Step 4: Equipment
When setting up your off-grid power system, don’t buy the cheapest inverter that you can find on Amazon! There are several excellent inverter manufacturers out there. I have some that I love, some of them are okay, and some I will not install. Keep in mind, as a professional installer, my reputation is at stake on every project I put in. There are some cheap ones that may perform well, I can’t go there. Some of the “good stuff” out there can power your retreat, automatically start and stop the generator based on battery status, and be tied into the ‘net. I know, some people don’t like that option, but on my end, I can monitor my projects, and even tweak them without leaving my office.
Whatever you do, make sure it is a pure sine wave inverter. They are more efficient, and your equipment plugged into them will last longer. Be careful of the cheap junk available on the ‘net. I have seen an 8000 watt (rated) 12-volt inverter in person. It said it needed to be fused at 200 amps. Simple math folks, power(W)=current(I) x voltage(E), so W=200 x 12, power = 2400 watts. What kind of alien technology are they selling you for the low cost of just $249, that it can output triple the power input?
My business is totally off-grid, powered by an old-school Outback Power Systems FX series 2000-watt inverter. It powers my air conditioner in the summer, and powers my pellet stove all winter. My house has a Magnum Power Systems PAE4448 that I took in on trade a few years back when a customer upgraded to an Outback GS8048 Radian. It backs up our several deep freezes and refrigerators. It will soon power the fan for my LP furnace, and the wife’s freeze dryer. I also have a twelve-year-old Xantrex Pro Sine 1800 that was in the house for a few years, then was bumped out to my shop for several years. Now it is in one of my service trucks. One of my work trailers has a little Morningstar 300W unit, which is fine for running tool chargers, lights, radios, etc. None of them have ever missed a beat.
I install more Outback inverters than all the others combined, not an ad for them, just simple truth. I like their equipment. Inverters are available to power most anything you can imagine. Three of mine are just 120-volt units, which are great for small projects, or can do an entire house or cabin. The other option is a 120/240-volt unit like my Magnum, the Outback Radian, or the Schneider (formerly Xantrex) units. These can feed a standard electrical panel that we use in the states, with no changes. These inverters typically also have a battery charger built into them to charge the batteries from grid or generator power if necessary.
I am writing this from a little Third World jungle, where I am using three Outback GS7048E inverters stacked to put out 21 kW of 230-volt 50 Hz power, with 24kW of solar, 4500 Ah of battery storage, and three diesel generators, a 23, 110, and 220 kW. Might sound stupid in the states, but down here they are paying over $0.42 per Kwh for electricity, when they can get it. They just went through 16 months with no grid power after Hurricane Maria! You want to practice for tough living? These people are living it, and for the most part life is going on.
Okay, we’ve made power. How do we store it? This question can be pivotal. I will admit, I am old-school, and prefer FLA (Flooded Lead Acid) batteries. They have a decent price point, can have excellent life, and can take some major abuse, if we give them some loving. I do use some SLA (Sealed Lead Acid) batteries, but typically only in instances where I feel maintenance may be an issue. FLA’s do not need a lot of maintenance, but with none, they will be an expensive lesson in frustration. Stay away from anything at a big box store that says “deep cycle” unless you want frustration.
Deep cycle batteries are not built the same as a typical car battery. An automotive battery is designed to give a big push of current for a few seconds to crank an engine, and then get charged back up right away. They typically use less than 10% of capacity, and as such, have many thin lead plates to give a lot of surface area to the chemical reaction. In an off-grid type scenario, we want batteries that will allow us to suck their life out, and then let them sit until the sun starts to shine again. This requires much thicker lead plates, so they do not warp when under the strain, and heat, of doing their job.
I use golf cart batteries in my solar powered golf cart. It hasn’t been plugged to an outlet in over two years. I use it to run around my property daily. In a building or house sized system, I start with L-16 size batteries, and go up from there. As you need more battery capacity, you are better off to go to larger batteries rather than more strings of batteries. Good practice is to not go beyond three parallel strings of batteries. I prefer to use two as a maximum, but will use three, if necessary. I typically use one string of large 2-volt batteries, using larger cells for more capacity, when needed. This way we can control the charging in that one string, and we don’t have strong and weak strings fighting with us. This leads to longer battery life, and if one should fail during a time that we have no option for a replacement, we can just jump out the one bad cell and change our charge controller settings down by 2.35 volts (approximately, check data sheet for your batteries), and just keep on running. If you lose a six- or twelve-volt battery in a string, well, good luck. You just lost a whole string, and either half or a third of your capacity, if not all of it. If you are debating on battery system capacity, (and you can afford it), err on the bigger capacity system.
