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The Reality of Home Solar Power

A community group in my area developed a bulk purchasing program for home solar energy. As a performance engineer, I’m particularly drawn to efficiency. Solar panels themselves are not that efficient. But my interest was piqued after listening to a podcast on DC power. The podcast was a “geek out” on a show called DotNetRocks. You can listen to it here.

What particularly excited me was the discussion on DC lighting. LED lights take DC power and its pretty inefficient to have each LED light bulb with an AC to DC converter built in. Then there is the 110 volt AC coursing through the walls that is dangerous to setup if you’re not an electrician. Then you have things like dimmer switches that would be a lot more efficient if they were using DC power. This video from LumenCache sums it up nicely:

DC lighting in the home with solar power going to a home battery like the Tesla Powerwall? Sounds good to me. Before you rush out to buy solar panels and CAT5 there are some realities you should be aware of.

Solar panels only make sense when subsidized

If you buy solar for your home, you can claim one third of the cost to install on your taxes. But you can only do it once. Without this, it would take too long to make your money back. Germany enacted a policy of heavily subsidizing solar panels nationwide, but that policy has since backfired. Do I have the same concern about the US? No. And even if I did, I’m not going to stop from buying solar panels because I think it might hurt the economy. The worry I really have is that subsidies will not always be there.

In Washington state, you can receive up to $0.54 per kilowatt hour generated by your home solar installation. This is on top of what the power company will pay you. However, this is only in effect through June 2020. With the problems in Germany, it’s unlikely that the policy will be extended. Also, in order to get the $0.54, you need to choose all materials built in Washington state. It’s a fair trade, but Chinese-built materials are cheaper.

Solar panels need direct light to be effective

Yes, you can generate electricity on an overcast day. It’s much less than you get from direct sunlight, but not bad. But even on a perfect, cloudless day like we just had in Seattle, your actual generation is limited to about 3 hours when the sun is at its apogee. A picture taken from my inverter will make this clearer:

Graph of solar power generation from inverter

I have a 3.8 kW inverter. My panels can generate a little bit more at peak, which is why it’s flat on the graph. The installer said the inverter can handle this and so far that seems true. 3867 Wp means I peaked at 3867 Watts. You can also see the curve based on the position of the sun. It would look like a nice perfect curve if not for some pesky trees. The installer estimated that cuts about 15% of my potential generation. The 12.63 USD is a calculation based on $0.54 per kWh.

If I lived closer to the equator and could get this year round, that would be nice. But we have to balance good days like these with the winter season where I’m more likely to get only 8 kWh.

Partial shadows kill your generation

This comes down to a choice you make when installing. You can get one inverter for the whole panel array or you can get micro-inverters. Micro-inverters are more expensive, require more maintenance, and have a higher rate of failure because they’re more exposed to the elements. The benefit though is that if some panels are covered in shadow, you lose less power.

With one inverter, a shade on one panel brings power generation on all panels down to its level. The reason for this is hard for me to explain. But if you’re interested, I think Richard Campbell has a pretty good description in this podcast.

You still will lose power when everyone else does

Even if your panels are generating electricity, if a power line goes down in your neighborhood, you will still be without power. This is because your solar panels are putting power back into the grid. A safety mechanism will prevent power from feeding back through the meter when it detects no power coming from the grid. It’s a safety mechanism to make sure the utilities workers attempting to fix the downed power lines are not electrocuted by what’s coming from your panels.

You are financially discouraged from going off-grid

If you decide to install a battery to store your solar power for use when the sun is not out, you will not get any subsidies, which pay way better than the offset to your electric bill. Plus, with a bulk buy program like I used, the installer will not do batteries. They have a contract with the power company. If you get solar panels, you essentially work for the power company!

Installation is expensive

Installing solar panels is an equivalent amount of money to a good kitchen remodel. If you intend to sell your home in the next few years, you’ll get way more value from a kitchen or bathroom remodel than from solar. My community program also partnered with banks to provide financing. I think if you have to finance it, you probably shouldn’t do it as the interest will delay your return on investment.

So, should I buy solar panels?

It is possible to get return on your investment. With one third paid through a tax break and five years worth of $0.54 extra per kWh, I may be able to break even in five years. Beyond that is gravy. And it raises the value of my home. So for my situation it makes sense. If you have a south-facing roof with a clear line of sight, subsidies, a reduction in installation cost due to a bulk-buy program, the cash to front the bill, and will stay in your home for a few years, then I think it’s a solid investment. If any of that doesn’t apply to you, consider mutual funds. :)