What You Really Need to Know Before Buying a Solar Inverter
In today's market, there are so many solar inverters to choose from, each with its own specifications and features, it can be hard to make a final selection decision. This article aims to provide some insight to help you make the right choice. There is a useful data table comparing popular off grid inverter models from the top manufacturers: Apollo Solar, Exeltech, Magnum Energy, OutBack Power Systems, SMA America, and Xantrex Technology. But what do all those technical specifications mean? Read on!
The first decision you need to make is what type of appliances or equipment you intend to run off your selected inverter. Modified sine wave inverters will work just fine for most small electric draws. However if there is a chance that you might be running any equipment that has a small motor (e.g. power tools, refrigerator, or perhaps sensitive sound equipment), you will want pure sine wave form for optimal performance.
Once you decide on pure sine vs. modified sine wave, there are several definitions you need to understand in detail before making your purchase selection They are outlined below.
Max THD (%) (only applies to pure sine wave): THD stands for Total Harmonic Distortion. Ideally, you want this as low of a percentage as possible. This is a measure of the harmonic distortion (electrical frequencies in the output) present in the signal when running your equipment off an inverter, which tends to increase with the more equipment you run off of the same inverter. Therefore, you want an inverter that minimizes this effect and maximizes controllable power flow. A measure of 0% would equal a perfect sine wave, and anything deviating > 5% really isn’t a pure sine wave inverter.
Rated Continuous Output Power (W): This figure is total available output specified over an indefinite period of time, and can be specified in documentation as either VA (volt-amps), which is a total inverter output, or watts (W), which is the power consumed through electrical loads. In cases where your selected inverter provides a continuous output power as expressed by VA, it is important to also identify the rated power factor. The power factor is a ratio of W:VA, the difference of which does no good. Where the ratio equals 1 (unity), all power is being used. As an example of this, the Magnum Energy MSH-RE Series inverters have a continuous rated output power of 4000 VA, with a power factor >.95.
Output Voltage: This is the value of voltage that the inverter will provide after stepping down the voltage from whatever supply voltage is being received. In North America, 120V is standard. Some inverters can handle 120 or 240 loads for specialized equipment.
Peak Surge (AC Amps): Appliances with motors typically require a temporary pull of a large amp draw at startup. For example, an average household refrigerator may consistently draw around 5 amps, however upon motor startup, it may temporarily draw as many as 10 amps, known as “peak surge”. Actual specifications depend on each model. If you know the peak surge of the appliance in watts, you can easily calculate the peak amp surge, where A = W/V. Your off grid inverter specifications should tell you the maximum available peak surge in AC amps for a specified maximum period of time, usually in seconds or milliseconds.
Stackability (Parallel, series, 3 Phase): In many cases 2 identical power inverters can be stacked to either double their capacity to draw amperage from the battery array (parallel stacking), or double the VAC output (series stacking). In parallel stacking, the positive input from the primary inverter is connected to the positive output from the second inverter. Likewise, the negative input from the main inverter is connected to the negative output of the second inverter. Using 2 120VDC inverters and stacking them in a series (via connecting the positive input from the primary inverter to the negative output of the 2nd inverter) will yield 240 VDC and enable the inverter to handle larger loads. It is critical that stacking not occur unless the inverters are specifically designed with the correct overcurrent protection. A special cable is needed to make one inverter primary and the other secondary. A 3 Phase inverter goes the extra step to provide a more balanced and efficient electrical load. This configuration off-sets electrical oscillations and minimizes a wide variation in voltage to your appliances. 3 Phase power distribution is required for heavy equipment that runs on 3 phase motors. Before purchasing an inverter, one should carefully consider not only total power requirements, but potential desire to expand the system later on where stackability may play an important role.
Inverter Peak Efficiency (%): The higher an inverter’s efficiency, the lower the amount of energy wasted during the AC to DC conversion process. However, don’t be fooled. Peak efficiency does not equal operational efficiency. Instead of focusing on this value, look at each inverter’s published efficiency curves and choose an inverter with strong efficiency ratings across varied output wattages.
No-Load Draw (W): All inverters, when connected to a battery source, draw power even when there is nothing plugged into them, or when connected appliances are turned off. High no-load draws can really have a negative impact on your power system, especially for smaller batteries. Consumers who do not pay attention to this value often find themselves in a tight spot, having their batteries drained more than expected.
Search Power (W): This is essentially also known as “sleep mode” for the inverter. This power saving feature that power downs the electrical draw of appliances when they are turned off. While in sleep mode, the inverter uses a small amount of energy to scan the system and determine if there are any new/active loads. Keep in mind that any off-grid system that has even one continuously running AC load will not be able to take advantage of the sleep mode feature, and will be defaulted to the higher no-load draw when appliances are turned off.
Battery Charger Max Current (DC Amps): This is simply the max number of amps that the inverter can draw from your battery connection.
Battery Temp Sensor (included or not): Whether or not a battery temperature sensor is built into your chosen solar inverter is really important, as you may need to buy one separately. The role of the temperature sensor is to provide temperature data to the charger, and adjust voltage set points accordingly. In this way, your system prevents undercharging in low temps and over charging in high temps.
Generator Start (optional, no, yes): A generator start provides for the option to start or stop power draws based on given criteria, such as battery voltage, state of charge, pre-set time and so forth. It’s a nice feature to have, but not required by any means.
Metering (optional, yes): This optional accessory provides information on the state of your inverter/battery system such as battery voltage, AC amps loaded from items plugged in, battery charge amps from the power source, and error notifications. If your inverter is programmable, you will want the metering option as your user interface.
Remote Display (optional, no): Inverters need to be installed within very close proximity of your battery, which is typically not near open living spaces. Remote displays make it easier to visually monitor your inverter and/or other system metering details.
Integrated system available: There are some important benefits to having an inverter that can be included as a part of packaged systems, which will ensure that all the individual parts of your off grid system work and fit together. Inverter purchase decisions are often based on how they fit with solar charge controllers and circuit breakers/boxes in terms of knockout holes, box sizing, wire sizing and so forth.
Warranty: Inverter manufacturers offer warranties against defects up to a specified period of time. It is important to register your purchase directly with the manufacturer so that you can reach out and resolve functional issues directly with the manufacturer during or even after the warranty period. Out of warranty services typically will have an added cost established by the manufacturer.