Photo-voltaic Power Generation System

Estimating annual electric power consumption should be easy: just look at the electric utility bills for a twelve month period.

For a number of reasons it wasn't that easy:

• Our electric utility, Potomac Edison (First Energy) often goes several months between meter readings, and their "estimated" readings can be wildly inaccurate.

• We recently (within the past year) replaced a vintage air conditioner and heat pump with two ground sourced heat pump systems. The existing oil furnace was (hopefully) relegated to backup status (the house also has three wood stoves, further complicating the heating picture). Preliminary measurements indicated that both heating and cooling the entire building year-round would consume less electricity than just cooling with the older air-source air conditioner, but without a full twelve months data it's difficult to know for sure.

• We recently replaced a conventional electric resistance hot water heater with a hot water heat pump (HWHP), and didn't have enough operational history to estimate the annual usage of the HWHP. Both ground sourced heat pumps also provide hot water via a "desuperheater" arrangement.

• An unusually large number of computer systems were powered on for extended periods for some one-time testing. For instance, one rack of mostly customer owned equipment was drawing 7.5 amps continuously for a period of several months, enough to really show up on the Potomac Edison bills.

• Spot testing of individual appliances showed that the three portable dehumidifiers that saw fairly frequent use were consuming a lot of electricity, more than the old hot water heater. With the new ground sourced heat pump running more or less continuously in the summer months no additional dehhumidification appears to be necessary. So far this summer (as of mid July) no portable dehumidifiers have been utilizied.

• A new wing was recently added to the house. It was far more heavily insulated than the rest of the building, and had top of the line energy efficient windows. A decision was made not to provide separate active cooling or heating, on the presumption that natural air exchange with the existing structure would be sufficient. As a precaution a radiant floor system was installed on the lower level.

  The downstairs room also has three ceiling fans. So far that presumption appears to have been correct and the radiant floor system has seen only occasional use.

In the year prior to specifying the PV system (2011) the total electric consumption was roughly 22,000 KWh. Looking at earlier years and attempting to adjust for the various factors noted above left us with a guesstimate of about 18,000 KWh per year.

During the process of quantifying past electric consumption levels we also were on the alert for any opportunities for additional convervation. Short of dramatic life style changes, that is -- we're neither masochists nor sufficiently noble to voluntarily self-inflict any significant hardships. Unfortunately we saw few opportunities for easy gains. The building has long since been converted to conventional and compact fluorescent lighting. we're reasonably well disciplined at turning off unused lights and appliances. The conventional air conditioner and heat pump have been replaced with ground sourced heat pumps and the conventional electric hot water heater with a hot water heat pump. In winter we have the capability of heating with wood, with three air-tight wood stoves and (most winters) an ample supply of firewood (note the issue of switching between wood stove and heat pump heating is a complex topic in its own right).

We have also invested a non-trivial amount of effort in improving the insulation of the building, including a whole-house leakage and thermal imaging test. The 70s vintage structure does have a fairly high level of air infiltration. Over a period of years significant additional fiberglass insulation has been added to the attic floors, unfortunately there are no non-disruptive options for improving the insulation in the 2x4 platform framed exterior walls which have (real) stucco siding.

Constant operation appliances -- a freezer, two refridgerators, radon fan, and some 7x24 computer gear -- were individually measured. A high efficiency server (SSD drives and Intel "Atom" processor) was acquired to handle some critical network services, but we concluded that the consumption levels of the freezers and refridgerators was not high enough to cost justify replacemment (a second freezer usually remains unused). Our measurements did show that the three portable dehumidifiers used an astonishing amount of electricity; one was replaced and we are making a conscious effort to minimize their use (so far this summer of 2012 none of the portable dehumidifiers have been used).