In addition to the main system on my home, I have a one-panel, 130W, stand-alone solar PV system on my garage. This doesn't contribute a significant of energy; it's mostly just for fun. I use it to maintain rechargeable tools and power some electric tools in the yard and garage. It would also provide a small amount of backup power in case of a utility outage.
Components
The roof contains a single Kyocera KC130GT panel, mounted on Unirac SolarMount Light rails, wired via a small weatherproof junction box through a 20A cutoff switch and into a Morningstar SunSaver SS-20L solar charge controller. (The switch and charge controller are big enough to allow me to add a second 130W panel in the future.) The charge controller maintains a small bank of used absorbed glass mat (AGM) valve-regulated lead-acid (VRLA) batteries:
- The two batteries in the grey pack on the left side of the shelf are fairly new. They are for my cordless electric lawn mower. Plugging them directly into the battery bank charges them after mowing but also provides additional amp-hour capacity for other uses between mows.
- The battery in the middle is one of my old lawn mower batteries. Its mate died, but this one still had some life left. Since one battery isn't any use in the mower, it's getting some use here instead of going to the recycler.
- The battery on the right is my old car battery. When it became too weak to start the car in the winter, I replaced it, but it isn't totally dead yet. Unlike the other deep-cycle batteries, this one is a starter battery, so it won't survive complete discharge. I built a diode protection circuit for it.
- Each battery is fused to protect the rest of the bank if one of them finally dies.
I estimate the bank has a combined capacity of about 60 amp-hours (or nominally 720 watt-hours). So, if my charge controller is 85% efficient, this bank would be fully recharged with about 7 hours of full sun, which is 1.5 to 4 days on average (depending on season).
Connected to the battery bank and mounted on the lower shelf is a Xantrex Xpower 1750 Plus modified sine wave inverter (no longer available, but the Xpower 1500 is very similar). (I had this sitting around in my garage gathering dust. What? Doesn't everyone have one sitting around?) This allows me to plug in just about anything with an AC power cord. Currently I've plugged in a power strip with a bunch of cordless tool chargers, but I also plug in an extension cord when I want to use solar power to run some corded tools, such as my hedge trimmer or saber saw. This inverter can provide 1500 watts continuously, so it can power just about anything I would normally plug into a 15A circuit.
Performance 
When I installed this system, I didn't have any good way to measure its performance. This inverter doesn't have any cool data-gathering features, and its only displays are battery bank DC voltage (which gives me a rough idea of how fully-charged the batteries are) and a bar graph showing approximate AC load.
However, in May 2010 I added an RC Electronics' Watt's Up DC power meter between the charge controller and the battery array, which measures the amount of energy transferred from the solar panels into the batteries. It reports this as a number of watt-hours accumulated over time (very similar to the utility company's power meter on my house), so I can read it periodically and know how much usable energy I've been making (i.e. after the losses inherent in the charge controller).
Unlike the solar panels on the house, which feed their energy to the unlimited "battery" of the local power grid, the solar panel on the garage can only store its energy in my batteries, and once they're full, the panel can't make any more. This means the amount of energy I make is limited by the amount of energy I use from these batteries. So, if I can manage to use all of the stored energy each day, the batteries will be able to accept all of the available solar energy each day. This would represent 100% efficiency. I estimate how much solar energy is available by taking the amount of solar energy generated in any particular month by my house and dividing by the ratio of the wattage of the two solar arrays:
| (available at garage) = (generated at house) x |
(130 W on garage) |
|
| (3720 W on house) |
So, now I can measure the total amount of energy generated by the garage solar panel (which is the same as the amount of energy used by the equipment in the garage) and estimate the efficiency of this system (which is the percentage of the available energy I'm using).
