ExhaustNotes Off-Grid: Which Is The Best Inverter For Me? Part 1

Here at ExhaustNotes we are not earthy crunchy, granola-eating oil-haters. Far from it as most of the fun things we like to do involve burning refined oil or playing with toxic products derived from oil. Hell, our entire 45-year working career depended on an oil-based economy. We’re not about to turn our backs on our old friend Petroleum. At the same time we enjoy building off-grid power systems isolated from The Man and his ever-increasing system development surcharges, power outages, readiness to serve fees and base facility charges.

It’s a shame that off-grid energy has been politicized along predictable fault lines because it really is nondenominational and serves everyone equally regardless of which clan they voted for. ExhaustNotes Off-Grid is not going to try to convince you to wear a tie-dyed shirt or stock up on ammunition for the coming civil war, but if you can shake off those tribal chains we think you’ll find off-grid energy appeals to the anti-government Far Right insurrectionist, the communist Far Left insurrectionist and that vast sea of reasonable anarchists who find themselves trapped between two crazy extremes.

At ExhaustNotes we are not electrical engineers, but with modern, home-based off grid equipment you don’t have to be.  It’s so easy a child of 35 could do it. We go off grid not to make a point but because it’s more reliable than grid power, it’s nearly on par with grid cost-wise, the watts per dollar only get better the longer you operate your system, and it’s fun. One beauty of going off grid is its scalability: You can go full hog and cut the cord forever or take baby steps to energy independence.

This blog series will cover standard, easily-available systems found in houses or sheds. Grid-tied systems are not part of this series. There’s nothing wrong with grid-tie, but to my mind grid-tie systems defeat the purpose of going off grid in the first place: The Man still has his sticky fingers in your business. I’m going to ignore grid-tie because I don’t build those systems and I don’t want to get bogged down in things I know nothing about.

I’ll kick off with inverters. I have a bunch of the damn things. Inverters are kind of like backwards battery chargers. Instead of taking relatively high voltage alternating current (AC, the standard house power from the grid) and converting it into relatively low-voltage direct current (DC) to charge a battery, inverters change DC into AC. I say relatively high or low because the voltages we will be working with are 240-volt down to 12-volt. In the big picture, 240-volt is not that high. Many different voltages can be used in off-grid systems and sometimes the DC voltage is as high as the AC voltage.

AC voltage is fairly easy to step up or down using simple transformers. It’s the reason Tesla’s AC system beat out Edison’s DC system in the early years of electrification. Changing DC into AC and stepping up the voltage takes more electronic components. It requires an inverter.

The simplest and cheapest inverters are called square wave inverters. These are getting harder to find due to inherent issues with their power output. Like all non-rotating inverters they flip the DC polarity back and forth 60 times a second (in the USA) to create alternating current then step that up to a higher voltage resulting in a square wave pattern. The null period between the two voltage peaks is blazingly fast, essentially zero. These inverters produce a waveform that is nothing like what you are getting from the grid. Square wave inverters will operate a lot of things but certain electronic equipment may not like it. Brush-type power drills, incandescent lights, resistance type heaters should work with square wave but I’m not promising anything.

If you test a standard, 120-volt square wave with a voltmeter it will probably show around 90 volts. This is due to that waveform switching back and forth instantly. Power under the curve is the closest way I can describe it: a normal grid waveform spends a lot of time rising and falling 60 times a second. The voltage rises, drops to zero then rises in reverse. For grid power 120 volts is an average. What this means is that over 1/60th of a second your electrical component is getting 120 volts some of the time, more than 120 volts some of the time, and no power at all some of the time. Electrical devices are built to this standard. A full, 120-volt square wave would most likely overheat whatever you plugged into it.

Modified sine wave inverters are an attempt to more closely mimic grid power. While a modified sine wave inverter is an improvement over square wave they are still sort of crude. Modified sine wave inverters are popular and will run most things but there’s still the possibility of frying a device if it happens to be really sensitive. Modified sine wave inverters cost a bit more than square wave inverters.

True sine wave inverters, also called pure sine wave inverters, are the most expensive and the closest thing you can get to grid power, maybe even better. These inverters produce a nice, clean waveform that looks the same as grid power to your electrical equipment. True sine wave inverters will run anything that you can run from grid power assuming the inverter has the capacity. True sine wave inverters used to be very expensive but the price has come down quite a bit. They still cost a lot more than a square wave.

For example, a 12-volt, 5,000-watt modified sine wave inverter like this one costs around 400 bucks.

While a 12-volt, 5,000-watt true sine wave like this one costs 1200 dollars, three times as much.

If you can afford it, always get a true sine wave inverter. If you’re really skint get a modified sine wave inverter, just assume the risk when plugging anything like computers or microwave ovens into the thing. If you just want to run incandescent lights, maybe fluorescent lights and simple, no-circuit-board electrical things get a square wave but be warned: I nearly fried my air compressor on a square wave and burned up a cell phone charger on one. I don’t buy square wave inverters anymore.

Inverters are sized in output watts and input volts. They are labeled sort of tricky some times. You’ll find a unit may be 12,000 watts surge capacity but only 5000 watts continuous. To make life simple ignore surge capacity and use the continuous rating. Now that you’ve decided to get a true sine wave inverter (if you’re smart), determine what your average operating load will be in watts and plan for an inverter with twice as much output. For a small house or shed I’d go with 5000 watts.

