AN OFF-GRID PERSPECTIVE

STC, PTC, CEC, CEC-AC What Does It All Mean?

If you’ve spent any time looking at solar panels or doing research on solar power, you’ve come up against many new acronyms. Some of the most confusing aspects of solar power are understanding how much power a solar panel, or more correctly the solar module, will produce. This is especially important when comparing systems and evaluating how much power a system will ultimately produce.

Standard Test Conditions (STC):

So let’s start with the manufacture. Below is a typical spec sheet for a 170,180 and 190-watt solarmodule. When the manufacturer sells this module, they list the maximum power that the module can produce. It’s listed in this table as maximum power or Pmax in watts. If you note under the title (Electrical Performance) they specify that this module be tested under STC or standard test conditions.This is 1,000 watts per square meter solar irradiance, 1.5 Air Mass and a 25 degrees C. cell temperature.

That’s all fine and dandy, but most modules don’t operate in a laboratory. The important wordin the STC rating to look at is cell temperature. It is tested at 25 degrees C. That’s 77 degreesF. Most module cells will operate around 20 degrees hotter than the ambient outside air temperature. That means that this test assumes that the outside temperature is 57 degrees F. So why do they want to test in such a cool environment? Simple, if you look down on the chart you’ll see that the temperature coefficient shows that for every degree above 25 C you’ll lose .45% off of the stated max power. 

So let’s do the math: at a roof temperature of 100 degrees, not that uncommon in most of the USA,your 190 watt module is really only producing around 161 watts. That’s a 28% reduction, not so good for marketing. 

PVUSA Test Conditions (PTC):

The solar industry needed a real world test of the modules so the PVUSA testing center in Davis, California, developed the PTC rating, which is typically 10-15% lower than the STC rating. The PTC rating represents a more real life condition based upon 1,000 Watt/m2 solar irradiance, 20 degree Celsius ambient temperature, and 1 meter/second wind speed. Note that this test is at 20 degrees C ambient which works out to a cell temp around 40 C or 104 degrees F. The facility in Davis takes modules from different manufacturers and puts them head to head and records the output data, an independent test of output. The PTC rating is great for consumers to look at when shopping for solar panels. 

The CEC or CEC-AC watt rating:

The California Energy Commission (CEC) adopted the PTC rating to measure nominal output power of photovoltaic cells or modules to determine the system’s rating in order to calculate the appropriate incentive level. So you may see a module’s power output listed as a CEC rating which is really just the PTC method. The CSI (California Solar Initiative) program uses the CEC method as well. After all these ratings, is this the real measure of power coming from your system? No.
This is where CEC-AC comes into play. We still have to take into account the location of the array geographically, the tilt angle, azimuth of the array and the loss of efficiency going through the inverter. What comes out the other end of all the equipment and is fed into the AC grid power at the home is the CEC-AC wattage rating. The CEC uses this to determine the incentive or rebate a consumer can receive.

So in review, you start with the STC rating as a marketing number that the manufacturers use. Use this number as a general size of the module. The PTC rating will give you a more realistic amount of wattage that a given module will produce. Use this number when figuring out how much DC wattage an array will produce. And finally, because the rebates are predicated on the KW power going into the grid, the CEC-AC wattage is used for figuring out the rebate amount for grid tie customers. The CEC-AC KW can be used as a direct comparison against your utility bill’s daily KW usage. The expected annual CEC-AC KW production can be compared against your actual annual use.

Helpful Tips and Stuff We’ve Learned Living Off Grid

Battery Maintenance

With summer sun and higher temperatures comes increased gassing of your flooded lead acid batteries. It’s time to do a little battery maintenance.

Here’s our recommendation for battery maintenance. First, make sure that your batteries are fully charged. Remember, as the charge of a battery goes down, so does the electrolyte level. The plates within the cells absorb the electrolyte as the battery is discharged. So a fully charged battery will be at its highest electrolyte level. This is important only because adding water to a discharged battery ends up with water overflowing out the cap when the battery gets fully charged. Not only do you end up with an acid spill, but also your battery is losing valuable electrolytes.

