and undergoing a half power test.

Once turned on, the system took about three minutes to sync to the grid and begin producing power. Operation is automagic including all the necessary safety features like anti-islanding to protect utility personnel.

A rear view of the new array.  The system produced 60.5 KWHr's in the first 30 days of operation.  This was below expectations but due entirely to weather conditions.

The former stand-alone system was converted to stand-by service with this small 100 watt hour peak array providing sufficient charge to overcome the small drain the inverter places on the batteries.

Lessons learned. Much has been said in the trades about how utility companies are being intentionally obstructive and hindering the advance of net metering and distributed generation. Perhaps in California and other states this is true - but not here in Helena Montana.  MPC staff were accessible, approachable, and forthcoming with information and suggestions. The State electrical inspectors office was equally helpful. All in all an experience that neither raised our tempers or tested our patience. We're on-line and producing clean electrical power that flows back into the grid.

Would we do anything differently? No, not really. The system is straight forward as far as design and construction go. The inverter is simple to install and requires zero effort on the owners part to operate. One of our best decisions was selecting the Advanced energy inverter for this effort.

Last word. Don't go guerilla. If you live in MPC territory you'll just get a bigger bill.  This isn't a difficult road to travel to get a legal hook-up.  Get a copy of the net metering agreement from our resource page, fill it in and mail it. You should hear back from MPC within two weeks.

Thanks to David Ryan of MPC, Daniel TwoEagles of Advanced Energy, and the crew here at The Solar Harvest for their generous help and contributions.

Look at the Resources page for links to more information and organizations that can help.