I recently purchased a small 600 square foot cabin that is wired for 12 volt dc. It has eight small florescent lights, a car radio and I also plug in a 19 inch 12 volt TV occasionally. In addition I purchased a used refrigerator Nova Kool model 3800 24volt DC for this cabin.
I am currently carrying two 6 volt golf cart batteries back and forth (for recharging) when I stay at the cabin on the weekends. I would like to make this process easier and am considering purchasing a Suntech 175W 24V Solar Panel, a charge controller, two more golf cart batteries and a power converter 24 volt DC to 12 volt DC. I plan to keep the lighting and power outlets at 12 volt DC and have 24 volt DC to power the fridge.
Am I on the right track? Do you know if Suntech solar panels are good quality or not? Can you recommend a charge controller and power converter for this application? Any other advice you might have for me?
Thanks,
Bob Kruckenberg
Bob,
The solar module you are considering is a 24 volt module, and cannot be connected to a 12 volt battery system unless you purchase a $400+ MPPT solar charge controller that allows a higher voltage solar array to charge a lower voltage battery. If you use a standard charge controller, it will force the solar module to operate at half its normal voltage which cut its watts output in half.
Although I do not buy or install any solar hardware made in China for more reasons than I can discuss here, I will say that SunTech is one of the oldest and best respected solar manufacturers in China, but I was not pleased with workmanship on the few that I have purchased.
If you stay with a 24 volt battery to match the 24 volt solar module, you will need four (4) golf cart batteries, and I think you will find the lower cost voltage converters to be very light-duty for this application. We have had several fail when loaded near their advertised ratings.
Since this is a small cabin, I would keep the batteries and all wiring 12 volts DC as you can find almost anything in an RV or boating supply store that will operate on 12 volts DC. You cannot find much of anything to run directly from 24 volts DC unless you use the voltage converter, but then if it fails you lose everything.
I have 2 40w panels wired in parallel for my camper. Can I connect a 20w panel in parallel with them?
I have been told I can only use panels of the same wattage. Is this correct? If so, can you explain the reason?
Thanks for your help.
Yours,
Rodger Newgreen
Roger,
Although it’s always best to match all solar modules in any array for the best performance, it’s possible to mismatch wattages in smaller systems under certain conditions.
The key is not the “wattage” of each module, its the “voltage”. You want all modules to have the same nameplate voltage ratings under open, short, and peak power. If the voltages match and they are all wired in parallel, the current for each will be based on each module wattage.
If wired in series, you may have problems if the smaller wattage module cannot handle the amp current passing through from the larger modules. There is also the possibility that the lower wattage module in the circuit will “draw down” the other modules so and reduce their output, much like what happens when you install one old battery in series with two new batteries.
Thanks for all the great information you provide BHM readers. I’m planning to install a small PV system using the instructions in your article in issue #116. This will be a very small system and used primarily for back-up during a black out. I have a Uni-Solar US-32 panel and Sunsaver 10 amp controller. I’m installing the panel on a carport roof, (I live in Massachusetts), running the wires to connect to one or two 12-volt deep-cycle marine batteries in my attached garage. I’ll be using the 2-pole fused disconnect and correct wire size for the panel and ground wires you recommend.
I have three questions.
1. What can I use as a bracket to mount the panel to the roof? I’ve seen small RV racks for sale and wondered if there is an alternative mounting bracket that can be purchased at a hardware store.
2. The wiring diagram in your article shows the load wires going to two 12 volt lights, hooked-up in parallel. I plan on wiring one light for the garage and continuing the wires about 30 feet to be run to the inside of the house. Those wires will be hooked up to a wall plate with a 12-volt cigarette plug receptacle mounted on the wall. I want to use the plug for a 12-volt TV or any other 12-volt appliance/charger that works using the auto style plug. What do you think of this arrangement and do you have any suggestions or recommendations.
3. Do I need to vent the batteries outside if they are in an un-insulated garage? I plan on putting the batteries in an insulated box for the winter and wondered if I can drill a hole into the top of the box and let the batteries vent into the garage.
Thanks for the help.
Ernie Smith
Ernie,
I can give you some general answers, but since we would not know everything about a specific installation, we cannot be too specific.
