Unclean Power - Don't Make This Off Grid Mistake
Let me preface this by saying that any off grid system can produce power that is just as "clean" if not "cleaner" than what you get from the power company--IF you choose the right inverter. That's why I'm letting you know about the 'unclean power' option so you don't make the mistake we made.
Background for Newbies
First, a little background for those who are new to off grid systems. If you've never heard of an inverter before, it's a component in a renewable energy system that converts DC power (what is stored in batteries) into AC power (what you use in your home). If you want to learn more about it, I covered this question in my blog post: Renewable Energy Systems Overview. Most quality inverters also have a charger built in. The charger is used when your battery bank may need to be manually charged with a fuel powered generator. There are two main types of inverters--sine wave and modified sine wave.
A quality sine wave inverter will produce power that is just as "clean" if not cleaner than what you get from the power company right now. Hands down, that is the first choice for any off grid system. Then there are modified sine wave inverters, which may produce a varying "quality" of power (i.e. better models may produce decent power, while the cheapest models put out awful electricity). For years, the only reason why someone would consider a modified sine wave inverter was because of price, which was roughly half the cost of a comparable sine wave inverter. This is what I call 'unclean' power.
Why This Could Impact You
But things have changed over the last few years. Now, the cost of sine wave inverters has dropped and the price of a quality modified sine wave inverter has risen to where there is not a large difference between the two. If that is the case, why would anyone even consider buying a modified sine wave inverter? Why am I even writing this article?
If your budget allows buying a brand new sine wave inverter, then there is no point in reading any further. This article is for those who are trying to reduce their budget by cutting corners with their inverter choice.
Because so many people have switched from a modified sine wave to a sine wave inverter, it is quite possible that you may come upon a really "good deal" on Craigslist or eBay for a modified sine wave inverter. It can be very tempting when your budget is tight, but I just want you to be fully aware of what you'll be dealing with.
You may decide that a modified sine wave inverter is perfect for you, and if so, that's great! But I want you to make an informed decision. And I can speak from lots of first hand experience on this issue, having lived for extended periods of time with both a quality sine wave inverter and a well regarded modified sine wave inverter. During the years I have used a modified sine wave inverter, it was only out of necessity, but hopefully it will provide some insights to you.
What Impact Would This Have On Your Lifestyle?
First, you need to decide if you will be operating sensitive electronics that could possible be affected by poor quality electricity. Here is a partial list of items that MAY possibly be negatively affected by a modified sine wave inverter:
- Newer front loading washing machine
- Cordless drill chargers (especially Makita)
- Sewing machine variable speed control
- Blender with variable speed control
- Really, almost anything with variable speed control
- Digital clocks
- Light dimmers
- Laser printers
- Satellite receivers
- Plasma TV
- Among other things
This is not to say that every one of these items will be destroyed or will not function with a modified sine wave inverter. There are many variables. But it means that there is the potential of encountering issues with any of these appliances when using a modified sine wave inverter.
Here's My Experience With Modified Sine Wave...
Theoretically, motor driven appliances and devices with a little black transformer cube should operate just fine. I do have suspicions about the impact that a modified sine wave inverter will have on the lifespan of these appliances and have personally seen some destroyed. There are varying thoughts on this, but one concern is the extra heat that is created as a result of the "dirty" electricity, and some say that could have a negative impact on the lifespan of your appliances.
After having a very nice and relatively new electric toothbrush die, LED lights fade to the point of obscurity, and a handheld massage device die (even producing smoke!), we became more careful with what appliances we would expose to modified sine wave electricity. I suggest you do the same if you find yourself using an inverter of this variety.
But having said that, out of necessity we have used everything from computers to hard drives to simple blenders (not adjustable speed) to hair blow dryers with no noticeable issues. It makes me nervous, but we have done it.
[WARNING: This section gets a little technical, so if I lose you here, just skip to "The Lesser Of Two Evils" below]
Let me give you my experience with a very popular modified sine wave inverter of yester-year. The Trace DR series, which was bought out and became the Xantrex DR series until they changed the name to Xantrex TR series. Then Schneider bought it out and renamed it the Schneider TR series until it was recently discontinued. You are likely to run into one of these inverters on the used market. In a nut shell, don't touch it unless it is free, and even then I would discourage using it for anything but a seldom used power system. I have extensive experience with this inverter, and here is why I say this.
ISSUE #1
The charger function leaves much to be desired. With the 24 volt model, it will charge at full power until the voltage reaches an arbitrary, non-adjustable 29.0 volts. Then it will hold it there until the charger backs off to 5% of the battery capacity. So if you tell the inverter that your battery bank is 1,000 amp hours, it will hold the voltage at 29.0 until the charger backs off to 5% of 1,000 which is 50 amps. Then the charger thinks your battery is full and ends the charge. In reality, your battery is far from full at this point. Don't believe me? Test the specific gravity and see--I've done it!
