How to Choose the Best DC to AC Power Inverter

I love the freedom that comes with an RV, be it a popup camper or small motor home. The idea of covering vast distances and experiencing new people and places, while maintaining control over what to me is essentially my most important personal space, the place where I sleep, is quite appealing. tent camping

Whether your passion is camping, travel, RV'ng or just hanging (hiding?) out in your remote cabin in the woods, at some point you are going to want to (or wish you could) generate 110 volts A/C electrical power. So, you are going to want to know what to look for and how to choose the best DC to AC power inverter for your needs and budget.

Jim, We Have a Problem Already!

There is, however, a big problem right off the bat as it were, with converting the 12 volt vehicle or camper power into AC current of the type at home. The problem is, quite simply, that it is cheap and, therefore, easy to do. Truly, the actual problem is that there are hundreds (thousands?) of cheap inverters on the market that will do the job of DC to AC power.

The problems common to these cheap devices are with the low quality AC electrical current they produce, their low conversion efficiency and, therefore, the heat they create when doing power conversion.

Additionally, the modified or square wave form of the output power of the lower quality inverter is often a poor match for the electrical appliance or tool using that power.  This results in less than optimal performance from the device, more heat, reduced service life, etc.

Circuit of push-pull DC to AC converter
Circuit of push-pull DC to AC converter

 

All AC Electrical Power Is Not Created Equal

 

The electrical appliances, tools, and electronics in your home were all designed around the use of AC or alternating current of a certain frequency. In the United States this means a 110 volt electrical ‘pressure wave' that is reversing direction 60 times every second, is available for tapping into wherever you see a wall receptacle.

The only reason we survive at home near our wall outlets is that air is a poor conductor. But stick a conductor (metal is a good one) into an outlet, specifically the smaller slot on the right and you will feel the power of AC. (No, don't ever do that!)  man being electrocuted

We can produce our own, portable electrical power away from home, and in fact we do it every day when we drive our cars. It is the conversion of this power to AC, and more than that, the right amount and with the right frequency wave that our electronics require for their proper efficiency and best performance. These are details that create the most mischief.

Producing AC power that is ‘noisy' causes inefficiencies in the tool or appliance, greater heat and thus more ‘wear and tear'. In certain electronics such as televisions, stereos and other amplifiers, this is noticed as problems with the screen image or a buzzing sound through the speakers.

Tools run hotter and do not perform as well compared with power that is not noisy.

A Wave Form is Worth a Thousand Words…

A brief illustration is in order of what ‘clean' electrical current looks like versus ‘noisy' current.

Remember that DC current just flows in a straight line and does not oscillate, so it has a frequency of zero. AC current ‘wiggles' for lack of a better word. The amount of wiggle, when plotted on a simple graph, can be assigned numbers both in terms of how large the wiggle is relative to the Y axis, and how far apart are the wiggles from point to corresponding point on the next wave, such as the very peak of the wave to the very peak of the very next wave.

At home, these waves come through the wall receptacle in a nice, smooth, analog or continuously changing wave form. This is represented like this: So every one second, you could count 60 peaks in the voltage wave at your wall outlet. This is what is referred to as the power frequency, which makes sense when you think about it.

Now, making AC power on a budget is great in a pinch, however, this usually results in a wave form that is not smooth and natural, but rough and simulated. Here I am talking about square waves, rectified waves, half waves and other modified sine waves. This is what the above graph looks like when you don't throw enough money at optimal AC power production:

Why does the modified sine wave always remind me of southwestern art?

Anyway, you can still make out the overall oscillation or cycling with half of the wave above the mid-line and half of it below. But missing is the smooth transition or wiggle as with the pure sine wave in the first illustration.

Don't misunderstand me, the modified wave gets the job done, but if your electrical equipment could talk, it would gripe about it. You will notice the extra heat, and the reduced efficiency would also require you to operate fewer appliances simultaneously than you otherwise would with a pure sine wave power source.

Your Vehicle's Battery is Doing it Wrong

If only there was an easy way to store some of the AC power you receive at home to use later at the cabin or campsite.

Wouldn't it be neat if standard equipment on a car's dashboard were the standard household wall receptacle? I have always thought this idea was a no-brainer million dollar maker for work trucks. But I digress!

Most car batteries produce only about a tenth of the voltage required to operate a household appliance, and its current flows from the battery without any oscillation or frequency, at least not until you add other components to the circuit to which it is connected.

The vehicle battery was simply not designed to power your blender and hair dryer at the campsite – its job is to provide the high amount of direct electrical (DC) current to your ignition system and starter motor, so your big-block Chevy engine can spool up and run.  car with raised hood

We can, however, use all that energy stored in the battery to produce the electrical current we need for our DC powered ‘stuff' by incorporating a couple basic concepts. The first is keeping the amount of energy stored in the battery from becoming too depleted and causing battery damage. We do this by running the car's motor and alternator to charge the battery, or perhaps connecting one or more solar panels and a charge controller to it and replenishing it that way.

We also can convert its DC power into usable AC power by attaching an inverter.

Why Not Just Make Your Own Inverter?

Imagine a very crude system where you connect your car's battery to a simple switch – like a broken doorbell that never stops making sound and instead alternates between two pitches. This switch reverses the flow of current back and forth about 60 times a second. (don't worry, you would have to push the button only half as fast, or 30 times per second for this to work!).

After the doorbell in this circuit you would connect your positive and negative wires to a copper coil which has been wound around an iron core. This Primary copper coil shares the core with a second copper coil, called the secondary.

