Friday November 29, 2013: Updated December 30, 2014
Grid-Tied Solar Panels -- Where Do I Start?
Grid-tied Solar panels are an awesome, low maintenance way to eliminate your electric bill. But where do you start? Follow these steps and you be on your way to never paying for electricity again.
1. Find out how much electricity you use in a year, (total up your monthly usage for the whole year, eg 12,000 kWh and divide by 12. This is your average monthly usage.
2. Reduce your electrical usage through increased efficiency, (LED lighting, air seal your house, more insulation, more energy efficient appliances, sleep your PCs, get an energy monitoring system, turn off lights when not in use). Why pay for a 6 kW solar system when with a few simple efficiency improvements a 4 kW system will suffice?
3. Calculate the amount of solar panels you will need. For example, assume you use an average of 1000 kWh/month, (This is a fairly large system). 1000 kWh/month / 30 days / 5 full sun hours = 6.6 kW of solar panels. 5 is the average full sun-hours in northern Utah. You can check how many full-sun hours you will need here.
4. Determine how much shade-free south-facing roof space you have by looking at your roof using Google Earth and measuring the length X width of the south side. As a rough rule of thumb, you can fit 12 watts/square foot of south-facing, sun-free roof space. 6.6 kW / 12 watts = 550 square feet of roof space for solar panels.
5. Measure your roof space and determine how you want the panels to be laid out. Horizontal or vertical, Three rows of eight or six rows of 4?
6. Measure the orientation of the roof where the panels will sit. It must be within 15 degrees azimuth of 180° (due South) to produce the most power power (and to qualify for utility rebates). If your only option is to mount east or west facing, you will lose about 20% of your capacity.
7. Measure the roof pitch on the side that the panels will be mounted. You can download a pitch gauge onto your smart phone and lay it on your roof to measure roughly the steepness of the roof. The ideal pitch in degrees is the same as your latitude in degrees. In Utah, that is roughly 41 degrees in elevation. You need to be within 15 degrees of that number. Your panels won't generate as much power if they lay too flat or too steep (you won't qualify for utility rebates either), but it's not the end of the world if your roof is off of that.
There are three main types of panels. Mono-crystalline and poly-crystalline and Thin Film.
Mono-crystalline (15-20% efficient) are made from a single grown ingot of silicon and are typically higher quality and slightly higher efficiency. Each cell looks like a square with the corners cut off. This is the kind that I installed on our house.
Poly-crystalline (13-16% efficient) are made from a cast ingot of silicon. They are usually less expensive because they are slightly lower quality and lower efficiency.
Thin Film (9% efficient) are very flexible and are generally attached to the roof by sticking them directly onto the roof surface.
9. Make sure what-ever type you get, they come with a 25-30 year warranty. You don't want to invest a lot of money in panels only to have them die the following year. Research the company selling the panels. Do they have a good track history? Good panels that are considered, "investment grade" will be more expensive but they won't give you any problems either. Do you really want to be troubleshooting a bad panel and pulling it off your snow covered roof?
10. Check the negative tolerance rating. This guarantees the maximum output power of the panel. For example if you buy a 100 watt panel with a 5% negative tolerance rating, you are guaranteed that under full sun, it will generate at least 95 watts. The smaller the tolerance number the better (1% is better than 5%). Also your state and federal rebates will be higher with a smaller tolerance rating since the rebate is based on your maximum rated output power.
11. Inverter Type: Decide if your solar installation will require the use of micro-inverters, power optomizers or a single inverter? You may want to pay extra for power optomizers because they are more energy efficient than micro-inverters, won't clip the panel power like a wimpy micro-inverter, will produce more power and allow you to monitor individual panel performance.
Single Large Inverter
Single Inverters are super easy to wire and generally less expensive but there are some disadvantages:
- 6-20 solar panels are all wired in series and fed into a single inverter. If only one panel has shade on it while the rest are bathed in sunlight, that one shady panel will degrade the rest of the array.
- If a single panel has reduced performance, it will take down the rest of the array. There will be no way to determine which panel is bad without checking each solar panel individually, one at a time.
- If the inverter goes bad, the whole system goes down, (with a good quality inverter, this almost never happens).
Micro-inverters are generally more expensive than single inverters but they are superior to a single large inverter.
- A micro-inverter is installed on the back of each and every solar panel. If one panel is being shaded or has degraded performance, rest of the panels continue to operate normally.
- There is no single, large inverter.
- Each micro-inverter will report back how much power its solar panel is generating. A degraded panel can be identified quickly and easily.
- If an inverter goes bad, only one solar panel goes down while the rest of the system keeps on making power.