In an effort to "trick" the charger into fully charging my battery, I tried programming my battery size as smaller than it was in an effort to force it to charge for a longer period of time. For example, my battery bank is 1,000 amp hours, and the charger turns off after backing off to 5% or 50 amps. So I program in a battery size of 500 amp hours instead. 5% of 500 is 25 amps, which means the charger will have to run longer until it finally trips off at 25 amps. Unfortunately, that didn't do the job either. I would have to restart the charger multiple times before getting close to being charged.
The problem is two-fold. First, the charger will not allow you to adjust it's absorb voltage set point--it is an arbitrary number that will not work for very many batteries. Second, there is no way to adjust the period of time the voltage is held at the absorb set point. Bottom line? Have fun trying to fully charge your batteries with this charger!
ISSUE #2
Most inverters have a search mode that is akin to "sleep" mode with some appliances. Basically, if it detects that you are not using any power, it goes to sleep in order to save power. But while asleep, it is constantly searching for any hint that you are trying to use power so it can spring to life again. There is an adjustment for the search mode that controls how sensitive the sensor is. That can allow the inverter to go to sleep even if a small amount of power is being used. Personally, I like keeping an inverter at the most sensitive setting, so that even the smallest appliance will "wake it up". Unfortunately it doesn't work that way with this inverter.
Even in the most sensitive position, our CFL or LED lights would not turn it on. The computer would not. Even the blender would not! Only certain motor driven appliances seem to reliably pull it out of search mode. Needless to say, it's a big hassle!
Sorry if it seems like I'm belly-aching. I just want to save you from an unpleasant experience.
The Lesser of Two Evils - Or Neither
If I had to choose a modified sine wave inverter, the only one I would even consider is the Magnum RD series. It has a great charger and is the best you can get in the modified sine wave world.
But I fail to understand why anyone would go that route. Since there is a pretty decent sine wave inverter of similar size, AND it doesn't cost much more (the Schneider Context SW)! So if you could stand a slightly higher budget, both the Magnum MS series and the Outback VFX series are top notch options that I highly recommend. If you are interested in those two, I did a comparison of them here: Inverter Comparison: Magnum MS series vs Outback VFX series.
Just my two pennies worth.
I hate to dominate your time but I forgot another question. I have asked this everywhere that I can think of including a solar installer and no one has an answer for me. I hope you do. Forty five years ago, I bought a stone age inverter at a yard sale. It put out 110 VAC @ 100 W. It was a comparatively pure waveform for a unit that looked home made, had no brand label, used a 2 huge transistor analog oscillator circuit and a massive flyback transformer. It weighed 10 pounds, drew 2 amps @ 12 VDC at no load and 10 amps at a full load of 100 W. It was good for a drop light or soldering gun but you didn't want to run them too long without starting the engine. Mind you, I have a working knowledge, (mostly), of how inverters work, waveforms and that the newer ones work at higher initial frequencies so they can use smaller components and transformers to make a 60 Hz output. I also know that one can't get something for nothing. My question to you is, on a 3 Kw inverter with a 12 volt input, for example, (it has to be 12 volt for reasons that are unimportant right now.), what would the no load idling and near full load working input current draws be on average? I am at a loss to understand how one can plan battery bank capacity without knowing what the maximum potential full load current draw requirements from the main inverter would be. The installer I asked must either be using a cheat sheet or professionally made kits or something. I also understand what you meant in earlier newsletters by planning your total KwH needs at 120 VAC. The only thing I have to go on is the above mentioned 100 W inverter of days gone by and I have a Walmart special 750 W Schumaker unit that is internally fused at 75 amps. For the record, it works well on lights and tools with brush motors, (not so well on induction motors), but this doesn't tell me what I need to know. Any insights you may have will help me sleep better at night. Thanks again. Jeff M.
Hi Jeff,
Sometimes it helps me to convert everything to watts so you can compare apples with apples easily. As you are well aware, the actual amount of power that an amp represents is dependent on the voltage, but a watt is a watt either way.
So with that in mind, I can tell you that the Magnum MS series inverters use less than 8 watts when in search mode, and 25-30 watts when they are on but there is no load. With the 100 watt inverter you mentioned above, it's no load power usage of 2 amps would work out to around 24 watts. So roughly the same as a 3-4,000 watt inverter.
As far as the maximum potential full load current, I'm not certain if I understand your question. Are you wondering how to figure how many appliances you could run at the same time? Or are you wondering how to plan how much cumulative daily power you'll be using in order to plan the size of battery bank that is needed? Or something else?
If you are concerned about simultaneous power usage, then you would use a $20 Kill-A-Watt meter to check the power usage of all the appliances that you could foresee running at the same time, and add them up, and then compare that to the continuous power rating of the inverter.