So what about battery voltage? For a small system, there are some advantages to using a 12-volt system. On a 12-volt system, we can get many small 12-volt appliances from the RV and OTR trucking market. We can recharge them with jumper cables from a car or tractor. If we are going to do a whole house, I would suggest going to a 48-volt system. That is the maximum allowed by the National Electrical Code in a residence. There are some commercial projects where we go higher voltage, and some of the new lithium systems are able to go higher (300-400V) as well, but that is not what we are discussing here. With modern inverter technology, we can use standard off the shelf lights, appliances, and wiring, with no need to run special DC circuits for a 12-volt system. We like to run the highest battery voltage we can, as this allows more power with less current, translating to smaller wires, less copper, and hence more pocketbook friendly.
I will readily admit that I feel the Lithium technology is going to be a game changer. I have worked with a few companies that are starting to repurpose Lithium batteries from electric cars into stationary applications. This greatly reduces the initial cost to get into this technology. Most of the major brands of solar charging equipment are already putting the charging algorithms into their controllers to work with this technology. To date, we have installed one new commercial lithium system and are encouraged by what we see. I am also extremely curious about the Iron Edison equipment, but have no experience with them as of yet. It looks like an excellent option, more up front, but longer life, and not a new player in the game. I try not to push stuff that I do not have real world experience with.
Solar PV panels are presently the cheapest they have ever been. At this time, the most common panel is going to be a 60-cell module, which will measure about 40 inches wide and 65 inches high. The frames can vary from around 1.2 to 1.75 inches thick. The average solar cell generates approximately .5 volts when exposed to full sun, regardless of its size. As the cell gets larger, it can output more current, but still at .5 volts. The 60 cells in a typical panel are wired in series to make approximately 30 volts, depending on cell efficiency, and the temperature. Temperature is a very important thing we need to take into account. A solar panels efficiency is inversely proportional to its temperature. This is a fancy way of saying that the colder it is, the more efficient it is. When it is -20 degrees outside you may see your voltage go past 45 volts on the same panel we just talked about. When it is 90 plus degrees and the sun is wailing, it is possible that your panel will only be putting out 25 volts. There are several good books out there as resources with the formulas for calculating this information, and most any reputable charge controller manufacturer will have a string sizing tool on their website that you can put your numbers into, and it will calculate this information for you.
Due to the temperature coefficient, and my northern climate, I prefer ground mounted solar rather than mounting to a building roof. A ground (or pole) mounted array is going to be low enough to the ground that you can easily sweep snow off your panels with a broom. This can be difficult at best or downright dangerous when on a roof. The other advantage with ground and pole mounts is that we can put the panels where they will receive the best sun and have the proper orientation to maximize output. When we place the panels on a roof, we are typically stuck with the angle and direction that the building has, which is most of the time not ideal. In the northern hemisphere we would like to face our panels south and have them angled the same as our latitude. If your mount is adjustable, we can add 15 degrees in the winter, and subtract 15 from that in the summer. That will help us to maximize our annual production. Most roofs nowadays either don’t face south, have minimal pitch, or tend to have so many dormers and additional peaks on them that they become a solar guy’s nightmare. When we do mount PV panels on the roof, the racking they attach to is typically going to space them 4-6 inches off the roof surface. This is to allow air to circulate between the two, thus keeping the panels and roof decking cooler. This can’t compete with the cooling of an open-air rack on the ground.
When trying to live off-grid with a solar system, it helps to get a little more in tune with nature. Try to use your big electrical loads when the sun is shining during the day, rather than at night or on overcast days. This allows you to use a smaller battery bank. By using the big loads during the day, the solar will power most of the load, and the battery will be a buffer to make up any difference. If you run the big loads when there is no generation, it will all come from your batteries. This requires a larger battery bank in order to not overload the system.
Hopefully this answered a few questions and provided some basic food for thought, without getting too complicated.