Energy efficiency, photovoltaics & near zero-energy homes

As the sixth report in its 'Building America Best Practice Series', the US Department of Energy published a guide on using solar thermal and PV systems (attention: large pdf file!) in combination with good insulation as a best practice for zero-energy homes.

The term 'zero-energy home' is used loosely in the report to mean houses with at least 50% reduced utility bills compared to standard practice.

The combination of energy efficiency with solar is cost effective compared to conventional homes as the reduction in energy bills is larger than the increased mortgage payments from the additional investment. This is a bit misleading though, since insulation will produce the largest part of the cost savings, whereas solar consumes most of the additional investment. Still, it may make sense to a home owner to use part of the cost savings from good insulation to help finance solar technologies.

To the builder, near zero-energy homes are premium homes. They offer the benefits of being more attractive to buyers, and may command a price premium. Especially for the high-end of the housing market, the additional cost of going solar is relatively modest.

To home owners, these homes are premium homes as well - they provide a hedge against raising electricity prices. The combination of the four riders of reduced energy bills, increased environmental performance, possible increased value of property and solar subsidies provides a powerful incentive.

However, solar systems require capital, which is often in short supply when building a house, but much less so 5-10 years later. Therefore, the guide offers a checklist to make homes solar-ready for later installation:

  • Design the orientation and pitch of the southernmost facing roof to maximise solar gain, although there is a great deal of flexibility in both the direction and angle.
  • Design the roof vents, chimneys, gables or other obstructions to sit to the north side of the planned array. Do not shade the part of the roof where a potential PV array or solar thermal collector could be installed.
  • Ensure that the roof structure is strong enough. Design structural support into the roof to handle the weight of a rack-mounted system.
  • Design space for inverters and disconnects near the main service panel.
  • Make sure the main service panel has space to handle a power input breaker.
  • Pre-wire or install an empty metal conduit from the roof to near the main service panel to handle wires from the future array to the future inverter. A 1” conduit would support most residential systems. A conduit will also be needed from the inverter location to the main service panel. If you are preparing a very large house, you may need to run two or more conduits from the roof.
  • Provide enough room in the breaker box for a double-pole 30 A breaker (solar electric feed).
  • Provide a vertical wall area to mount an inverter in the mechanical area of the house.
  • Minimise the distance (wire run) from the array to the inverter.
  • Install an electric disconnect switch for a potential future solar electric system.
  • If stand-off mounts or racks are needed, install them before the final roofing material is installed to ensure proper flashing.
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