Selecting a Power Inverter for When the SHTF

September 13, 2012

Solar Energy

using a power inverter in a grid down situation

The following article has been contributed by a fellow prepper named Mike. It has been published with permission of the author. It does not necessarily represent the views of

In recent articles, we discussed the use of both solar and wind power; living off the grid when the SHTF.

For years I have been a 12 volt man aboard our boat.  Twelve volt DC systems were always simple, safe and served their purposes.  But after a cruise aboard our boat some months back where we had to continually power up the generator to supply electricity to the 115 volt ac freezer to maintain temperatures, I began to consider alternatives.  We put over 90 hours on the generator in just 5 days; time for another oil change!   And when the SHTF, running a diesel generator for prolonged periods is not going to be a good idea; diesel fuel will likely be in short supply.

So I began to consider installing an inverter to power the freezer.  But what is an inverter anyway?

Selecting a Power Inverter

An inverter is an electrical device that turns 12 volt battery direct current (dc) power to 120 volt alternating current (ac).  But there were so many selections from which to choose, where do I start.

When it comes to choosing a power inverter for your boat/home the selections can be confusing. With so many choices on the market it is important to understand what the differences are so that you can make the best choice for your own application.

Your first consideration will be wattage. Inverters are sized by the wattage they can handle.  Consider what items you will be running and how many amps each item requires. You will need to also consider how many of them you will be running at once to determine which size inverter will be the best for your needs. You do not want to overload your inverter by plugging in too many high wattage items at once. You will probably notice that the cost of inverters rise with the amount of wattage.  You can get the wattage by looking at the manufactures label on the appliance or if only the amps are there use the formula (amps x 115 volts= wattage) to convert to watts.

The next consideration will be whether you should get a true sine wave inverter or a modified sine inverter. A true sine inverter is one that will give you a nice current just like you get from the power company.  A pure sin converter will operate virtually everything you may have where a modified inverter may not power certain devices.

A modified sine inverter is slightly less pure.. The modified sine inverter was developed as a more cost effective alternative to the true sine inverter. You will find that although the pure sine inverter will provide the best electrical current, it also will be the most expensive.  An 1800 watt modified sine inverter will cost you about $350.00.

Although the modified sine inverter is cheaper it has its own drawbacks. For most small appliances, such as a refrigerators or hair dryers, a modified sine inverter will be sufficient. However, with some items like plasma televisions you may notice a lack of precision. In this case a true sine inverter may be a better solution so that you can have a better quality output.  It is also important to remember that a modified sine inverter cannot be upgraded to a true sine inverter.  You will want to make sure you are buying the correct inverter for your needs at the start.  As a side note, we found out that a modified sin inverter will not power an induction cook top.

It is a good idea to inquire about overload shut off indicators or switches on your inverter just in case you over load the inverter; most inverters will have them as standard equipment. You will also want to check to find out if the inverter has thermal and short circuit shut down to prevent those types of emergencies also.  In addition, some inverters have built-in shut down capabilities if the battery storage levels get to a certain low point.

I chose to install a Xantrex 1800 watt modified sine inverter capable of 1000 watts of continuous load.  This inverter has a built-in transfer switch that transfers between shore and inverter power.


The final consideration is the battery bank you will be drawing from.  Your inverter will be pulling from this battery bank and you must be able to calculate how long the inverter can operate before a recharge must take place.  Look at your battery to find the amp-hours for them.  Amp-hours mean amps time hours.  A battery bank of 2 8D batteries will have about 580 amp hours.

So I know that my freezer pulls 7.5 amps and runs for 20 minutes each hour which equals about 8 hours per 24 hours.  So the total amp-hours the freezer will use per 24 hours are about 60; all the other usage gives us a draw of about 112 amp hours.  .  So technically we could run the inverter for about 5 days without a recharge, but can we?

It isn’t good to run a battery all the way down to zero.  Battery life is directly related to how deep the battery is cycled each time. If a boat battery is discharged to 80% every day, it will last about twice as long as if it is cycled to 50% of full discharge every day.  If cycled only 10% of discharge, it will last about 5 times as long as one cycled to 50%.

This does NOT mean you cannot go to 80% occasionally. It’s just that you must have some idea of the loads, you should figure on an average discharge of around 50% for the best storage vs cost factor. Also, there is an upper limit – a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the lead dioxide tends to build up on the positive plates.  I prefer to cycle my batteries down only 30% from full before recharging.

Monitoring your batteries can be estimated but I prefer to use a Trimetric 2025 monitoring device.  This electronic device allows me to monitor the battery banks amp-hours use precisely. I gave you the location to buy this in the solar panel article.

With some careful planning an inverter can make your SHTF a more pleasant experience. After you choose the inverter that is right for you, make sure to have it properly installed. If you are not sure about working with electricity, it will be best to have a professional do the job for you.

We took another 4-day cruise over the last New Year holiday with our new inverter and I am proud to say we only used the generator 6 hours in 4 days.  Life is good.

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8 Comments on “Selecting a Power Inverter for When the SHTF”

  1. Jack Says:

    This is a timely article for me. I just purchased a15 watt solar panel, and am looking at Costco for two or three deep cycle marine batteries. Probably two due to cost, and an inverter, and the proper wiring… I bought a “Cube Igloo Cooler” ( on sale for 29.00 at Lowes. Because of the handle, I am dropping the batteries in there, and will also mount the inverter inside on the lid and create a mount for the top of the “cube” for the inverter if I want to mount it externally if the power goes out. I’ll then run wires from the solar panel/charge controller to the cube and the batteries…


  2. TLSmith Says:

    Jack, a 15 W solar panel will charge your batteries in about 2 weeks. Also, do not mount your inverter in the same box as your batteries. The out-gassing of the batteries will corrode your electronics. Just add a little more solar power and you will be happier with it.


    • Jack Says:

      Great advise! Thanks so much. So, would you say 30 / 45 watts of input would be sufficient for accelerated charging?


      • TLSmith Says:

        45 watts @ 12v will give you 3.75 amps. So on a great sunny day that is what you would get for a few hours. You get about 80% efficiency into a battery so figure best case getting 3 amps going into the batt. If it is a 100 ampere batt that would be about 33 hours to full charge. So say 6 hours solar charging per day would make it about 5 days to charge.



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