What you have with the two copper coils wound around the iron core is a transformer. By varying the thickness of the wire and amount of wire used, the transformer changes the electrical frequency to a higher or lower voltage. Really pretty simple.

With your broken doorbell switch sending and receiving power to the primary coil, what you will end up with at the end of the secondary coil is ‘transformed' current, or in this case electrical current stepped up to 110 volts AC current. This happens due to the switching of the direction of the DC input current and the variation of the copper wire winding around the iron core they share.

This is a rudimentary description of the work done inside the inverter. The inverters being sold today contain sophisticated circuits that include components such as capacitors and inductors to filter and adjust the waveform of the current. Low-pass filters that remove harmonics from the waveform, rectifiers and diodes to control the semiconductors (switches) so they perform properly with inductive loads (motors).

You do not need an electrical engineering degree (although it would help!), to understand the basics of what is going on inside your inverter, and the fact that there are a staggering number of variables that determine the quality and therefore price differences between inverters. This despite the fact that two inverters rated at 1000 watts will each produce generally the same amount of electrical power.

So You're buying an Inverter – The Bigger, The Better, Right?

Not so fast!

To avoid the disappointment that comes with doing all the research, spending the money and time to acquire and mount your new inverter only to realize that it is mis-sized, you will want to think about your average power requirements and select the inverter with the best fit.

Renogy 2000W 12V Pure Sine Wave Solar Inverter
Renogy 2000W 12V Pure Sine Wave Solar Inverter

Some attention to detail here will avoid your inverter being flogged to produce too much power or by being mostly idle and gobbling up resources for essentially, not much work. Either case is bad, but over-demand may be worse because will be constantly tripping and resetting breakers, and you will be continuously coordinating which tools and appliances are being used to avoid another self-inflicted black out.

Selecting a pure sine wave inverter that is about 20 to 25 percent larger, expressed in wattage, than your average power demand (within practical reason) would be best.

Keep in mind that even the best inverters lose about 10% of rated power in the conversion process. Other words, a 1000 watt inverter has a useful power output of about 900 watts. However, the better inverters will be able to deliver those 900 watts all day long. The budget ones may well leave you wanting as they struggle with heat and inefficiency and therefore are constantly flirting with disaster. With luck, your cheap inverter is only popping breakers or fuses and once it cools, you can use it until the next (hopefully) temporary failure.

Without unnecessarily complicating this, you inverter must have a higher VA rating than your required power requirement expressed in watts. This VA (volt-ampherage) to wattage ratio is referred to as the power factor.

I would suggest you consider simply adding up the total wattage of those things you will want to be able to run simultaneously and add 25% to that total, to arrive at the VA number you need. This should be close but also conservative with respect to your actual power factor ratio.

What About the Battery?

Renogy 12V 100Ah Deep Cycle Pure Gel Battery
Renogy 12V 100Ah Deep Cycle Pure Gel Battery

Batteries introduce yet another set of variables that can make, or break, your power inverter system. A battery with a low Ah performance (Amp-hour) will sabotage the best inverter by draining too fast to be practical.

Your options here are buying either the highest capacity battery possible, commonly around 200 Ah, buying two batteries and a two-battery inverter, or both.

Personally, I like the idea of a two battery setup for the extra reserve power such a setup affords as well as the redundancy of the second battery which adds a layer of safety and usefulness when I am far afield.

Keeping the battery or batteries charged while on the road is the final link in the chain. Producing your own DC power is best done by mounting a solar panel, or several in parallel, (all positive leads wired together, all negative leads wired together) and connecting them to a charge controller, which in turn connects to your battery or battery bank.

The important point here is that battery ratings are great for comparing one battery to another. However, real world application means something entirely different.

Take a hypothetical 100 Ah battery. This may mean that it can produce 100 amps of power per hour, but you would destroy the battery if you could even draw that much from it! You should never discharge a battery beyond, or really very close, to 50 % of its rated capacity. Violating this rule will damage the internal plating inside the battery and impair or destroy its ability to store power.

Batteries should be chosen carefully based on their capability, and monitored closely to keep them at full performance for the longest possible service life.

It is difficult to determine the proper battery, inverter, and solar panel sizing for every situation because of the many considerations relative to both the power generating and storage equipment, the variety of appliances to be powered and the environmental conditions in which this all occurs.

To complicate things even further, when planning an inverter system,  every wire, junction, terminal, etc, adds its own load or bit of resistance to the circuit. The beautiful-mind guy may be able to work it all out on paper beforehand, but even he would likely have to field-test his setup to see if he got it right!

 

Conclusion

I may have actually created more questions in your mind than answers regarding how to choose the best DC to AC power inverter, but that was actually my intent. This subject is much more difficult to tackle than it at first seems.

Hopefully you now at least have a basic grasp of the different inverter types and the issues you will face when producing your own portable AC power.

Choosing an affordable system that is also capable enough to live with without frustration is no small feat.

If you are brand new to this and have an immediate need for a solution, I would suggest looking into a package that has been pre-designed to address the obstacles I have outlined in this article.

There are plenty of solar ‘cabin kits' on the market at really any price point, where the assumptions about usage and input, mounting, wiring etc, have all been made.

Renogy has a few as well, here is their 200 Watt, 12Volt Solar RV kit, which you could pair with their inverter and even batteries to make a complete system that will perform well.

Renogy 200 Watt 12 Volt Monocrystalline Solar Starter Kit
Renogy 200 Watt 12 Volt Monocrystalline Solar Starter Kit

 

The possibilities are truly endless. Quality and durability are always at the heart of any successful system. The best advice is to choose sine wave inverter unless you have a tight budget and very temporary requirements, and try to approximate the power requirement to your actual needs.

 

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