“The meter’s running backwards!” It’s 1 p.m. on July 27, which means the sun is at its zenith, and the contractor has finally connected our solar system to the power grid for the first time. My wife Vicky is hypnotized, watching the black mark on the edge of the disc in our electricity meter move from right to left—moving pretty fast, truth be told.
This meter, the one that came with the house, is temporary. We’re signed up with the utility for “Schedule 7” rates. During the “middle” of the day, the cost of electricity will be around 28 cents per kWh. (The definition of “middle” depends on the season.) At other times, the rate will be around 4 cents per kWh.
We hope that when we’re putting power onto the grid, our electric bill will decrease by the larger figure. When we’re consuming grid power, much of it will be billed at the lesser rate. We still will be sending money to the utility every month, but we’re hoping it will be substantially less than it was before we got our solar system.
But capital and labor costs were pretty high. How long is the payback? I don’t know. Fifteen years is what’s bandied about. I figure there are too many unknown variables to say with any certainty. But if you’re interested, I can give you some basic numbers.
On the roof, there are 16 Kyocera KC190GT modules on a Pro Solar rail-mount system (Fig. 1). Solar panels are rated according to “PV USA Test Conditions” (PTC), which were developed at the Photovoltaic (PV) USA test site at the University of California, Davis.
The PTC rating refers to the output of a panel under conditions of 1000 W/m2 solar irradiance, 1.5 air mass, and 20ºC ambient temperature at 10 m above ground level and a wind speed of 1 m/s. Each module on my roof has a PTC rating of 167.7 W dc. The dc output of the total panel array is nominally 2683 W (under PTC conditions). Our array appears to be better than that, based on voltage and current measurements made by the inverter (Fig. 2).
That dc-ac inverter with the built-in measuring capability is on a wall on a second-floor deck. It’s a Fronius IG-series unit rated for 3 kW. The inverter’s nominal efficiency is 94%. The ac output from the inverter goes through an emergency cutoff switch located next to the house’s service entrance and goes on to a pair of breakers in the electrical panel.
The system’s estimated peak ac output is 2522 W. With good weather, it should produce 4500 kWh/year. If all of that output decreased our electric bill to the tune of 28 cents per kWh, that would amount to $1260/year.
So what did it cost? That’s where it starts to get tricky.
To mount the panels, we needed some plywood under the shingles. The brackets for the mounting rails attach to that. But the old roof needed replacing, not just under the relatively small area where the panels would be mounted, but everywhere. All told, the roofing work came to around $10,000. I might arbitrarily allocate $2000 of that to the solar system.
The contractor charged $24,600 for materials and labor, but we’re supposed to get a $7000 rebate from the utility. (I’m doing some rounding-off here.) So non-roof costs were $14,600. A county permit (we’re outside the city) cost around $750, and the contractor charged $250 for steering that through. (Permit charges for solar installations have been eliminated or reduced to trivial amounts all around the area, but not in San Mateo County.)
So our costs sum out around $17,600. If we actually reduced our utility bill by $1250 a year, payoff would be a little more than 14 years. Ask me in 2030.
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