Backup Power System, Part 2

Continuing on the subject of a backup power supply for your house, I want to discuss the inverters a little more. There are things to look for and things to watch out for!

I have owned lots of inverters, ranging from 150 watts to 12,000 watts. Some were high end and some were junk. I have even built my own inverters like the 5548 that is on my 48v system and a 3648 for the electric jalopy. In inverter parlance, the “55” in 5548 means 5500 watts and “48” means 48 volts. Given my Scottish ancestry, I am always looking for a bargain, but I have enough experience with this stuff that I can tell you that the ones with the higher price tags may be the better bargains.

One thing you will see in inverter specs is the type of waveform the unit outputs. The power from the power company is a sine or sinusoidal waveform. If you don’t know what that means you can look up a photo on the internet. It is very smooth. All of your equipment likes it. Early inverters used a square wave. Light bulbs and some appliances were ok with that, but motors, chargers and other things were not happy at all about that. These are pretty much gone. They were followed by the “modified sine wave” inverter, which actually makes a modified square wave, giving the peaks needed for most equipment. I have heard all kinds of warnings against running your microwave or other things on them. I have run just about everything on them and never had any trouble, but your ceiling fans will buzz! These are rapidly being superseded by sine wave inverters as the electronics technology has advanced. Motors run cooler on these and these are your best bet in the bigger inverters.

Another term you will see is low frequency or high frequency. The power coming out in a USA model is 60Hz (pronounce Hz as Hertz) or cycles-per-second, as we old timers say, and that’s with the high OR low frequency model. The low frequency unit creates the 60Hz directly through a great big transformer. Doing so gives a little bit of an advantage for surge power when starting motors like a saw, air compressor or A/C. The high frequency models use little transformers that are very efficient at high frequencies to step up the voltage and then switches it back and forth at 60 Hz to form the output. Most of these have the modified sine output, but there are pure sine models on the market, too.

Which do you want? It is really like having a pickup truck with a conventional V8 next to one of the new ones with a turbo 4 cylinder. One lugs along slow and steady and the other buzzes along at higher rpm, but both will haul the load. If you want to carry it around, you’ll probably want the high frequency model. If you are going to bolt it down for the long haul get the low frequency unit. Your bigger models are likely to be low frequency.

Some inverters, like the ones I built, just invert. They just step up the power from your battery to 120 or 120/240 volts. That may be all you need. Consider, though, the advantage of an inverter charger. If an outage runs long, or if you are using in a solar installation, you may need to top up the battery with a generator. The inverter charger can help there. MAKE SURE that the charger will let you set the charge rate. This is very important. If you have, say, a 5548 inverter that can charge at 80 amps and only at 80 amps and you have a 2kw generator, then you have a problem because 80×48 = a whole lot more than 2000 watts. The better inverter chargers will let you set a percentage charge rate so you won’t injure your generator or the batteries. I would love to add that feature to my homebrew inverter, but that would require thinking. These days, thinking is reserved for remembering where I left my phone and keys.

An automatic transfer switch (ATS) is another grand feature to have and it is in most of the bigger inverters. What this does for you is it lets utility or generator power go through to the house and switches to inverter/battery sourcing if the primary source fails. Most of these have some selectable variations. For example, Tom runs his house on the inverter anytime the batteries have enough power and these are charged by a 10kw solar array. He does not have a big battery stack, so if the A/C runs a lot on a summer night, the inverter may switch to the power company at a preset state of charge. Alternatively, the inverter could start a generator if a hurricane took out the utility power and the batteries got low.

Some inverters also have a “sell” mode. This lets you tie a solar system into the grid to sell power to the power company and/or use the grid to store your power for later. This requires some paperwork and an agreement with the power company, especially where these new smart meters are installed. Tom’s power company won’t allow grid-tie and buy his power, so that is why he uses both, and the inverter figures out which source to use at any given time. You don’t need sell mode for a backup system and you may not need some of these other features, but they may come in handy if you get the urge to add solar, later!

Now, let’s go over some things of concern in buying an inverter. First is rated power. A lot of inverters you see on Ebay don’t even come close to spec. I saw one rated at 3000 watts, about the size of your hand and priced at $33. Folks, you will not find a 3000 watt inverter for $33 and you can’t make one that small. I blew up the photo and saw that the label said “300 watts,” which is more likely. I corrected the seller. Instead of correcting the rating, he photoshopped the label! Somewhere in the fine print there was a mention that it was 50Hz. You don’t want that if you are in the USA. Some of these cheap rigs are 240 volts, too, and I don’t mean split phase 240/120. You don’t want 240, I bet. Heaven help you if you order an inverter from Thailand or Malaysia!

Look for continuous rated power. A lot advertise at a peak rating, so that 10,000 watt inverter might be good for 2500. A unit that John sells rates it in levels. This is a good sign. You may see that it says

4800 watts forever, 5500 watts for 20 minutes and 15,000 watts for 3 seconds. These aren’t exact numbers, but this is pretty typical for a good unit. Look at the weight, too. Real watts require real pounds of iron and copper in the transformer. My big inverter (rest in pieces) weighed 185 pounds! One well known Ebay Chinese inverter uses great electronics, but they have gradually used smaller and smaller transformers. My 8kw Chinaverter has one transformer…they used to have two and they weren’t big enough, then. The result is that it can put out 1600 watts continuously and the transformer smells bad if you try to use more for a long time…but it doesn’t weigh very much! On the other hand, my ancient Trace 2524 will put out 2500 watts all day, but bring a friend if you want to move it…it has a big transformer.

Can you parallel the inverter? That means run two of them lashed together for twice the output. Tom bought a 5500 watt inverter from Sun for his entire all-electric house and he had to compromise with the loads. He could not run stove, A/C and clothes dryer at the same time, for example. He added another in parallel and can run anything he wants, now. If anything happens to one of them he can fall back to the other until repairs can be made. It is a nice feature in case you want to expand, later.

One other concern that you might not have considered is service. My 12kw unit came from an American company. I bought it used and did not do much research. Tom had used it for a number of years and it worked great. After the lightning strike, I went to order schematics and parts. It quickly became clear that the guys at the American company did not have schematics or know anything about it. They just screened their name on a decent Chinese inverter and marked it up. The price of replacement circuit boards indicated to me they were robbing parts out of a new inverter. Looking under the hood, I discovered the circuit boards were designed by the same guy who designed my wimpy Chinaverter!

I used to design circuit boards, so I can spot a designer’s trademark. So, the moral of the story is, search around on the internet to see if anybody does service or sells parts and manuals. I checked one of John’s nice rigs and the signs were good. They may have some more info on that subject at Sun, so it would not hurt to ask.

A final consideration is a seller who will back his gear. Consider anything you buy directly from China as disposable. Trust me. Again, I don’t work for Sun Electronics…I am a customer and this is a true story: something I bought from John two years ago went bad. I contacted John to see how the warranty worked with that product. He said he didn’t know, but HE would replace or refund it. He put me in touch with Louis and after sending in some photos showing the problem, Louis said he would give me a credit. No hassle. Done. Where else are you going to find that?

Next time I’ll continue onto the subject of batteries and other considerations. In the meantime, if you just can’t wait to get started, you should at least have some ideas on what you want and need. Call John or any of the sales engineers at Sun Electronics and they can get you started.

By: Neal Collier