Efficiency is a way of life for everyone living beyond the last power pole. When we moved off-grid 20 years ago, the first thing that was drilled into our heads was the necessity of making efficient use of energy at all times.
It was good advice. But, as we’ve discovered over two decades of off-grid living, it was just a little overstated. Yes, I know it sounds like heresy. So before opening that can of worms any further, I believe a short discussion on the efficiency of off-grid systems would help clear the way.
How efficient is an off-grid solar power system?
In those rare times when I get to speak with people on the subject of renewable energy, questions about system efficiency always come up.
Are solar panels getting more efficient?
How efficient is an off-grid inverter compared to a direct-tie converter?
When will batteries be replaced with something more efficient?
The point of these questions, aside from the fact that most renewable-energy enthusiasts have developed a preoccupation with energy efficiency, is to get a rough idea of just how efficient mankind has become at converting solar energy into usable electrical energy.
The short answer is, “Not very.”
Let’s start with solar panels. While some experimental solar panels have reached efficiencies upward of 40 percent, and some commercial mono- and polycrystalline panels boast efficiencies of 22 percent, the less-expensive crystalline panels most of us buy weigh in at around 17 to 18 percent.
Amorphous, thin-film panels, by contrast, are a few points less efficient, which means it takes more surface area to produce the same wattage. But because they outperform their crystalline counterparts in low-light conditions, they’re often preferred when extra space is available.
Since crystalline panels are used in most installations, we’ll assume a starting efficiency of 18 percent. When is that efficiency reached? Not in the early morning or the late afternoon, and not on warm and/or hazy days or when the panels are angled more than 5 or so degrees from perpendicular to the sun’s rays. In other words, not all that often
It’s all in the details
Let’s be optimistic and imagine that, over the course of a year, a solar array averages around 12-percent efficiency at turning solar energy into useful electricity. But there are still three more components between the solar array and the wall outlet, and each takes a bite out of the system efficiency.
The first is the charge controller, the component that modifies the high-voltage current coming in from the array to match the lower battery voltage. The newer MPPT (Maximum Power Point Tracking) charge controllers, which convert excess array voltage into usable amperage, do so with around 95-percent efficiency, meaning that the energy leaving the charge controller is down to 11.4 percent of the solar energy hitting the solar array.
Next stop is the battery bank, which is comprised of flooded lead-acid batteries in the vast majority of off-grid systems. Because batteries store and convert energy through the making and breaking of chemical bonds, they’re not nearly as efficient as we would like. But healthy new batteries can generally perform their duties with 85-percent efficiency, a number we should be happy with, even though our overall system efficiency is now down to 9.7 percent.
The last stop is the inverter, which converts the low-voltage direct current (DC) from the batteries into the high-voltage alternating current (AC) we use in our homes. Off-grid inverters generally perform this function with 92-percent efficiency, taking our hypothetical system efficiency down to 8.9 percent. If we then include a fudge factor for line losses and other niggling little heat traps along the way, it’s safe to say that the average overall efficiency of an off-grid solar-electric system is less than 8 percent.
How does this compare to that big, imposing power plant down the road?
Well, these plants all operate by burning fuel to boil water to produce high-pressure steam. The steam, in turn, runs giant turbines, providing power for electrical generators. Coal-fired or nuclear plants convert fuel to electricity at around 35-percent efficiency, while combined-cycle natural-gas plants can reach efficiencies up to 60 percent.
But there’s one very big difference: Solar energy is free. To keep your off-grid home up and running, you don’t have to drill a well, lay a pipeline or a railroad track, strip-mine an entire mountain, enrich tons of uranium or deal with tons of nuclear waste, all processes that require massive expenditures of energy not factored into the original calculations.
You just have to wait for the sun to come up every morning.
Making the most of that 8 percent
When energy is free—in the sense that the supplier of the energy doesn’t ask you for money every month—the scary notion of efficiency loses some of its sting. If you’re dependent upon a system that’s inherently 8-percent efficient and you’re continually low on energy, you either increase the size of the system or find more efficient ways to use what you have.
Does this mean you should go out and buy the most efficient appliances and tools on the market? No, but maybe some of them—the ones that tax your battery bank most.
First on the list should be your refrigerator. It runs day and night, whether you’re home or not. It doesn’t care if it’s been cloudy for five days and your generator is out of gas. It just keeps plugging away without a care in the world.
Around the turn of the century, there was a big push to make refrigerators more efficient. We managed to find a Kenmore that uses less than 1 kilowatt-hour (kWh) per day, and we’re still using it. Today, the general trend is to make refrigerators bigger and loaded with goodies, so it might take some looking around, but they’re out there.
Units with top freezers are the most efficient for the simple fact that the cold air they produce descends into the main compartment. Contrary to what you may have heard, an ice maker is fine, as long as you can turn it off when the sun isn’t shining.
What about other appliances? Are the really efficient ones worth the higher cost? It depends.
In the early days of solar energy, solar panels cost around $5.0 per watt, as opposed to the $0.75-per-watt price tag they carry today. Solar arrays were smaller back then, so every watt was truly precious. Today, however, solar arrays are generally far bigger than they really need to be, resulting in periods of virtually unlimited energy during sunny days.
This leads to a division between appliances: those that often draw power from your battery bank (i.e., at night) and those that don’t. In the former category (in addition to the aforementioned fridge), you may have a freezer, a furnace or boiler, innumerable light bulbs and possibly a number of components relating to a satellite internet and Wi-Fi system.
Then there are the charge controller and inverter; they both use a baseline wattage, even when in standby mode. The less that incessant energy drain, the better. And the well pump would also be on this list, unless you can pump nighttime-use water into a cistern or large holding tank during the day.
In the latter category, you have clothes washers and dishwashers, microwave ovens, vacuum cleaners, and quite probably more power tools than you care to count. If you can restrict the use of these things to sunny days, then efficiency doesn’t really matter that much.
This is because, assuming you have a solar array capable of charging the batteries to full capacity in just a few hours of sunlight, you will be left with a sizeable surfeit of energy that will otherwise be shunted away from the batteries because they’ve reached their limit.
This energy is free in every sense of the word. As long as you’re not using more energy overall than the array is producing, you’re not taxing the batteries (at most, your activities will simply slow the charging rate) or putting excess wear and tear on the rest of the system. It’s a use-it-or-lose-it proposition. So, you might as well enjoy yourself. Personally, I make ice, and my wife plugs in the crockery cooker.
As self-evident as all of this may seem, there’s one thing you need to keep in mind: Discharging the batteries a few percentage points and then quickly recharging them is not the same as never discharging them in the first place.
Batteries lose a small amount of charging and storage capacity with each use. So running, say, the dishwasher on a cloudy day that promises to clear later is not the same as being patient and waiting for the sun to come out.
Still, we all have our extravagances. I’m certainly not going to tell my wife she can’t use the microwave after dark to heat up her neck warmer, and she won’t tell me I can’t use the coffee maker before sunrise. Sometimes you just have to break the rules—even your own.
Suggested resources for preppers and homesteaders: