First Full Month of Solar

As April ends, I can now assess the results of our first full month of having solar power. Specifically, I was very interested to see if my projections for solar production were even close. If you remember, I wrote this blog entry back in October 2009 as I was trying to determine how big a system to get, what sort of output I could expect and what my savings would be. I created this Google Docs spreadsheet to help with all these calculations.

For the month of April, the spreadsheet predicted for the 3.68kW system we got, production of 513 kWH (this actually comes from the PVWatt tool mentioned in the spreadsheet). It didn't look like we'd get there as the 27th and 28th were overcast and rainy days, but the last 2 days of April turned in the highest production so far. Actual production came in at 538 kWH. Based on this, it looks like the analysis I did in the spreadsheet turned out very accurate and as a result, I'm very pleased with our purchase so far.

Overcast Day Has Greater Solar Intensity???

It seems counter-intuitive, yet on a cloudless, sunny day, our panels peaked at a lower electricity output than on a day with overcast. In fact, on the cloudy day, our panels maxed out the inverter, causing it to clip. See the graph below where I combined the charts of 2 different days.



The red line shows the nice production curve on a cloudless day, but the green line shows the output on a cloudy day. The green line maxed out earlier in the day and even later in the day than the red line.

The sunlight hitting earth must be reflecting back up to the sky, but the overcast is causing it to reflect back down again. This results in more solar energy hitting the panels. I would assume that the glass over the panels is like this as well - allowing the sunlight to pass, and some light that reflects off the panels would get reflected back to the panels by the glass.

Solar Array Powered Up and Some Graphs

The installer powered on the solar array on Tuesday after the inspection. After he left, I installed current sensors for the solar breakers and ran those along Cat-5 back to the ECM-1240. I was debugging the charting and got it pretty much sorted out by Wednesday night. Here's the first few RRDTool graphs from Thursday.


You can see it generated nearly 21 kWH yesterday. I'm really happy with that as we typically use 22-25 kWH/day. I didn't expect to get that much production in March. Does this mean the assumptions I made in my Solar Spreadsheet may have been a little pessimistic? There's still a lot of rain & overcast in March (of which we're seeing today), so it will probably average out. We'll have to wait for April to get a good full month to compare.


With a southeast facing roof, you can see we catch the morning sun very early. The array woke up around 6:22AM and peaked around 11AM. Once the sun passed over the top of the roof around noon, production started to drop and around 2PM it started to decline rapidly. By then, it produced about 80% of the total for the day. I wish I had room on the west facing roof to catch the afternoon sun, but it would only fit maybe 4 panels - good for another 1kW. Maybe when we get an EV ;) Production stopped almost exactly at 6PM. You can see some of the spikes in production as the sun was behind some clouds.


This is just a net usage graph. Solar production for the day caught up to our daily usage by noon. Between 3-4PM, production fell below our usage and about 9PM, all our credits were used up. For yesterday, we basically paid the utility for electricity for the 3 hours from 9PM-12AM. Pretty cool.

Solar Panels Installed!

Here is this final product - a Sunny Boy inverter and 16 230W Sharp panels. Installation took only 1 1/2 days. It's all ready to go, just waiting on the utility to swap out the meter and for the city inspector.

Solar Contract Signed & ECM-1240 Ordered

We signed the contract a couple days ago with Real Goods Solar (Regrid Power) for a 3.68kW system. Projected install date is late February and power on in March. Might as well have it come online when there's more sunlight.

I also ordered the ECM-1240 power monitor from Brultech. I decided to put current transformers on the lines from the solar array, but I will have to extend those via Cat-5. They should be able to go 100 feet, but it looks like it will be very, very close to 100 feet or possibly more. Hopefully, it will work. I've already opened up a hole in the wall next to the sub-panel, installed a mudring and drilled through the stud next to the subpanel so I can pass CT wires from the subpanel to the mudring. I also ran a couple Cat-5 wires through the wall behind the subpanel to a closet in the adjacent bedroom. I'll need those to connect to the Cat-5 drops in that room to extend the solar array's CTs and possibly get the ECM-1240 on the network via a Lantronix MSS-100. With luck, it'll get here before the Thanksgiving break so I'll have time to work on it.

Cleaning Solar Panels Uneccesary?

