In an industry where size is everything - larger engine, smaller accessories, and lighter weight - it’s easy to make inaccurate assumptions and create undesirable problems for yourself. There are many instances of undersized inverters being used by operators in an effort to save a buck. When operating below recommendation, the electrical system is far from efficient and can prove troublesome. So … the million dollar question for this edition of Tech Doctor is: What size inverter SHOULD I have in my system? And the obvious answer is … well, it’s really NOT so obvious! The truth is, this decision all depends on exactly how you want and/or intend to use your inverter. Let’s consider some questions to help you understand how to make the best choice for your unique application.
It’s important to look at the wattage draw of ALL of the appliances, plus to leave some room for future additions. If you have a 1000W A/V system, for example, and install a 1000W inverter, you’re probably asking for problems. You see, inverters are rated for their optimum performance under optimum conditions. If the inverter gets hot, or the battery voltage drops, the inverter’s ability to feed a constant 1000W may be compromised. Also, surges in power draw (such as turning on a motor) can affect the ability of the inverter to maintain proper output. Consider this example and accompanying formula. If you are going to plug-in two devices simultaneously, add up the total wattage of both devices, and then add at least 50% more to account for peaks or spikes in the power draw.
Obviously if you know for sure that two loads are never going to run at the same time, your calculation can include the higher draw load exclusively. My advice? Use common sense here, but always err on the side of the inverter. After all, nobody wants to have to remember to turn off the entertainment center when using the microwave to pop the popcorn! If there is any possibility of the loads running concurrently, include both in your calculations.
Some loads, like motors or other inductive loads, have an extremely high demand at start-up, some as high as 5-times their rated power (check with appliance manufacturer for more information). Most inverters have a fairly high surge rating which is their ability to feed short-term high power to get these loads started. There’s no sure-fire way of knowing the appliances surge demand, without testing, since such data is not labeled. While high frequency inverters are cheaper and more efficient, low frequency inverters can surge better, and for a longer period of time. However, depending on your load mix, it may be better to use a larger, high-frequency inverter than a smaller, low-frequency inverter to provide instant start-up current for some loads, plus available power to run all loads at once.
It is not directly related to inverter sizing, however you must take into account the type of electronics and appliances you want to run using the inverter. Let me briefly explain the two types of inverters. You could refer to my previous article “Sine Wave v/s Modified Sine Wave: Which is better?”. True Sine Wave Inverters produce AC power that is similar to power available from the public utility grid system. They are expensive compared to modified sine wave inverters, but they produce quality output that operates the most sensitive and sophisticated electronics. Modified Sine Wave Inverters produce AC power that is sufficient to run most electronics. However, the laser printer, fax machine, laptop, plasma television set and medical equipment may not run properly with modified sine wave power.
One of the most common tech questions debated today includes the best choice for inverter installations between true sine wave (TSW), (sometimes referred to as pure sine wave) and modified sine wave (MSW) technology. Let’s begin by addressing what alternating current (AC) means. AC literally means that the current alternates its direction over and over again (every 8 milliseconds for the US power grid). As is demonstrated in the accompanying chart, a TSW, like shore power, alternates smoothly with a sinusoidal curve that has a rounded peak and a clean ‘zero cross’ to a rounded valley. MSW produces instantaneous peak voltage for a few milliseconds, down to zero for a few milliseconds, then back to the valley. Now that we’ve covered the foundation of how AC operates, let’s address the bigger issue.
MSW (modified sine wave) power is usually sufficient to run many electronic devices with some distinct exceptions. One example is a typical digital clock. Some clocks tell time by using a charged crystal that has a consistent pulse which a microprocessor uses to calculate time with a simple algorithm. These clocks are not affected by MSW power. Other digital clocks use the incoming AC current to calculate time. The processor ‘counts’ how many times the voltage reaches zero, or ‘zero crosses.’ TSW (true sine wave) waveforms cross zero cleanly, while MSW waveforms ‘rest’ on zero for a few milliseconds. The processor could interpret this rest as multiple zero crosses resulting in the time being calculated incorrectly. Many electronic devices utilize digital time calculations as a function of their operation. Other issues that can occur with MSW inverters are many models of electric blankets, coffee makers, laser printers and other devices that regulate heat using a microprocessor that may not operate correctly.
Modified sine wave products are initially more economical than true sine wave products. In addition, MSW inverters have the advantage when the load is a simple induction load like a motor, or a resistive load like a light bulb. MSW inverters easily fill this role and typically use DC more efficiently than their TSW counterparts. However, with today’s technological advancements and the rapid proliferation of sensitive electronics that require true sine wave power to operate correctly, operators often now prefer the TSW inverter in lieu of the more limited MSW inverter, particularly when it can now be purchased for roughly the same price.
To answer that question, you really must consider how you want to use your inverter. The more complex or state-of-the-art your demand is, the more likely you will want and need to consider a TSW inverter. If, on the other hand, your demand is simple power and you have no aspirations of utilizing today’s sensitive electronic devices either now or in the future, a MSW inverter is the more economical choice.