Monday, January 28, 2013

Strategies for Using a Photovoltaic System to Achieve Net Zero

We have designed a 1400 square foot 2 bedroom house to build near Platt Park. The levels of energy efficiency are comparable to Passivhaus standards, but our main goal is net zero energy cost.  The only way to achieve this goal is with renewable energy. A photovoltaic (PV) system is the best choice.

Now, we could make some reasonable assumptions about projected energy usage for the house in order to size the PV system.  In reality, it's impossible to predict the actual usage for a house, so let's look at the whole problem for other clues to size the PV system:

1.  The roof size places a practical limit on the system size based on the actual roof size and how much photovoltaics will physically fit on it.

2.  In new construction, Xcel Energy limits the size of your proposed system based on how much energy they think you will use.  They make a very primitive and arbitrary guesstimate, but the good news is that they will let you oversize the system and pay you more than retail for all the energy you collect and put on the grid. Since Xcel pays you retail price for the energy you generate, they get to say what the maximum size is.

3.  For an existing home, Xcel will let you size the system based on your last 12 month's actual energy usage.  They will let you build it 20% larger than that if you really want to (because you're SURE to get an electric car here in the next two years, and you need the extra capacity).  For most homes, the roof will be nowhere large enough to provide all your energy needs with PV

Xcel used to pay a one-time cash rebate on your system as soon as it was installed.  Nowadays, the rebate system is performance-based. Your contract with Xcel simply obligates them to pay you, as the system owner, $0.13 per kwh generated, up to the contract maximum per year for ten years.  The contract maximum is also the expected annual output of your system.  This strategy protects Xcel because if your system doesn't perform as expected, they don't have to pay.  It also gives you a big incentive to get your system functioning properly.

By entering into a typical power purchase agreement (PPA) with a solar leasing company such as Sun Run, you can get in the game immediately with no cash out of pocket