Yup, I'm still on my solar tangent. Getting close to deciding on a contractor, but not yet. I ran across this Google study on their solar installation. Their study shows that tilted panels, like virtually all residential rooftop mounted systems, don't need to be regularly washed. Rain does a good job of cleaning off the dust. This would discount SunRun's solar panel washing benefit. Panels that are installed horizontally do benefit greatly from a cleaning, however. To see more details on the Google study, check out their slides.

Solar Spreadsheet

A lot of the solar quotes I've gotten will take your current usage to show a cost-benefit analysis. What I want to see is what happens to that benefit when my usage changes. When sizing our prospective array, I want to see how much more electricity I can use before the costs really blow up. Ideally, I want to keep grid usage in the lowest 3 tiers. So I created this spreadsheet to help with planning.

It's based in Google Docs. It's read-only but if you go to File->Make a Copy, you can save yourself an editable copy. There is a 2nd sheet in the spreadsheet called 'info' that contains instructions on how to get the data for your specific scenarios. The information is based on California's PG&E standard residential service. You can adjust the formulas, tiers, rates, etc. based on your situation. Also note that many solar households go to the time of use rate schedule instead of the standard schedule, so that will affect calculations. For simplicity, I've chosen the standard schedule. Let me know if I've mucked anything up.

Ladders & Solar Financing

Our home is two stories and you need at least a 15' ladder to reach the lowest part of the roof. So far, of the 2 companies that have done a roof survey, both of them have showed up with a 13' adjustable ladder. When the first guy visited, I saw him pull out his little ladder and I figured it wasn't going to be high enough. After a couple minutes of watching him reposition his ladder, I offered my 17' ladder. When the other installer came, I was in the house watching him on our cameras while he tried to figure out how he was going to get up on the roof with another 13' adjustable ladder. Again, I came to the rescue and offered my ladder. These guys seriously need to think about carrying a longer ladder. If I didn't have a long enough ladder, these guys would have really pissed me off. I would have taken time off from work for nothing and would have had to reschedule.

Anyway, all these solar installers are pushing this long term lease thing - the most common is by a company called SunRun. There are a couple different options - but they both have an 18 year term. The idea is they pay the installer for the system after its installed - so they own it. Then, they lease it to you over an 18 year period. At the end of the 18 years you can sign another lease, ask them to remove the system or purchase the system at some yet to be determined amount. You can pay either a lump sum amount or monthly. Its lure is that your initial outlay, if you do the lump sum, will be less than your cash purchase price before the federal tax credit.

So for example (assuming you live in California like me), if you purchased a $22,000 system, you would receive about a $3500 state rebate. Typically, you sign the rebate over to the installer and the out-of-pocket cost would be $18,500. Then a 30% federal tax credit shaves the net cost down to about $12,500.

With SunRun, you would pay a lump sum of about $13,000 for 18 years. Of course, they throw in a few more goodies. They extend the warranty of the system from the 10 year installer warranty to cover the entire 18 years. They provide monitoring for the duration of the lease (some installers offer it for 10 years and others charge for it). If they detect something wrong, they'll send a crew over to fix the problem. They'll also come out and wash the panels once a year. Finally, they'll replace the inverter since they typically have a 10-12 year lifespan. They want you to believe this is an added $6200 worth of goods.

They say monitoring is worth $2000, but since we've already got a home automation system & server running, I can get a Brultech monitor for about $250 and monitor it myself. Maintenance is supposedly a $2700 value, but that's hard to quantify. The system is supposed to be maintenance free, and I suppose I can climb up on our roof and hose the panels down once a year (but will I do that when I'm 50 or older???). Finally, they peg the inverter replacement at about $1500. If these inverters are $1500 in 10-12 years, to me, it assumes that there have been no technological advances in that time span. In all, I would derate the maintenance costs by 50% and say the inverter will cost $500 when it needs replacing. For our situation, this gives SunRun's perks a value of $250 (monitoring) + $1350 (maintenance) + $500 (inverter) = $2100, about 1/3 of what they claim.

If we take those "perks" and add it to the net purchase cost of $12,500, we get $14,600. Compared to SunRun's $13,000 cost, the purchase will cost $1600 more, but I get years 19-30 for "free." With SunRun, after 18 years, I could either fork out some unknown amount for a new lease or to buy the system from them. Either way, it's going to cost more than a purchase. Otherwise, how would they make money?

