In the previous articles in this series, we covered the essentials of selecting solar panels, batteries and chargers. Be sure to go back to the first article in this series for a complete overview of how to design an off-grid energy system. In this article, we cover some of the important aspects of inverters to make the most of the available solar energy and battery capacity. Since this series is focused on off-grid applications, we are only interested in inverters appropriate for that task.
Not Just For Solar
Calling the inverter portion of our design solar is really a misnomer. Once the energy has been stored in our battery array, we no longer care how it got there. The batteries could have been charged by a generator or a hamster wheel for all we care. The important role of an inverter in our off-grid system is to convert the low-voltage DC power in the battery array into a higher voltage AC power equivalent to that available in a normal wall outlet.
Inverters are classified by what kind of output power they produce. Most inverters typically use one of three different technologies to produce their output power. These three classifications are square wave, sine wave and modified sine wave, as shown on the diagram to the right.
Square Wave OutputThe square wave, the blocky waveform shown in red on the diagram, is the simpliest form of inverter output. This waveform, while simple to generate and found in the most inexpensive inverters, is only suitable for small, resistive loads, such as incandescent lights or heaters. Attempting to operate inductive loads, such as motors, fluorescent lights, or even some new LED lights, from this kind of output will likely damage the equipment it is powering. This waveform is also not suitable for long distance wiring as this tends to make the inductive effect even worse and causes interference with electronic equipment.
Pure Sine Wave OutputThe sine wave, the smoothly varying waveform shown in blue on the diagram, is the premium form of inverter output and most closely matches the waveform available from the normal grid utilities. This waveform is expensive to generate, but can operate any type of equipment or appliance that can normally be operated from grid power. It can also be transmitted over any reasonable distance that most off-grid arrays will support. As mentioned above, since most new LED light fixtures have an internal inductive power supply, even these need a nice, clean sine wave input for long-term use.
Modified Sine Wave OutputThe modified sine wave, the stairstep waveform shown in green on the diagram, is a compromise between the square wave and the sine wave outputs. For practical purposes, however, the modified sine wave has the same limitations as the square wave. Because of the marketing appeal of the term modified sine wave and the relatively small additional cost to make this type of inverter versus a simple square wave inverter, square wave inverters only rarely appear for sale.
For those with the 12 volt battery array design, there are some nice low-cost modified sine wave options that provide USB charging ports intended for use with car batteries when camping or so on. So, one of these in your system would be great for that. Certainly do not run long extension cords for questionable loads with these. Also put some tape over the AC sockets to keep you from plugging your freezer into it! Because these units are designed and marketed for automotive use, they usually are not available for 24 volt or higher battery array voltages.
Many of the inverters designed for USB charging or small loads (under 100 watts or so) are intended to plug into a cigarette lighter, so an adapter with battery clips (about $4), will be required. This can be a very convenient low-cost solution for many tech accessories. Unless the power drawn is very small compared to the battery array capacity, don't be tempted to use these across one battery in a 24 volt array, this could wear the battery string in ways beyond the scope of this article.
One other use for a cheap modified sine wave output is for high resistive loads such as hair driers, curling irons or coffee makers. For these applications, a high-wattage, low -cost modified sine wave unit is a perfect match. Again, clearly label the unit to keep it from being used otherwise.
Bottom line: when in doubt about the kind of load your inverter needs to drive or what qualifies as long distance wiring, choose a pure sine wave inverter.
Now that we have an understanding of the types of inverters and how to choose intelligently between them, let's consider some of the inverter options on the market. Fortunately, unlike with chargers, battery arrays and solar arrays where the power, current and voltage ratings weren't obvious, with inverters, the nameplate rating is what you get. And in fact, most inverters can even supply a little more than nameplate power, which helps some inductive loads during startup. As a result, the only real design decisions are to match the inverter input voltage with the battery array voltage (such as 12 V or 24 V), pick the right kind of output power (110/120 VAC for most purposes), and overall power rating (such as 1500 W).
This chart shows just a very small sampling of the inverter options found on the web. We've included some modified sine wave options as a comparison for price and features, but we highly recommend spending extra for pure sine wave. That way, you won't forget one day and blow out the compressor on your freezer full of meat when you use the wrong circuit in the middle of a stormy night.
Also remember that if your battery array can't supply the power, then the inverter can't deliver it, at least not for very long. All the previous articles have talked about how to size your solar and battery arrays and charger to meet your continuous power needs. The inverter is where the rubber finally meets the road, or where the power hits the plug.
Two things are important to note in the above comparison. First, the price difference between a 12 volt and 24 volt inverter in a given performance class is very small. As you will recall from the discussions of the implications of battery voltage design and charger selection, a higher voltage gives more charging current capacity effectively for free. There is, however, a significant jump in cost from 24 volt to 48 volt. Second, additional capacity is typically a small increment in price as well. The implication of this observation is that, if in doubt, it makes sense to pay for a little more capacity than you think you need right now, even if the remainder of your system isn't currently sized to supply a larger load. Fortunately, several inverters can be easily attached to the battery array for additional circuits as your system expands.
Now that we understand the inverter options and how to make an informed buying decision, it is time to discuss the accessories needed to form a complete solar system. This is the topic of the last group of articles in this series: Solar Safety Accessories ...
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