Although most solar dealers offer an “approved” angle “foot” to attach any solar module to a roof, I have taken 1-1/2 X 1-1/2 aluminum angle and installed the leg standing up to the side of the solar module and the flat leg bolted into the roof framing. Keep in mind that if you do not use the pre-drilled mounting holes on the back of the module you will void the warranty, so you might try a combination of two short lengths of angle back to back to make a “Z” shape. This will give you a way to utilize the bolt holes on the back of the module.
As long as your loads are 12 volt DC and wired in parallel, they do not all need to be at the same location. However, I would increase the wire size to the more distant load as there is 10 times the voltage drop at 12 volts DC that there is at 120 volts AC.
Finally, a few RV or golf cart batteries out in a garage will not give off enough gas to cause a problem due to the large space, but don’t locate them next to a gas fired hot water heater. Hydrogen gas is only explosive when highly concentrated like in a small sealed up closet or battery box. If you do place the batteries in an insulated box, you will need a vent at the top. For larger battery systems we use a 1-1/2″ PVC pipe vented outside, with screening to prevent insects from entering. The pipe needs to slope uphill since gas rises, but you will need something to keep out the rain.
Putting together an off-grid system, I am inquiring as to how best choose/purchase the solar panels for optimal efficiency in relation to their voltage output. There are panels that put out similar wattage, however the vp/imp can vary greatly.
Let’s imagine for example the system objective is approximately 2kw using a 24v to 110v inverter with battery back-up.
Which would be the best solar panels to use for maximum efficiency?
As we say on the web site, we cannot answer specific design questions because there are too many variables we would not know about your specific application, and we are trying to keep this a free service. However, I can provide some general answers that should help you narrow down your choices.
First, there are many things that go into the design process and the first is inverter input limits if grid tie, and solar charge controller input limits if battery based. In other words, if your solar charge controller has a limit of 75 volts DC input, this will require the number of solar modules wired in series to be well below this high limit. The NEC Article 690 provides more specific design guidelines related to safety multipliers that you must use.
The maximum input amp rating of the solar charge controller (or inverter if grid tie) will determine the maximum number of parallel strings and total amps from the array after the required NEC design multipliers are added.
Once you know the maximum voltage and maximum current your system can safely handle, this will define the solar array that your system can handle. Once you know this maximum total array wattage, you want to select the highest quality module you can buy that has the lowest cost per watt.
Take the total delivered price of a specific module and divide by nameplate wattage to get the $cost/watt.
If this process does not give you the solar array you want, then you will need to make changes to the inverter if grid tied, or the solar charge controller if a battery based system. Normally I first select the exact solar array wattage and physical size I want for a specific budget, then I select the inverter if grid tie, or the solar charge controller if battery based, but it sounds like you are working backwards and trying to base you design on comparing output voltages and amp ratings of the modules to fit a specific requirement.
In other words, its like taking a car and trying to replace the engine with a bigger engine to get more power. However, if you do not also upgrade the transmission, tires, drive shaft, fuel system, exhaust system, and brake system, then the vehicle cannot take advantage of the higher horsepower.
Have noted that some distributors are selling slightly blemished solar panels. When advertised they indicate the panel meets UL/CE specs and carries the full warranty period as a normal solar panel. They also indicate the blemish will not hinder the output of the solar panel. In your years of experience are you aware if a blemished panel would have some drawbacks?
Thanks,
Jon
Jon,
Depends on the cause of the blemish. For example, dents and scratches on the aluminum frame or backing is usually not a major concern. If one or more cells have failed and the voltage output has dropped, this could be an indication of possible further failures most likely doe to poor quality control during manufacturing.
If the modules still have a UL certification, and the seller documents they warrant the wattage at some specific level, then most likely you should be OK.
I’ve been wondering something. Is there a significant difference in the quality of the PV panels and equipment from better known suppliers over the panels seen at places like Harbor Freight and Northern Tool?
Harbor Freight recently had a sale/coupon for a 45 watt panel with a few extras, such as a couple of 12vdc lights for about $180 each. I passed it up fearing the quality of the panels, as I cannot put them to use right away, and wouldn’t be able to return them down the road.
Rick
Rick,
What a great question! Actually, for some reason the old adage that you get what you pay for is actually still true and surprise – it also applies to the solar industry. First we need to separate out the solar modules that were not built or intended to last 30 years. This would include the solar chargers and solar cells for portable devices and to charge car batteries, cell phones, MP3 players, ect. Typically, these will be under 10 watts in size.
