# How can I cheaply evaluate how much solar energy I can collect?

I'm nearing the end of the life of my asphalt shingles on my house (the inspector estimated 2yrs left when I bought the house 2yrs ago). Since my house is 100% electric and my state (Nebraska) offers Net metering (but no other incentives), I'm considering the option of having solar shingles installed.

At this time I'm not considering any other solar collection devices (either shingles or nothing). My thinking is that I'm already going to have to endure the cost of the new roof and (AFAIK) solar shingles offer the same physical protection as asphalt shingles (aka, they are a direct replacement) with the added bonus of generating solar power. So, the money NOT spent on asphalt shingles would help offset the cost of the solar. Of course this only makes sense if the cost of solar shingles is less than the total cost of asphalt + solar panels (an assumption I have not yet verified).

I have a decent amount of un-shaded roof space available (probably more than I can afford to cover). It is a hip/hurricane style roof so part of it faces in all directions. I haven't taken measurements yet but my guess is ~400sqft facing directly south and another 1600 facing east or west (800 each way). Very little of my roof is facing North and that is where the little bit of shade is.

There are hundreds of calculators and estimators available on the internet but I would prefer to deal with some real world numbers.

What I would like to do is this: Put a small (maybe 1sqft) solar cell (or multiple...up to 5 depending on cost) on the roof and measure the amount of electricity generated at various locations on my roof. I could then extrapolate the surface area and efficiency of the devices vs shingles to get some real-world expected values. Ideally this would be attached to some sort of data collection device (I'm thinking arduino perhaps) so that I can measure over time (at least a month). Of course I would also like to measure this in the winter as well but I may be forced to replace my roof (and thus make the decision) before then.

-edit- Location is roughly Sioux City, IA (42.5000° N, 96.4003° W)

So, does any device like what I've described currently exist? Or am I going to have to piece this together myself if I want to do it?

• It would help if you put your location into your question or profile, as useful resources tend to be somewhat local. As one answer suggests Australian links but you are apparently in the US that would be handy to have up front.
– Móż
May 4, 2016 at 4:17
• @Mσᶎ good point. Updated May 4, 2016 at 18:58

A simple short term test like the one you are thinking about will reveal little information. Many factors affect the output of photovoltaic cells.

Results from a cloudy day will differ those for a sunny day. Also the angle of the sun relative to the horizontal changes throughout the year: low during winter and high during summer. The angle at which sunlight strikes a PV cell affects the amount of electricity the cell will generate. The pitch angle of your roof is fixed. Generally, PV cells are installed parallel to the roof.

Most PV cell rooftop installation have the cells installed at a fixed angle, so they rarely perform at maximum efficiency. Better results are obtain if the orientation of the PV cells can track the sun. This is expensive and why most PV cells are installed at a fixed angle.

• Tracking systems are also much heavier and catch the wind more, so they're not ideal for roof mounting. Apr 23, 2016 at 17:23
• @Fred Thanks for the feedback (and sorry for the delay responding). Upvoted because its good information even if it's not quite what I'm looking for. While not explicitly stated, I did mention the desire to measure during the winter as well so I understand the need for an extended test and I'm also aware that the angle of the sun changes during different times of year. I'll edit the OP to be a bit more specific. Apr 27, 2016 at 3:26

You are much better off using one of the "solar calculators", specifically one that uses actual data from installed systems. You might get lucky and find one of the tracking sites has an install near you that is very similar to what you want to do.

For our system I used a randomly searched calculator like this one at EnergyMatters, because those numbers are just physics and it really doesn't matter who produces them. Better ones (like that one) use weather and existing installs to improve their estimate.

I also used the online output tracking/sharing tool PVOutput to find other systems in our area and see whether they gave markedly different results. Once I used Google Maps to remove a couple of badly-sited systems the answer was no. There was one installed under a tree, and one or two with odd orientations (people are strangely reluctant to turn their houses to get better solar performance!)

Our system has performed as expected from the above, BTW.

• Thanks for the info (+1). Every solar link I click on at this stage gives me something new to consider. Unfortunately, I can't find a system within 100mi (160km) of my location using PVoutput while spot checking any decent sized city. I wish they had a better search tool. Unfortunately the EnergyMatters link doesn't give ANY details as to what they base their calculations on so I can only assume they are based on ideal installations of the systems they sell (which may or may not be relevant here in the US) May 2, 2016 at 14:21
• As a followup, I realized you did not actually back up your initial statement in your answer. More specifically, Why am I "much better off using one of the solar calculators"? Is it simply because they are Good Enough? May 2, 2016 at 16:19
• For the USA pvwatts.nrel.gov/pvwatts.php seems likely to be somewhat impartial and has at least got a caveat on the end about variability. I suspect Ⴖuі means that compared to spending money and waiting for two or more years worth of data to build up, any online calculator is likely to give you a free answer that's 90% as good as what you will actually measure. You also have the problem of finding a sample solar cell that matches the array you will buy in two (or more) years time. Better to buy now and if you care a lot about performance, measure what you buy and look for better later.
– Móż
May 4, 2016 at 4:18
• From my vague, remote knowledge of the US market I get the impression that buying name brand systems is much more important in the US than in Oz and Aotearoa because your consumer laws are weaker. But even here I suggest (and bought) the better gear rather than risk cheap stuff failing. In my case LG panels (premium) and a Xantrex (mid-tier) inverter, installed and warrantied by an experienced, well regarded company. Inverters last 5-10 years, panels 20+ so I spent my money there.
– Móż
May 4, 2016 at 4:24

Using PVWatts, as suggested by @Mσᶎ, really is going to be your best bet. NREL (the National Renewable Energy Lab) has been collecting solar irradiation data for years, and have a very rigorous method for scrubbing the data to make it generally applicable for regions surrounding the weather stations from which data is collected.

Sioux City seems to be swamped with weather stations:

PVWatts lets you trace the outline of the proposed panel over the satellite image of your rooftop, though you'd have to do it three times for the different angles you're working with.

User @Ⴖuі also noted using the tool by Energy Matters to plan a solar array, and that

[The] system has performed as expected.

The PVWatts system is designed for and used by solar installers and those seeking solar installations to plan and justify the cost of a solar installation system. You can add all necessary detail about the type of system you want to install, and the manner in which you will install it. It is the best you can do short of installing the actual system you're proposing.