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Tested: Living with Five Years of Solar Power

By Loyd Case

We had 27 photovoltaic panels installed on our roof in 2008. But generating electricity is only part of the power consumption equation.

In 2008, 27 Sunpower 225W solar panels were installed on our roof, capable of generating 6KW of electricity. Actual power distribution after inverter losses is roughly 5KW. Back in 2008, the entire installation cost about $37,000 after rebates and tax credits. Of course, I wrote up the process and rationale for installing solar. I followed that up with a one year update on how the installation was running, and overall cost savings.

27 of these panels are on my roof now, generating power every time the sun shines.

Today, my solar installation has been humming along for five years. Other than occasionally climbing on the roof and hosing down the panels, the system has required zero maintenance. That first year, our power bill for the entire year was $1,461. However, my power bill for the 2011-2012 year was $1,020. And for the 2012-2013 year, my power bill was just $654.

What happened? Clearly, solar power wasn’t solely responsible for the steep decline in my power bill. When it comes to energy usage, power generation is only part of the equation. Other technologies and changes in how we use power are also key factors. But first, let’s take a look at the numbers for the solar installation itself.

Five Years and Going Strong

The Sunpower panels used in my installation are warrantied for 20 years. However, that warranty specified a maximum decline in output over that time–something like a 10% decline in per-panel generation over ten years, and a lower rate of decline after that. When the system was first installed, I would religiously climb on the roof once a month and hose the panels down, since the inevitable layer of dust would cut down on power generation. However, I noticed that normal dust accumulation really had a minimal impact, though ash from grass fires in the Santa Cruz mountains one year cut power generation by nearly 20%.

A little dust doesn’t matter, but maybe it’s time to hose down the panels again.

The greatest loss in efficiency comes at the inverter. The inverter is capable of handling up to 7KW, and is rated at roughly 90% efficiency. Think of this as the giant sized version of the inverter you might use in your car to power your laptop.

That's a lot of capacitors...

The other factor in solar power generation is weather. The winter following the installation was a little wetter and cloudier than average, so power generation dropped substantially.

Looks like the 2009-2010 year was pretty cloudy.

The drop isn't as steep as it looks, since the Y-axis starts at 7,400KWh. It’s really less than a 9% drop. It’s recovered a bit, but still hasn’t hit the lofty levels of 2008-2009, which was a very dry, sunny year. Of course, some of that lack of recovery may simply be due to not hosing down the panels regularly, as I did that first year…

What about the falloff in power generation as the panels age? One way to look at this is to chart the biggest power generation day of the year, over multiple years.

Over a five year span, the highest power generation day of the year decreased over time.

I’m guessing that the overall power generation will stabilize over time, but this also suggests that maybe I should hose down the panels more frequently. Since it’s been raining less in the past couple of years, there’s likely been greater dust accumulation on the panels.

Note that we have no local storage of power–no big batteries or capacitors. If we generate more power than we use, that excess is fed back into the grid, and we get credited by PG&E, our local utility. Last year, we had two months of negative power generation.

Given that the overall power generation of the array has declined a bit over time, why has my power bill dropped so precipitously?

Technology, Habits, and Power Use

After that first year, where our annual bill was roughly $1,400, our power bill settled down to about $1,250 for the next two years. Then it dropped even more, first to $1,050 and then to $667. What happened?

In the winter of 2011, we remodeled our kitchen. Prior to the remodel, our 45-year old kitchen had an old-style electric range, which used metal coils heated up by passing electrons through them. It was a slow and inefficient cooking process at best. The kitchen and dining area used a total of 750W for halogen lighting.

The new kitchen uses LED floodlights and an induction range for cooking. The induction range is substantially more efficient in its electricity use, and that efficiency is made even better by how fast food heats. A total of twelve LED floodlights supply lighting in the kitchen and dining area, consuming a total of 156W (versus 750W previously.). If we add in the 20W for under-counter lighting, overall power consumption of for lighting is still under 200W. I’ve also replaced the R20 can lights throughout the house with LED equivalents and updated the home office with lamps that either use CF (compact fluorescent) or LED bulbs.

Those are the technology changes. But living habits have changed as well. In Fall of 2009, our oldest daughter headed off to college and last year, our younger daughter left home for the shores of higher education as well. I’m also not running simultaneous benchmarks on four high end gaming PCs much these days, either.

So a combination of life changes, changes in habit and upgrades to more power efficient technology all contributed to using less overall power.

What About Today?

Since our original solar installation in 2008, the price of photovoltaic panels has plummeted, partly due to competition from low cost Chinese manufacturing sources. Today, you can buy an 8,000W solar power kit from Home Depot for under $19,000–including the inverters. Also, changes in financing have had a revolutionary impact. Companies like Solar City now install photovoltaic panels for free, and you pay them by the month, as you would any utility – but you’re paying less than you would to your local utility company. So you can buy or lease your installation.

Given the precipitous decline in prices, it’s easier to justify a solar power installation these days, if you live in an area that gets a reasonable amount of sunshine. In the end, though, installing an alternative power generation system is only half the battle. If your lighting or other power consuming tech inside your home is inefficient, you won’t get back nearly as much as you might with a few other upgrades.

In the end, changing how you use power plus upgrading to a solar power installation will have the biggest impact on your power bill. The good feeling that comes with that is a bonus.