They really push the upfront cost advantage of SunRun vs. purchasing, and that really gets people interested. Then they throw in their perks/scare tactics, to help push you over the edge, which seems to work because solar systems are not very common (and people still buy extended warranties for tv's and other household items). It seems like an OK way if you don't have the cash for a purchase, but you could probably do a home equity loan and get the tax write off for that.

More Solar Thoughts

The system I'm currently leaning toward consists of 16 Sharp 230W panels and a Sunny Boy 3kW inverter. Apparently, due to what's called stringing limitations, I cannot install a Sunny Boy 4kW inverter with 16 panels. If I chose 14, 20 or 22 panels, it would work. That changes my expansion plans.

One suggestion would be to install 16 Enphase micro-inverters instead of a central inverter. The microinverters would allow each panel to turn on individually, instead of having to wait for the array voltage to reach a certain level before a central inverter turns on. Microinverters could possibly boost output 5-10% because the panels will generate sooner and stay on later and are supposed to work better in shade and on hazy/cloudy days. Also, if one dies, the rest of the array will still produce. However, there appears to be a $2000 premium based on my system size and they are relatively new technology (but have a 15 year warranty). There is also the issue of replacement in case of failure - someone would have to hike up to the roof and possibly remove panels to get access to a failed microinverter (depending on how they do the installation). I have also been told that around 10-12 years, normal inverters will probably need replacement, so I assume around 15 years, the microinverters will need to be replaced - and there is extra labor associated with their replacement. Finally, with so many microinverters, there is an increased number of points of failure vs. a single regular inverter.

Panel brands is another topic of interest. One installer is very high on Evergreen Solar and how their panels are rated 0-10% above spec vs. other brands which can be +/- 5% or so. Sunpowers are high end, high efficiency, but somewhat restrictive with their installation. Perhaps this is why none of the 5 quotes I received used Sunpower. Sanyos are also high efficiency and high priced, but are manufactured/assembled all over the place. Obviously, each dealer will tout the panels whose manufacturer they have deals with or which provides them the largest profit margin.

For the consumer, it ultimately comes down to cost, wattage and size (room for future expansion) - at least it does for me. When you start to use the more exotic panels, it's hard to justify the longer breakeven period. I chose the 16 230W panels instead of 20 175W panels due to density - I want to have room on the roof for future expansion. The 20 175s would nearly occupy the entire back roof. I could get 16 Sanyo 215W panels for a $2000 premium (and 240W less output) vs the Sharp 230W. The Sanyo's are about 8" narrower so I could squeeze 2 extra columns - or 4 more panels - in the space of the 230W Sharps. Overall, I could squeeze up to 27 Sanyo's vs 20 Sharps - 5.8kW vs. 4.6kW output - a pretty large difference.

I am leaning toward the 3kW inverter at this point. In the future, if we need more capacity, I could then go with microinverters for the additional 4-6 panels that there's room for. With today's panels, I have room to go from 3.7kW to 4.6kW, but in 5 years, panel density will be better - so I should be able to get more power out of the remaining roof space.

Planning to go Solar

I just had a site evaluation with an installer we're considering. He checked a few things out - the roof - and it's in great shape and the electrical panel - which can easily accommodate the extra amperage. Then he ducked into the attic to check the structure and take a few pictures. While on the roof, he took measurements and made note of obstructions like vents. Some short vents can be cut even shorter and panels can be installed over them. The lowest efficiency we'll have would be about 98% due to our back neighbor's Italian Cypress, which will cause some shading in the early morning. I may ask the neighbor if we can cut it down a little. Our back roof has a SE facing, while not optimal, is still very good in terms of sun exposure. Based on this, we should qualify for 92% of the California solar rebate. Next step is for the installer to send a couple plans with different panels and outputs. We're looking at a 3.5-4 kW system which will reduce our yearly electricity bill by about 72%. I also need to think about future needs and roof space for expansion like an EV car or 40" LCD touchpanels for automation ;) The installer has a bit of a backlog, combined with the winter weather and holidays, the soonest they could start would be January. Power on might not happen until March due to inspections from the city & utility. I have 3 other installers to look at, but I'm leaning toward this one as they did a great job on my co-worker's array several months ago and have had an answer for every one of my questions (I ask tough questions! ;)