Thursday, July 31, 2014

Solar Power Finally Makes Sense--Even Without the Tax Credits and Deductions

Here's the spreadsheet:

This involves buying this 400 watt system, which includes a 1000 watt pure sine wave inverter and 30 amp charge controller, cables, and mounting brackets.  It does not include a battery, which I really don't need since the goal is not to become completely independent of the grid.  It would, however, provide enough power to keep the well pump and pressurization pump running at least part of the day.

My assumptions, based on my experiments with solar panels at my location over the last few years, are 3.5 hours of full sunlight in winter (we get snowstorms, but many days of clear blue skies that are below freezing), 7 hours in spring and fall, and 10 hours in summer.  I am also assuming that the current Idaho Power electric rates are not going to drop.  The Idaho income tax deduction is 40% of the cost first year, and 20% for the next three years.  Strictly speaking, treating this as 100% first year is wrong, but inflation isn't so high as to make this too far off.  I am assuming that the costs of having my neighborhood electrician install this to my breaker box are not going to much at all, and I will install the panels on the roof myself.  I am assuming that the system will net 100 watts per panel, even with inverter losses, because a number of buyers of this panel report actually measuring 120 watts per panel.

Notice that it pays for itself in less than two years, and even without the tax benefits, in less than three.  (Tax credits can hide a host of bad ideas, but in this case, it makes sense.)  It looks like it scales pretty linearly if I install more equivalents in future years.

The primary goal is to reduce dependence on the grid, but it at least seems to make sense as well.

UPDATE: The 400 watt system above includes a charge controller (which I don't really need).  I might get the cost down by $30 by purchasing the inverter, cables, and mounting brackets with the panels separately.

UPDATE 2: In case you are wondering why this kit includes a charge controller but no battery, I suspect that it is because enough deep discharge battery to store very much of this power would take the price way, way up there. It certainly makes it nice to be able to just add as much battery as makes sense for you.  This 28 aH battery would store 336 watts--less than an hour of output from these panels (assuming no losses in the wiring and perfect output from the panels).  The cost of enough battery to store several days energy production is as much or more than the 400 watt system, which is an argument against using batteries unless you are seriously interested in getting off the grid on a regular basis.

UPDATE 3: The panels alone are about $600.  This 400W pure sine wave inverter is $144.95.  With the cables and mounting brackets, this could be done for about $800, which knocks payback down to a year and a half.

UPDATE 4: I am waiting to talk to my electrician, but I suspect that I am going to need a 220V inverter to feed power into the bus on the panel.   To my surprise, while pure sine wave 220V inverters are rarer than 110V inverters, they are not dramatically more expensive in the same wattage range.

UPDATE 5: I just can't take seriously an inverter with eye candy in the picture.


  1. UPDATE 5: But ... but ... she's making COFFEE!

  2. I will be very interested in the outcome. Your system price seems very low - it looks to me like a grid-connected system usually costs about $4.50 per peak watt of which only $1 is for the panels. Yours is closer to $2.00 for the whole thing, with $1.5 for the panels. However, it appears that system installation may be a big part of that delta.

    It will also be interesting to see how much power (okay, energy) you actually get compared to your calculations.

    Do your calculations include payment for power fed back into the system (which is a big hidden subsidy in most states) or are you just accounting for solar substituted for grid power to you?

    I used a system design program for here in Arizona, where we have a lot more sunlight. My costs came out to about $5/peak-W for a big system (40kW). I didn't calculate subsidies - it wasn't even close to break even.

    Anyway, looking forward to seeing how this works out - please post as you learn more and get experience with it. Hopefully it will be a good deal.

  3. Do you have a bidirectional meter installed yet?

  4. Mauser: nothing is installed yet.

    StormCchaser: the system is not a grid-tie--strictly substituting power from the solar panels for power from the grid. Because I already have a transfer switch in place for the backup generator to prevent power from going back to the grid, I believe that my electrician can plug the output of the inverter directly into the bus on my electrical panel, with very little effort.

    Installation will be putting some brackets on the roof, running the cables, and drilling a hole. This doesn't seem very expensive, especially because much of this I can do my self.

    The cost of PV panels has dropped a lot.

    I don't see that for a small system like this that there is going to be a lot of running the meter backward, so that simplifies installation a good bit.

  5. So when are you going to put that Wind Turbine up like that guy off of I84 between Micron and Mountain Home used to have? He had it turning the meter backwards.

    FYI, he did have a heart attack climbing those tall towers though (he died) and BSU took it over for their engineering program after that. Been years since I've been over that way, but I think it may have been dismantled.

    He told us on an HP/IEEE engineering field trip how much he spent on those surplus turbines. I forget the exact amount, but I think it was several hundred thousand (he had family investors). He did have to rig special cranes and design towers to get them up in the air. He was self-taught (grew up as farm boy type) that was creative but did things no college educated EE or ME would ever do. It was a challenge to figure out his setup since it was so unorthodox.

    Maybe you could start a power plant co-op with the neighbors.

  6. Clayton, thanks for your response.

    I'm a little bit unclear on how your system will operate. Will you be disconnected from the grid, or will it simply reduce (perhaps to zero at times) the draw from the grid?

    Re: cost of PV panels. In my system costing, they were lower than what you have, and yet the total cost was a lot higher (per peak-watt). That's while I'm asking about details.

    And, of course, I'm curious in general. I am most interested in the implications of unsubsidized solar power. One cost, which I find hard to quantify, is the iimpact of the variation in load/generation by solar/wind systems. I have read that this requires utilities to run low efficiency "peaking" generators all the time in order to make up for sudden changes in load/input. I'm not a power systems engineer, so I don't know the details of that, but it's a factor in total societal costs/gains from these systems, and I'd like to know more.

    Another potential cost/savings is in the distribution lines. The cost (fixed and maintenance) are tied to the revenue those lines generate. If a lot of people on a line went to solar, then the line would be operating at a loss (which would be a subsidy to the solar folks at the cost of others). I don't have a quantitative feel for that either.

  7. I will be connected to the grid, but the goal is not to run the meter backward (I will seldom produce that much power relative to my load), but to reduce consumption of Idaho Power's electricity. A second benefit is that if Idaho Power were to go out for an extended period of time, I would still have enough power to pump water from well to tank and then through the house.

    The trick is that my system will have no batteries to it, and I already have a transfer switch that disconnects me from the grid when my house is producing power (as the backup generator already does). The cost of batteries is also a substantial part of most systems that are completely off the grid.

    You are correct that utilities have to have generators available to make up for unreliable sources, such as wind or solar, but that is because they need to supply energy to many consumers at once. If my solar has a cloudy day, it is likely a day that my air conditioning isn't running anyway.

  8. Thanks for the clarification.

    Does your transfer switch only cut in when your house is producing *enough* power? How does it sense that power is being produced. After all, an unloaded power system can have full voltage on it.

    Or maybe? The inverter in your system adds power to the grid power, and the switch cuts over when the net current draw from the utility is zero or negative?

    Anyway, this should be interesting. Unfortunately, here in AZ, the summer daytime power drain is huge (in my new house, I estimate about 8kW).

    As an aside, I'd love to find a numerical explanation of the backing generator issue, as it is hard to separate political info (i.e. skeptics of unreliable sources discuss this, but even though I tend to be on their side, I can't trust them without actual facts and engineering info).

  9. Wait, there's an inverter in those pictures? :-)