When it comes to adding distilled water there are numerous devices and watering cans to do the job. And what a job it can be. With three to six cells per battery, a large battery bank can have 48 cells and use 5 gallons of water. So here’s the cool tip we learned through experience. We buy the large 2.5-gallon containers of distilled water and use a 6-foot length of 3/8″ clear plastic tube. Punch a hole in the top of the water container and set it above the battery bank. Place the tube in the water and using the good old siphoning trick, get the water flow started. Just pinch off the water flow as you work your way around to each battery cell. If a little water gets on the top of the batteries it’s ok. Just prevent foreign objects from getting in the cells. This technique works especially well because the tube can get into and around objects like battery cables and get to the hard to reach back cells. After all the cells have been filled, replace the caps and wash the tops of the batteries with a very mild soap solution and an old rag. Do not use baking soda. If the soda gets into the cells it will destroy your electrolyte.

An Off Grid Perspective

In the course of a day, we talk to many people either giving them advice or suggestions about their specific situations. Lately, we’ve been talking to people who are considering solar power for reasons other than saving on energy bills. They want information about becoming independent of grid power sources because they’re concerned about the safety and well being of their families in these uncertain times. Some customers have indicated that they want a place where they can be more secure than living in a city. It’s not a crazy thought. We understand their concerns, as we live off the grid and could go a month or more without outside power or help in an emergency. In fact our lifestyle wouldn’t change much.

Here is a recent e-mail from a client of ours that typifies the feelings that some folks may relate too. “Kris and I have decided that we no longer have faith that PG&E (the utility) will continue forever to be a reliable partner in providing our electricity, and we’re interested in modifying our PV system so that we can have power even if the electric grid shuts down. Not that I think anything dire is about to happen, but if things get dicey, I don’t want to need to scramble for a solution.”

Now is a good time to think about grid tie with battery back up or off-grid solar power. This decision has become easier financially and because solar is becoming more “user friendly” with falling prices for solar equipment and state rebates still on.

If you’re considering solar, read the article by Dana Hall in the San Jose Mercury News, November 30, 2009. She had a “Checklist for Going Solar,” which we found very informative. Here is a brief summary of her list:

• Take a good hard look at your electricity usage over the past year or several months.

• Take a look at your roof. Does it need repairs and is it situated for solar?

• Ask friends and neighbors who have solar power about their installers and the service they received.

• Learn about your financing options.

• Check out the cash back incentives and the Solar Federal Tax Credit.

We can also give you information based on many years in the solar business and from our own personal experience.

Helpful Tips and Stuff We’ve Learned Living Off Grid 

Make Hay While the Sun Shines

By programming our power-hungry appliances and running them only during the day, we significantly save our electricity usage without giving up their any of their usefulness. Here are some tips, which might seem like you’re saving just pennies, but they add up over time.

• Set your refrigerator on the lowest setting you can use without spoiling food.

• Don’t use the drying cycle in your dishwasher, which turns on a heating element. After dishes finish washing, open the door to dry them.

• Don’t run major appliances at night.

• Many models of microwaves have an option to turn off the light that turns on when you open the door. Turn it off.

• Replace light bulbs with compact florescent bulbs (CFL) or LED bulbs.

• Reduce your pool pump run time until a change in condition is seen.

• Turn off the display windows on appliances that stay on when the appliance is not in use.

• Program your washing machine to run during the day.

• Program the washing machine to use less water and have shorter cycles depending
  on whether you have clothes that aren’t heavily soiled.

Propane and Natural Gas Lower Your Energy Bill

Even though you might be already using solar power, or you just started to look into going solar, you will still want to reduce your electrical load. We found that using propane and natural gas to power appliances saves money. Propane and natural gas used to power gas stoves, clothes dryers, furnaces, pool heating, water heaters, etc. saves more BTUs and is more efficient and cheaper than electricity. Go to an energy savings calculator on www.usepropane.com/esc/ and compare the costs of using propane gas appliances and equipment to the cost of using electricity in your home.

An Off Grid Perspective

You can’t turn on the news today without hearing about another weather related story.

One hundred mile an hour winds in southern California, wild fires raging through the western United States, hurricanes pounding the southern states. Forget the snow storms, now we have earthquakes in the eastern states, and then hurricane Irene shuts down the city that never sleeps. What’s going on here? Well the good news is not more than ever before. Weather statistics show that severe weather is not getting worse. The bad news is we are just more aware of it through the advent of global media. The other factor is, with more of us living on the planet than ever before and more of us building in flood plains and coastal areas, weather is affecting us in ways we’ve never seen. It doesn’t even matter if you believe in global weather change or not, with more of us to affect, weather will become a major factor in our lives sooner than later.

