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Gall's Solar Home


This webpage provides some information on the solar cells on our roof. They work very well and we are happy with them. We got them installed in summer 2007. Later, we moved to a new house and bought a new system for the new house in spring 2012. This provides some interesting comparison. Our electric power meter runs sometimes forwards and sometimes backwards, depending on if the sun shines or not. Over the year, the system provides just about as much electricity as we need. This is true for both systems (the new house uses more electricity due to the pool pump). The electric portion of our power bill is $0. That is actually not fully true, we still pay the $16/month (as everyone else) for being connected to the grid.





The 3.78 kW system consists of 18 PV modules (210 W each) from SunPower and an inverter box in the garage. The array is 24x10 ft2 wide and covers about half of our roof that is facing south. It was installed by Renewable Power Systems, LLC which was handling all technical aspects as well as all paperwork. The system produces ~4400 kWh per year.

The 7.2 kW system consists of 30 PV modules (240 W each) from Canadian Solar and small Enphase inverters attached to the bottom of each panel. The array is 33x16 ft2 wide and covers most of the roof that is facing south. It was installed by Real Goods Solar which is actually the same company as Renewable Power Systems (see left), just after two mergers. The system produces ~7000 kWh per year (as it experiences ~17% shadow).


Total cost

(including all permit fees)


(including update of electric panel ~$1,200)


NYSERDA PV Incentive



($1.75/watt for 7kW)


Federal income tax credit



NY-State tax credit



Net cost:



Savings on electricity bills:

4,400 kWh x $0.15 = $660/year

7,000 kWh x $0.15 = $1,050/year


Our new (2012) system is almost twice as powerful (7.2/3.78 = 1.9 times larger), but was costing nearly the same, which is due to the steep decrease in the cost of solar cells. The overall energy production is only about 1.6 times larger for the 2012 system, though, because of some shadow. Nevertheless, the benefit per cost is larger for the newer larger system. Assuming a constant electricity-cost and no interest on the capital, it would take 18 years (2007 system) or 11 years (2012 system) to get even, and from then on we will make money. Naturally, it is somewhat naive to assume no interest and no increase in electricity cost. An alternative calculation is to consider the solar cells as an investment, for which we receive a tax-free 5.4% (2007 system) or an 8.8% (2012) interest every year. If the energy costs go up, the money gained is larger, if the system breaks (warranty on solar panels is 20 years), we will lose. Thus, there is a chance that these systems are financially a good deal, but this is not at all guaranteed. However, it is clear that it has a positive impact on reducing CO2 emissions and on conserving fossil fuels.

If you are interested in any aspect of our PV system, feel free to contact me at galld@rpi.edu, or to look at some other websites as those of my work colleagues Prof. Barb Cutler or Prof. David Borton, or at some solar energy sites like those of NY-State, a site summarizing tax-incentives in the USA, the site of the American Solar Energy Society, or the National Renewable Energy Laboratory.