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Well... no sooner did I fire off my last post
than I checked e-mail and found that Tim had done the legwork. I'd look
into further, but I don't expect to find that such standalone (no-grid)
systems are going to be substantially less expensive than the one Tim
found. Going strictly with an equivalent capacity calculation (that is, based on an apples-to-apples power supply capacity comparison), the arithmetic is pretty simple. The system cost is completely unreachable (for me) even without considering storage for no-sunshine periods, based on my existing 44kW grid-supplied capacity: 44kW ÷ 3kW = 14.67 = ~15 each 3kW systems...just to have full system capacity for part of the time when the sun is shining. (I say "part of the time" because I'm at ~35°N latitude, which means I'll never get the full incident solar power flux, even at the summer solstice.) That gives me little to no capacity when the sun is obstructed, which is... 3,094 hr/year ÷ 8,760 hr./year = ~35%... meaning that I'd only have access to my current system capacity approximately 35% of the time. I don't think that's going to be very convenient. In order to satisfy the criterion that the photovoltaic system be equivalent to my existing grid-supplied power capacity, I suppose I might as well do the full power supply capacity cost comparison, wherein I install enough collection & storage capacity to accommodate the no-sunshine periods: 125kW ÷ 3kW = 41.67 = ~42 each 3kW systemsHmmm... that could be a bit of a tough sell to my wife... I don't suppose there's a snowball's chance in (expletive deleted) that the 3kW systems include batteries. My real usage (28,500kWh/year) is approximately 3 times the usage that Tim calculated for a 10,000kWh/year home, so I would need to buy something like 120 batteries. The Trojan T-105 battery costs ~$80.00 + $16.00 shipping = $96.00; $96.00/battery x 120 batteries = $115,200 battery cost. $1,875,000 + $115,200 = $1,990,200 equipment cost...and that doesn't include the cost of system installation or the ongoing maintenance & operations costs, which are significant. The batteries require periodic replacement, and ongoing maintenance (adding distilled water), and they should be operated in a conditioned space for optimum performance and longevity. How significant are the battery costs? I saw one set of calculations done at Villanova University that estimated the energy storage cost at 14.8¢/kWh, which is more than I'm currently paying (~14.5¢/kWh) for grid-provided energy!! That means that the batteries alone are not a break-even proposition... and then there's that pesky $2 million system cost. Admittedly, there is some flexibility one way or the other in any of the numbers here, although most of the flexibility in the calcs thus far has overwhelmingly been in favor of the photovoltaic system. It seems pretty clear that no matter how you play with the numbers, they're just not in any kind of realistic ballpark for a person of my current means. Unless I've overlooked something significant in the calculations, it's pretty clear that a system of the kind described here is pretty much a rich-man's toy; IOW, the technology is not quite ready for prime time. Nothing is more revealing in that regard than the ROI calculation. Rather than go through the battery replacement frequency calcs, I'll just assume that... I dunno, somebody gives me the batteries for free everytime I need them, or something. That's probably not going to happen, but I'll assume the miraculous for simplicity's sake. As I said, my grid-provided energy cost is ~$0.145/kWh, which is about $4,100/year annualized electrical energy cost, so just take the initial system cost calculated above, and ignore the substantial system engineering, permitting, construction, and installation costs. How long would it take me to recover my investment based on my current annual energy cost? $1,990,200 ÷ $4,100/year = 485.4 yearsOK... now I see why it's important to focus on biology. We've gotta get those lifespan numbers up higher if we're going to be able to hang around long enough to realize payback on photovoltaics. ;-) All kidding aside, my calculations would have to be wrong (i.e., too high) by approximately two and one-half orders of magnitude to get the payback time down to a reasonable (~2-year) level. Even at two orders of magnitude (~5-year payback), I don't see that kind of potential error in the numbers. If anyone else does, I'll be receptive to hearing about it. Cheerth, Pete Tim Gwinn wrote:
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