If you are concerned about cumulative power usage for planning the size of battery bank needed, that is a little more involved, as you have to measure the appliance power usage (with a Kill-A-Watt meter) and multiply that times the number of hours (or fraction of an hour) that it would be used each day. To do it up right, it is helpful to factor it out on a weekly basis and divide by 7 days in order to account for some appliances that are not used every day. Then when a total number of watt-hours is figured up, you would divide that by the voltage of your battery bank to arrive at the number of amp hours needed. And since there are inefficiencies in everything you would want to take them into account (for instance, batteries are typically only about 80% efficient). Also, you don't want to discharge lead acid batteries more than 80% EVER, and 50% is much better on a regular basis. So all this has to be taken into account. It's a bit of a complex process but only has to be done once if its done right. In order to make life a lot easier, I developed a system design calculator for Off Grid Boot Camp students that does the math for them and takes all the inefficiencies into account, so they can accurately size their battery bank and also solar array (if desired). But if you are not using solar and are good with math, you can probably get somewhat close with the instructions above. It's kind of hard to describe it in written form, which is why the calculator is a lot easier.
Anyhow, did that make sense? Or was I misunderstanding your question? Hope it helps. Take care and all the best on your project!
Nick
I am presently trying to design a small solar system for a mini school bus camper that my sister and her husband are planning to start building this spring. She wants the best of two worlds in my opinion so I'm actually thinking about maybe a split system. They want to get away with a small battery bank but also want a system that will intermittently run a small microwave oven for quick snacks and meals to supplement the gas stove. I've been considering a heavy sine wave inverter that would be 50% larger than the current draw of the oven that they finally decide upon. This would also give her power to run a small vacuum cleaner when she's not cooking. I said a split system because I was thinking of having her turn on the big inverter only when cooking and vacuuming, using a cheap 300 W inverter for lighting only where power quality doesn't matter and a better, quality 500 W sine wave inverter for TV, DVD, cellphone charger, laptop ect. I figure this would avoid the use of the sledge hammer inverter for driving carpet tack loads thereby extending it's life and, at the same time, keep the idle power draw down to less than one amp for the other smaller inverters and avoiding the search mode blues that you mentioned in your article. What do you think? Thanks in advance. Jeff M.
Hi Jeff,
You could certainly do a 2 inverter setup like you described, but you would have to weigh the power usage advantages against the hassle--especially if non-techie people will be using it.
Honestly, in a small camper it's going to be pretty easy to deal with the search mode issues I mentioned. Where those get to be problematic is when you have a house full of appliances and you have to go around and switch off a bunch of items each night that you don't want to be impacted by the search mode electrical pulses.
We lived for years in a 1200 sq ft home that had a basic set of appliances and we would leave the inverter in search mode 24/7. During the day when we were using power, the inverter would be on automatically. At night we would turn everything off and it would go into search mode--thus using very little power. Occasionally we would turn everything off and it wouldn't go into search mode, so we would spend 30 seconds hunting for the appliance that was keeping it on, but that was the main inconvenience. That works really well if you are okay with taking a little extra effort to be super efficient. When in search mode, a good inverter (even large ones) use very little power. For instance, Magnum MS series inverters use less than 8 watts in search mode. The only reason we don't use search mode anymore is because we have an invisible fence for our dog and it needs to be on all the time so she doesn't run off at night.
So your plan could certainly work, and it might save a little bit of power during the day, since the small inverter would likely use less power when on than a large inverter, but for me the benefit would be far outweighed by the cost of having to continually fiddle with anticipating how much power each load would take and deciding whether I needed to turn on the big inverter and plug into a different outlet and all that. However, that is something that you or your sister will have to discuss and decide. But with an energy efficiency mindset like that, you'll be able to do fine with a much smaller system than most.
Take care,
Nick
My family has been totally off grid for four years, and around 90% off grid for three years before that. Our system runs multiple inverters, pure sine, LF pure sine, MSW & LF MSW.
They all have their advantages and disadvantages. However, one advantage of MSW inverters is that they are actually more efficient. There is solid physics behind this - it is far more efficient for electronics to simply switch a current on & off than it is to ramp it up and down. However, this is providing you use them with loads that can utilise that kind of waveform properly.
This basically means heating appliances and electronics that run from switchmode power supplies that would be just as happy fed with pure DC. Power tools with universal (brushed) motors work perfectly too, although as pointed out, speed controls tend to not function (usually just running at full noise!) as they rely on the rising sine wave to trigger the triac circuit. Although, there is actually a trick you can use to get the speed control to work, but that's probably getting a little too involved for this comment.
And do not ever run things that use capacitor dropper power supplies from a MSW inverter, as they tend to drastically overheat as the dropper cap passes more current than it usually would.