Monthly
Data |
|
2010 |
Energy
Generated
(kWh) |
Energy
Available
(kWh) |
Gen
Avail
(%) |
| Jan | -- | -- | -- |
| Feb | -- | -- | -- |
| Mar | -- | -- | -- |
| Apr | -- | -- | -- |
| May | -- | -- | -- |
| Jun | 7.8 | 13.3 | 58 |
| Jul | 6.3 | 16.1 | 39 |
| Aug | 5.2 | 16.3 | 32 |
| Sep | 4.9 | 13.1 | 38 |
| Oct | 3.8 | 14.2 | 27 |
| Nov | 4.6 | 10.5 | 44 |
| Dec | 2.8 | 6.3 | 44 |
| Total | 35.8 | 90.8 | 39 |
|
| 2011 |
E
Gen
(kWh) |
E
Avail
(kWh) |
Gen
Avail
(%) |
| 1.7 | 4.6 | 37 |
| 2.1 | 8.1 | 26 |
| 3.3 | 11.9 | 28 |
| 1.9 | 13.4 | 14 |
| 4.3 | 12.6 | 34 |
| 4.8 | 13.9 | 35 |
| 4.6 | 16.8 | 27 |
| 4.9 | 17.9 | 27 |
| 7.1 | 12.8 | 55 |
| 6.0 | 13.7 | 44 |
| 3.9 | 7.0 | 56 |
| 2.6 | 6.9 | 38 |
| 46.9 | 137.9 | 34 |
|
| 2012 |
E
Gen
(kWh) |
E
Avail
(kWh) |
Gen
Avail
(%) |
| 2.3 | 7.2 | 32 |
| 2.8 | 9.2 | 30 |
| 5.0 | 14.2 | 35 |
| 4.4 | 15.7 | 28 |
| 4.8 | 16.0 | 30 |
| 3.8 | 14.7 | 26 |
| 3.5 | 16.8 | 21 |
| 4.0 | 15.5 | 26 |
| 5.2 | 16.5 | 32 |
| 4.2 | 14.5 | 29 |
| 2.9 | 8.7 | 33 |
| 1.9 | 4.6 | 41 |
| 44.8 | 153.6 | 29 |
|
| 2013 |
E
Gen
(kWh) |
E
Avail
(kWh) |
Gen
Avail
(%) |
| 2.8 | 8.5 | 33 |
| 2.4 | 6.6 | 36 |
| 3.1 | 12.5 | 25 |
| 3.2 | 12.9 | 25 |
| 4.4 | 14.9 | 30 |
| 6.0 | 13.8 | 43 |
| 4.9 | 13.2 | 37 |
| 6.5 | 18.0 | 36 |
| 4.3 | 11.7 | 37 |
| 4.2 | 11.9 | 35 |
| 3.1 | 8.4 | 37 |
| 2.4 | 4.7 | 51 |
| 47.3 | 137.3 | 34 |
|
| 2014 |
E
Gen
(kWh) |
E
Avail
(kWh) |
Gen
Avail
(%) |
| 2.4 | 4.2 | 57 |
| 2.6 | 9.9 | 26 |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| -- | -- | -- |
| --- | --- | -- |
|
- Energy Generated is measured by reading the Watt's Up at the end of each month and subtracting the previous month's number.
- Energy Available is estimated by multiplying the energy generated on the house panels during the same period by the ratio shown above. This assumes the garage panel and house panels see the same amount of sun every day; however, the garage's roof isn't quite as ideal as the house, because the garage roof's slope is a little gentler (i.e. more optimal for the summer's higher sun but it won't perform as well in the winter) and the mature maple tree in the backyard shades the panel until late morning. However, I'm not trying to squeeze every last watt out of this system, and this estimate is good enough to compare the efficiency of different seasons of the year.
- Totals may not add up to the sum of their column because of rounding.
- As a point of comparison to check this estimate, I also estimated the amount of energy I should be able to make using PVWATTS July data for approximately 30° tilt and a guess that 30% of the sunlight is lost due to shading and then 15% of the power is lost due to system derating (net derating = 60%). The result was 14.1 kWh, which is pretty close to the above estimate of 16.1 kWh for July 2010.
- To help put this in perspective, my Energy Star refrigerator consumes only 33 kWh per month, but my garage system could never make more than about 15 kWh per month (even if it could capture and use 100% of the available energy), so it isn't even big enough to run my fridge continuously.
- Generated / Available is the ratio of the two previous numbers, to indicate how much of the solar energy reaching my garage is being captured and used. Data gathered during cloudy periods (in which the batteries never get full) indicate that the realistic maximum is only about 60%. (Most of the missing 40% is sunlight that is lost is due to shading.) This means a ratio of 60% would indicate that all available solar energy was being used. So a ratio of 30% would indicate I was only able to use half the available solar energy, and the rest was discarded because the batteries were already full.
- Since my average ratio for each year is between 30% and 40%, this means if I want to add anything on to my garage solar system, I should add batteries, not panels. (It also means I should find more ways to use the energy stored in the batteries on sunny days.)
I have used my P3 Kill-A-Watt to determine where most of this energy gets used:
- Recharging my mower (after mowing half the yard) from AC with a Deltran Battery Tender Jr. consumes 0.56 kWh. I probably have to do this six times in July, so that accounts for 3.4 kWh/month. We mow most often in June and July, so this tapers off in later summer and autumn.
- Maintaining two cordless tool batteries for a week and recharging the string trimmer after one use consumes 0.87 kWh, so that accounts for 3.7 kWh/month in the summer.
