Maine is a "net metering state." There are two meters on our house--one measures power "purchased" (in) from Central Maine Power Company and the other meter measures power "sold" (out) to CMP. If we export the same amount as we import, there is no charge (in theory). Also, these number reflect weekend usage of the house during the fall, winter and spring (all systems are running including the refrigerator) and full-time usage during holidays and summer.
This graph
represents our energy "in" and "out" from March of '95 to March of
'96. With a few exceptions, it is beginning to reflect the predicted
bell curve of available sunlight. In June of '95, for example, we
used 238 kWh from CMP and generated an excess of 303 kWh, leaving us
with a surplus of 65 kWh. At .025 cents/kWhr, that's a staggering
profit of $1.45 (they only pay us "avoided cost" on the net). During
the summer months, there are as many as 5 grandchildren and their 4
parents here, which means a lot of extra laundry use. As you can see,
July of '95 cost us money (.12 cents times 12 kWh or $1.44!). August
was a bear--a shortfall of 111 kWh! But we had fun with the 5
grandchildren and I gladly paid the $12.83 bill.
September saw us return to a surplus condition (44 kWh) even though the freezer section of the refrigerator was left open for a week! On July 5th, our 48vdc to 240vac inverter passed the 2 MWh mark! On October 1st, it passed 3.5 MWh. The inverter was down for a 6-week period in October and November, thus the dip in power production (it took two manufacturer-paid round trips to the west coast). The December numbers show an upswing in power production due to a healthy inverter and more sunny days. At the end of December it passed the 4 MWh level. February's numbers were puzzling. March was a winner! As we approached spring 1996, our inverter rocketed through the 5 MWh level.
The PV system has been up and running since March 3, 1995 but the first 30 days was a "test period" in which our output to the grid didn't count. |
The numbers on the left represent temperature, both outside air and water temperature inside the solar tanks. The day began with an outside temperature of 1 degree above zero, peaking at 28 by mid afternoon. The water temperature continued to increase until about 2pm (156F). The PV output is measured on the right, nearly reaching 4.0 kWh between 11am and 12 noon (peak output is calculated at 4.2 kWh). Quite an efficient day all around!