The remaining solar modules that are intended to provide primary or backup power for homes, businesses, and remote telemetering sites normally carry the standard manufacturers warranty which these days is 25 years. I have solar modules on my home that were manufactured in 1980 and they are still going strong, although now sun bleached in color.
This past year there was a house fire caused by “cheap” solar modules that were built by the installer. These shorted out after a brief rain on an hot summer day, and the non-glass covering caught fire and melted down which then caused the roof to catch fire. Although I doubt that you will find semi-homemade solar modules in a store, it does show that the solar modules must be made from quality materials. The best way to find this out is look on the back label. The label should have the logo indicating the module is certified by “UL” or the Canadian equal “SA” or both.
There is a flood of solar modules now entering the US market. SUN-TECH is a very well respected China product and although I cannot say what the long term life of their modules would be, the ones I have inspected appear to be very well built. Unfortunately, there are many other made in China brands that have only been out a few months and we do not know their quality or the quality of their manufacturer. Many countries are desperate for cash right now and solar modules are the only game in town that is still selling. This means you not only might get some really great prices on some discounted quality solar modules, but also on trash modules that were cheap to start with.
I enjoy your articles whenever they appear in BHM.
I have been hearing about “thin-film” voltaic cells in the press for the last few years and I think your readers would enjoy an article discussing the present state of this inovation and how long it will be before they reach the mainstream market.
Are they really going to be as cheap in comparison to the current product as the press would have us believe?
Thanks
Jerry T.
Michigan
Jerry,
Actually I have discussed thin film solar module technology in past articles. Thin film solar modules are called amorphous cells and are one of several research attempts to lower the cost of solar modules. Unlike standard modules which are assembled from individual solar cells and then wired into a module and vacuum sealed behind tempered glass, the amorphous solar module is made by a plating process which deposits a thin film of photovoltaic material on the back of the glass or on the surface of a metal plate. A laser is then used to divide the single large cell into separate cells to increase the voltage, and this process is much faster and cheaper.
Unfortunately, although an amorphous solar module is less than half the cost of a conventional solar module, so far they are less than half the efficiency so you need to buy twice as many and use twice the roof area for the same amount of collected energy. They are also developing solar paints or inks that can be applied like spray painting a car, but these also are much lower efficiency and have a shorter life. There will, of course will, be some applications where system life and efficiency are not as important, as long as they are cheap.
There are many things in the news these days about improving solar technology and lowering costs, but so far, almost all of these articles are talking about things that are years away from being turned into real products you can buy.
I recently built a small alt-e project with my daughter. It’s a shelf with a 6v van and a couple of led lites for over her bed. It’s powered by a 6v 36 amp-hour battery in an ammo can under her bed. I have a 4.5w, 300mA Coleman solar panel intended for trickle charging 12v batteries. It charges at about 17v.
Am I right in thinking that with a system this small and with a current that low that I can use this panel with this battery without frying wire or battery, or should I hold out to find a panel that charges at 8v.
Steve Sonntag MD
Steve:
In an emergency you could temporarily re-charge a 6 volt DC deep discharge battery with a 12 volt solar array, but I would not make this a permanent connection for the following reasons. This Wal-mart solar charger was designed to trickle-charge a large 12 volt battery, and does not have a built in solar charge controller, which is available as an option from the manufacturer. Supplying a higher voltage to a wet cell deep discharge battery is typical for periodic equalize charging, but it sounds like you have a sealed deep cycle battery. If that is the case, over-charging from a high charging voltage will dry out a sealed battery and these cannot be restored.
Also, without a solar charge controller between the battery and the solar module, you have nothing to prevent battery charge from discharging back through the solar module, although this small solar module may contain a blocking diode to protect from this. You may be able to remove the backing from this solar module and “split” the string into two groups of 6 volts, since many solar modules are made up from separate strings of individual cells like Christmas string lights. Find the wiring point between two equal sets of cells and cut this wire, then tie the positive side to the positive output wire, and tie the negative side of the cut to the negative output wire. Now you will have a 6 volt nominal solar module. Keep in mind that many commercial 12 volt and 24 volt modules have internal junction boxes set up to allow you to do this with these larger units.
If all else fails, buy a 12 volt battery, fan, and light. They are probably easier to find than 6 volt units anyway.