Since inverters need a DC power source, like batteries, you’ll also need to choose what input voltage your inverter will draw from your storage batteries. 12-volt DC input is most common for smaller inverters but as inverter output rises the input voltage usually goes up also. 24-volt and 48-volt DC input is normal on larger inverters.

Higher input voltage from the batteries helps keep wire sizes and voltage drop within reason. A 12-volt input, 6000-watt inverter running at full chat will be sucking 500+ amps out of your 12-volt battery bank. A 500-amp load requires huge battery cables to carry the current and puts a lot of stress on electrical connections. Terminal posts tend to get hot with 500 amps flowing through them. At max load that same 6000-watt inverter with 48-volt input will be sipping a mere 125 amps from your 48-volt battery bank. The reduced current at 48-volts allows for smaller and cheaper cables. The cost savings is significant. Big copper battery cables are expensive. You can use the money you saved on cables for more batteries.

With smaller inverters output power is usually single-phase 120 volt AC. They often have standard duplex outlets built right into the machine. These are a good way to gain a little independence from the grid without having to mess with high voltage wiring: You simply plug things directly into the inverter and go to town. You’ll only be able to run 120-volt equipment but most house stuff is 120-volt.

Another option is 120-volt/240-volt split phase output. These inverters usually require hard wiring to the output and you’ll need a breaker panel. No easy plug-ins. This setup is nice if you have 240-volt things like an air compressor or small South Bend Lathe you want to operate from the inverter in addition to regular 120-volt equipment. A split-phase inverter is kind of like two single-phase inverters joined at the hip, the hip being the neutral leg in this case. One half the total output is available on each of two, 120-volt circuits and all the output is available at 240-volts. On a 6000-watt split-phase you’ll be able to draw 3000-watts of 120-volt power from one side, 3000-watts of 120-volt power from the other 120-volt side or 6000-watts at 240-volts. Note that you cannot draw 3000 + 3000 + 6000 watts all at the same time. You’ve only got 6000 watts total no mater how many ways you divide it up.

In my off-grid shed I run a 24-volt Aims 6000-watt, true sine wave, 120/240 split-phase output inverter that feeds a standard, household breaker panel. From the panel I have circuits for lights, outlets, and 240-volt outlets. I went with 24-volt input because I run LED, DC lights and a DC water pump. It’s harder to find these items in 48-volt. My system has been operating for two years without any problems. The inverter is kind of a watt-hog as it draws 3-amps “on” with no load attached. The Aims inverter has a pulsing, sleep circuit that reduces the no-load draw to 1 amp but it only senses one side of the 120-volt output. Turn on a load that is connected to the non-sensing side and nothing happens; the unit won’t wake up and make power. I don’t use sleep mode but it’s a good energy saving feature if you can make it work.

Another farkle I don’t use on the AIMS inverter is the pass-through relay. The pass through relay automatically stops the inverter when generator power is feeding your system. The generator power passes through the inverter relay and powers your shed/house/whatever. In addition the inverter can be set up to switch to battery charge mode when the generator is powering your load. I do the switching manually because I like to keep the inverter’s job as simple as possible.

The AIMS battery charge mode is great. If you kill the batteries you can input 240 volts from a generator and the inverter will become a 24-volt, 85-amp battery charger. This charger function is useful for cloudy or snowy days when your solar panels aren’t working. Of course I have my system set up for manual operation of the inverter charging. Like most inverters the AIMS shuts off at a selected low input voltage so as to not kill the battery bank completely or harm the inverter from operating on low voltage.

To give you an idea of what 6000 watts will do, I can run a small, wire feed arc welder or a ¾-horse air compressor. I use traditional power tools, a concrete mixer and a spot welder. I have yet to find something it won’t run, I just can’t run everything at once.

A super easy way to take the first steps on your path to grid independence is this little 300-watt, true sine wave, 40-volt input Ryobi. It doesn’t have a lot of power but if you already own 40-volt Ryobi tools like I do it makes a nice emergency back up power supply. You can run a lamp or an Internet router, maybe a small monitor or television. Also you can charge phones with a couple USB ports. Ryobi makes a 150-watt unit but 150 watts is too small to be much use.

If you don’t have 40-volt Ryobi batteries this Bestek true sine wave, 500-watt, 12-volt input inverter will turn any 12-volt car battery into a 120-volt power source. If your power goes out during a big storm clip the leads onto your car battery and you’ll have back up power not reliant on The Man. Just remember to run the car every hour or so to recharge the battery or just leave the car running. At 500 watts you can do some damage with this thing (maybe even run a small refrigerator).

I mentioned earlier about rotating inverters. Back before the advent of fancy, complex inverters a brush-type DC motor turning an AC generator provided a simple, reliable method of inverting DC into AC. The waveform it produced was true sine. It wasn’t that long ago either as I worked on this type of inverter back in the 1970s. I’m guessing it wasn’t as efficient as the new ones and the frequency may drift a bit. The motor-inverter used a lot of copper compared to modern stuff but it worked ok. If you find an old rotating inverter in a junk pile grab it; it would be fun to mess with.

That’s enough inverter chat, it’s too much geeking for me. My head is spinning from typing about it. Hopefully this will help you decide which inverter will suit your needs. If you’re interested in this sort of stuff we’ll cover batteries, solar panels, generators, wind generators, loads, and how to tie the mess together in the future, so be sure to subscribe to our ExhaustNotes.us email alerts and you won’t miss a thing. Unless of course you want to miss a thing.

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