Power Inverters

​An Inverter takes the 12 (or 24) volt DC current available from batteries and increase the voltage by about 10 times to create 120/240 volt AC current, as well as changing the current from “direct” (DC) to “alternating” (AC). As part of that process, the inverter will either produce a Modified Sine Wave (MSW) or a True Sine Wave (TSW – also known as Pure Sine Wave/PSW).

  • Modified Sine Wave – This type of power basically steps the wave – similar to a staircase going up and down – in an effort to emulate the pure wave of AC power. Inverters which use MSW will normally run the majority of your household gadgets without issue. That said, they will likely have issues with “electronic noise” and/or some items may not run at their full power. Even more important, sensitive electronics need a pure sine wave to run properly and you may cause permanent damage if you try and run them with MSW – so think hard about what you’ll be using your inverter for before you make your decision.
  • True Sine Wave – This type of inverter produces a pure wave – similar to a rolling wave on the ocean – going up and down in a smooth line like AC power. While inverters using this type of sine wave are usually more expensive, they are significantly better at dealing with sensitive electronics and/or making sure your items operate at their top performance level. If you plan to use your inverter for charging a computer, using a printer, dimming your lights or watching that new flat screen TV onboard – you should select a TSW inverter.

In addition to the wave type, inverters are also available in different levels of output power – which will often dictate whether they are portable or fixed mount. Most of the smaller, portable units are made to plug into a 12 volt socket style outlet. These inverters will usually provide up to 300-400 watts of power and are suitable for powering laptops, cell phones or some small appliances.  

Larger fixed mount inverters can provide up to 4000 watts of output power, depending on the model and brand you select. Fixed mount units will normally be hardwired to your battery bank and may even integrate directly with your AC wiring – which is why it’s best to leave their installation to the experts! At higher outputs they may come with a remote panel and often will be combined with the ability to perform as a charger when shore power or a generator is being used – highly convenient!

​Determining The Right Inverter Size For Your Needs

There are really only two types of power you need to know to choose the correct size inverter: Peak, or surge power, and the typical or usual power. Average Power is not usually a factor in determining inverter size. 

  • Surge is the maximum power that the inverter can supply, usually for only a short time – a few seconds up to 15 minutes or so. Some appliances, particularly those with electric motors, need a much higher startup surge than they do when running. Pumps are the most common example – another common one is refrigerators (compressors).
  • Typical is what the inverter has to supply on a steady basis. This is the continuous rating. This is usually much lower than the surge. For example, this would be what a refrigerator pulls after the first few seconds it takes for the motor to start up, or what it takes to run the microwave – or what all loads combined will total up to. (see our note about appliance power and/or name tag ratings at the end of this section).
  • Average power would usually be much less than typical or surge and is not usually a factor in choosing an inverter. If you run a pump for 20 minutes and a small TV for 20 minutes during a one hour period, the average might be only 300 watts, even though the pump requires 2000. Average power is only useful in estimating battery capacity needed. Inverters must be sized for the maximum peak load, and for the typical continuous load.
Power Ratings of inverters

Inverters come in size ratings all the way from 50 watts up to 50,000 watts. The first thing you have to know about your inverter is what will be the maximum surge, and for how long.

  • Surge: All inverters have a continuous rating and a surge rating. The surge rating is usually specified at so many watts for so many seconds. This means that the inverter will handle an overload of that many watts for a short period of time. This surge capacity will vary considerably between inverters, and different types of inverters, and even within the same brand. It may range from as little as 20% to as much s 300%. Generally, a 3 to 15-second surge rating is enough to cover 99% of all appliances – the motor in a pump may actually surge for only 1/2 second or so.
  • General Rules: The inverters with the lowest surge ratings are the high-speed electronic switching type (the most common). These are typically from 25% to 50% maximum overload. This includes nearly all the inexpensive inverters in the 50 to 5000-watt range. The highest surge ratings are the transformer based, low-frequency switchers. Surge ratings on these can range up to 300% for short periods. While high-frequency switching allows a much smaller and lighter unit, due to the much smaller transformers used it also reduces the surge or peak capacity.
  • Pros and Cons: Although the high-frequency switching type doesn’t have the surge capacity of the transformer based, they do have some definite advantages. They are much lighter, usually quite a bit smaller, and (especially in the lower power ranges) they are much cheaper. However, if you are going to run something like a submersible high power pump, you will need either very high surge capacity or you will need to oversize the inverter above its typical usage, so that even at maximum surge the inverter will not exceed its surge rating.

Battery Bank Size Related To Inverter Capacity

​It is also extremely important to make sure your battery bank capacity is large enough to support these needs and that you can maintain your battery voltage while using your inverter. The higher the wattage and the longer you need to use the item may overrun your battery bank’s capacity to cover those needs – so you must take your battery bank’s amp hours into consideration when deciding what you can (and can’t!) run on your boat with an inverter.  


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