My personal off grid perspective is that I have one major concern out of the way when disaster strikes. Power. It all comes back to this when things get bad. Without electricity things get worse very quickly. Lack of water, food, communication, heat and air conditioning rule the way we live today. All of these basic needs are dependent on electricity. If you’re living off grid this is easy, but what if you’re connected to the grid and the power goes out?

We have the solution. For the past five years we have been installing and selling systems that we call hybrid solar that allows the solar electric system to continue to operate after the grid goes down. This hybrid solar market is becoming very popular with folks who want to be prepared for emergencies, when the grid goes down for a few hours or a few weeks. We have customized many systems for families and businesses that need power when the grid is down based on how much power they need to generate until the grid power comes back on line.

If you are preparing for the next El Niño or other natural event, call us so that you will be energy ready.  For me, when disaster strikes I hope to hunker down and ride it out with my lights ablaze and a cold beer in the fridge.

Helpful Tips and Stuff We’ve Learned Living Off Grid

I’d like to tell you about what I call the poor man’s solar tracker. I regularly get calls asking about the use of solar trackers. I have always said that I think that a solar tracker has its place, but for most installs I feel it does not make sense. The cost of the tracker and the continuing maintenance costs are far outweighed by the ease of just adding extra modules to the array. This in effect equals the added efficiency of the tracker for the same costs and without the maintenance.

So when does a tracker make sense? When you have a very small footprint of solar isolation available. Let’s say you have a small back yard with just one corner in the sun and your roof is not available. A tracker can work great to harvest the most sun in a limited space.

I have recently started suggesting the poor man’s tracker to my clients. In the last few years we have had the opportunity to install pole mount solar arrays that consisted of two pole mounts of equal size arrays. On a hunch I started to experiment with taking the two pole mounts, dropping the tilt to 20 degrees and facing one of them to the southwest and one to the southeast during the summer months. My suspicions where confirmed through testing that the resultant losses due to not being faced true south were very small, less than 10%.

Remember that in the summer when we have an abundance of power, and when the batteries are charged up, the solar array can only feed to the loads and all other available power is lost. But with our array skewed to the east and west, the overall result, was a dramatic increase in our solar window. What this means is that the southeast facing pole mount would pick up the morning sun much earlier and start recharging the battery bank early in the day and the southwest facing pole mount allows the solar array to power loads late into the afternoon saving the battery bank for as long as possible.

As far as a winter setting, we have found that a south-facing array at 40-50 degrees is still the best option. Changing the pole tops twice a year on the winter and summer solstice is easy and just takes a few minutes to do. This type of setup is only advantageous to an off grid installation and a grid tie system would not benefit from this.

Helpful Tips and Stuff We’ve Learned Living Off Grid 

For us old-timers in the solar world, search mode on your inverter was a must have feature and was used to reduce power consumption. Nowadays it’s hardly ever used. So lets take a look at what search mode is and when to use it.

First off, only off grid inverters have this feature and its purpose is to reduce the power consumption of the inverter when it’s not under load. As we know all electrical equipment uses energy. With that said even an inverter that has no load on it, is using between 12 and 30 watts. On early off grid systems this continuous load would be a factor. Let’s say you have a small 1000-watt array and it’s installed in Rochester, NY. Your average sun hours are 3.3 for the year. That’s 3300-watt hours per day produced from your array. If your inverter is using 25 watts per hour just sitting there when all your loads are off, you’re still burning 600 watts per day. That’s 18% of your daily power production going to just powering the inverter. So the inverter folks came up with search mode. In essence search mode puts your inverter to sleep when it doesn’t see a load. With the same example this would drop the daily watts to 150. Big difference. In this search or standby mode the inverter is not producing output voltage and as such any small loads like an electric clock or refrigerator would not receive power. In smaller systems with propane refrigerators and no need for small electric full time loads, this works just fine. While in search mode the inverter sends out a pulse of current looking for a load. When it senses a load the inverter starts up and away you go. So as you can see if you have loads that need continuous power then search mode is really useless but for the smaller systems it does help with system efficiency.