- Update: 11/18/2014: WARNING!!! Be very leery of installers that pair up a wimpy little 215 watt micro-inverter with a much larger solar panel. While their "experts" claim that this is OK, they are WRONG! This WILL clip the power output of the PV panel. Many times throughout the year, your solar array will produce 10-25% LESS than what it would otherwise. Do NOT pay a premium for a PV system that is designed for mediocrity. You are much better off going with a power optimizer design. In applications where the PV panels are matched and shading is not an issue, even an inexpensive series string inverter will outperform a micro-inverter every time.
- Update: 12/30/2014: A caveat to pairing up a large panel with a smaller micro-inverter is when there is major shading issues or the array is mounted facing due East or West and/or mounted on a flat roof or vertical wall. In these configurations the non-ideal solar exposure and orientation will limit production and allow a little micro-inverter to work just fine with a large solar panel.
Power Optomizers with Inverter
- A power optomizer is installed on the back of each panel (just like a micro-overter) but out-perform any other inverter technology in regards to efficiency and shading issues.
- All the power optomizers output a constant DC voltage regardless of panel size and shading issues. That voltage is then fed to a single, large inverter.
- They report back how much power its solar panel is generating. A degraded panel can be identified quickly and easily.
- They have a larger operating voltage range than what you will get with a micro-inverter and therefore will make more power over the whole day.
- They are rated to handle more power than a micro-inverter so you can pair them up with a much larger solar panel.
Our house roof has a full view of the sun with no trees or obstructions. I went with a SunnyBoy SMA7000 single inverter because at the time, it was $3000 less than going with 26 micro-inverters and power optomizers were not invented yet. Today's solar panels and inverters are extremely reliable and in the past 3-years, we have had absolutely no problems with our photovoltaic system. The twenty-six, 240 watt, Phono-Solar brand solar panels carry a 25 year warranty and the single inverter came with a 10 year warranty.
The only time of year I really wish we had a micro-inverter or power optomizer setup is in the winter time when it snows on the panels. When the snow melts, it slides off but stops short, slumping up on the bottom row of panels. Eventually it melts off but if we had a micro-inverter setup, the upper rows of panels would still be making power in this situation.
We used to lose about 120 kWh of solar generating capacity each winter just because of snow slumping on the bottom row of panels, killing solar production of the whole array. Not wanting to risk breaking my neck climbing on the roof, I bought a snow rake with a long handle to clear off the panels when it snows.
12. Find rebates in your area. Between federal/state tax credits and rebates as well as utility rebates, it could be possible to reduce the end cost of a solar panel installation to zero. Start filling out the paper work now and get it in the cue while these programs are still available.
13. Financing. While the long-term benifits are a no-brainer and there are tons of rebates to help out along the way, the upfront costs for a whole house solar system can be staggering. Even though home-equity loans are the devil, I took one out so I could buy a large $22K solar system. A year later, rebates and tax credits took care of about $10,000 of that. Today you can buy a 5kW solar system for under $9000 initially and for less than $4000 after federal and state tax credits (potentially for free with utility rebates).
14. Find a reputable installer. Keep in mind that if you can shingle a roof, you can install your own solar panels. You must take out a building permit for it and you need to let your home owners insurance know about them too. If you are in an HOA, make sure they are on board before you buy anything.
15. How old is the roof? If you roof is more than 15-20 years old, you should replace it before you get solar panels. They're going to be up there for the next 30+ years. The last thing you want 5 years from now is to be taking down solar panels just to re-shingle.
16. What about making my own solar panels? There are companies that sell plans and directions on how to make your own solar panels, claiming to save you thousands of dollars. While it is possible to make your own solar panel, I would not recommend it for any reason other than the DIY/hobby experience.
- In order to grid-tie, you must install UL certified panels. If you don't you will not be able to reap the huge benefits of net metering.
- Home-made panels carry no warranty. Are you willing to venture onto a snow-covered roof to repair a defective panel?
- Making your own panels is incredibly time consuming and labor intensive. That might not be a big deal for making 1 or 2 small, off-grid solar panels but attempting to manufacture 20 to 30 large, door sized panels is a huge undertaking.
- Currently you can buy a pallet of commercially made solar panels for less than $0.70 per watt, ($3500 -- before tax credits -- for twenty 250 watt panels). It will actually cost you more to buy the raw materials to make the panels than to simply buy the panels themselves. This holds true for any mature product being mass produced.
- Home-made solar panels don't qualify for any federal, state or utility rebates/credits. Depending on what rebates/tax credits are available in your area, a commercially made solar system may end up not costing you a dime.