Features of off-grid power, or what to do during rolling blackouts, Part 2: UPSs and inverters

Last time, we looked at relatively compact devices for storing electrical energy. They can help charge your phone, laptop, or tablet. But I can already hear the obvious question: what do you do when you need to power something more demanding, or something that requires AC?
So yes, today I’ll talk about uninterruptible power supplies and some of the finer points of using them.

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A few notes on terminology.
UPS (Uninterruptible Power Supply)—not to be confused with a switching power supply.


Inverter: a DC-to-AC converter. Technically, it can either step up or step down the voltage, but we won’t consider the latter option for now.


A UPS provides uninterrupted power to devices that can’t tolerate a loss of power. If the mains voltage fails or drops (or rises) below (above) a critical level, the UPS switches to battery power. Once mains power is restored, it starts charging its internal or external battery.
There are three main types:
1. Off-Line: when the power goes out or the voltage falls outside the set limits, it switches to battery power. These are hardly made anymore, at least for home use.

2. Line-interactive: when the voltage goes outside the set limits, it tries to stabilize it. If that doesn’t work, it switches to battery power. It can operate without switching to battery over a wider voltage range, so it’s better suited to areas with an unreliable power grid.

3. Online: because the inverter runs continuously, there’s no transfer time, making these units a good choice for powering highly critical loads.


Inverters.
Essentially, an inverter is just one component of a UPS. But since there’s demand for them, especially among motorists, they’re also sold separately. An inverter converts one voltage to another—for example, 12 V DC to 230 V AC.
Some inverters have a built-in charger, but they’re much less common. Unlike a UPS, they have a simpler voltage-monitoring circuit—in essence, they’re very basic Off-Line UPSs.
There are also inverters that combine a UPS with a charger for solar panels or a generator, but those are a whole different class of devices, and I’ll probably cover them another time.


Both UPSs and inverters can produce different output waveforms.
1. Square wave, also known as a “modified sine wave”
2. Sine wave (Pure sine).

The regular model is on the left, and the one with a sine-wave output is on the right. This is usually stated in the description or on the case. If there’s no label, it’s a regular converter.


So, these devices are usually needed to power things like:
TVs, computers
Lighting
Power tools, coffee grinders, mixers
Refrigerators, gas boilers, range hoods

There’s a reason I’ve divided the loads into these groups. They all have their own specific characteristics.
The easiest loads for a UPS are the first two groups. TVs, PCs, laptops, and similar devices contain switching power supplies, which don’t really care about variations in frequency, voltage, or waveform. Lighting often has a built-in switching driver, or is simply a resistive load.
A little more challenging are power tools. Frequency and waveform aren’t especially critical here either, but the UPS or inverter does need to handle the inrush current.
The most challenging loads are devices with induction motors. They need not only relatively stable voltage, but also stable frequency—and especially a suitable waveform. Refrigerators are the hardest to run because they also have a high starting current, 5–10 times their operating current.
We won’t cover loads like heaters, water heaters, washing machines, or underfloor heating for now, because powering them for extended periods gets very expensive and isn’t particularly practical.

So, to figure out what to do, we first need to decide what we actually need.

Let’s start with a typical scenario: protecting equipment from voltage spikes. You don’t need a UPS for that at all; a voltage relay is enough. By the way, I strongly recommend buying and installing one. Given the current situation, voltage fluctuations can happen when power lines are switched, when the neutral burns out, and so on. I’ve already seen my own grid drop to around 100 V and rise to around 260 V. And you absolutely must set the power-on delay—it’s critical for refrigerators!

I’ve already reviewed several voltage protection relays, and I can recommend any of them.
ZUBR-D2-63
RN-263t
RN-119
VC-115
There are also more advanced devices—EM-126T and EM-129



There’s no point in considering protection against brief power outages: in practice, it will only help your home PC. So I’ll move on to systems capable of providing power for longer periods.

The cheapest, though not the simplest, option is to modify a standard UPS to work with an external battery. It’s both simple and complicated at the same time; let me explain the details.
Most typical low-cost UPSs have a square-wave output rather than a pure sine wave, so they’re suitable for powering devices in the first and second groups (TVs, lighting), somewhat suitable for the third (power tools), and practically unsuitable for the fourth (refrigerators, pumps).
Also, a typical UPS is designed for short-term operation, so the heatsinks inside are often just blocks of aluminum. They rely on their high thermal capacity to keep the transistors from overheating for 5–10 minutes.
The charger in these UPSs is usually low-powered too, with a charging current of around 0.4–1 A. That means the car battery you’ve prepared and stored in the garage will take... 2 to 4 days to recharge after being fully discharged. Will a UPS like that help you? Of course not.

Inside, you’ll find three main components: a battery, a large transformer, and a control board.


The power transformer in the UPS shown above is hard to miss. But here, people have asked me, “Where’s the transformer?” It’s simple: there is one here too—that little yellow one on the right. In this case, though, a high-frequency 12-to-310 V converter is used, followed by a stage that generates the familiar 230 V, 50 Hz. Based on my experience, I’d say that personally, I prefer the design with the “classic” 50 Hz transformer shown above.


Unfortunately, I don’t have much data on specific UPS models—I’ve never really needed to keep track of it. The situation is further complicated by the fact that even UPSs from the same manufacturer and product line can sometimes differ considerably.
For example, below are two UPSs made by Mustek, which used to be very common. One is rated at 400 VA, the other at 600 VA, but in reality they differ in many more ways. For instance, the 400 VA model uses a push-pull topology and has a battery charging current of 400 mA, while the 600 VA model uses a bridge topology and charges at 1 A. The first model uses a common LM317-based circuit for charging, while the second uses a very basic PWM regulator based on a comparator.


So if I were choosing a UPS for myself, I’d definitely go with the second model. Also, keep in mind that more or less modern UPSs shut down when the load is very low. And of course, it’s a good idea to tape over or disconnect the buzzer right away.
Interestingly, two models with power ratings that differ by a factor of 1.5 can have transformers inside that are similar in size—and therefore in power capacity.


So, you need to keep your loads powered without interruption. What are your options?
The simplest option is to buy a UPS designed from the outset to work with external batteries. Some have an internal battery but also terminals on the back for connecting an external one.
For example, we have a DELTA N-SERIES UPS at work that supports external batteries out of the box, with a dedicated connector on the back.
It has six 12 V batteries inside, and an external battery can be used to extend the backup runtime.


Another option that’s no more complicated is to buy a UPS with no internal battery, designed to work only with an external one. These UPSs usually have a higher charging current too. For example, a model from Novatek-Electro charges at 8 to 15 A—compare that with the feeble 1 A of a standard UPS.


A reminder: if you plan to power a boiler or refrigerator from your UPS, you need a model with a sine-wave output, such as the one above—although it may not have enough power for a refrigerator. And as I mentioned earlier, manufacturers usually make a point of highlighting this on the front panel first.


If you have the tools, some experience with electrical work, and want to save money, one option is to modify a regular UPS to work with an external battery.
Basically, there’s nothing complicated about it: extend the wires that go to the original battery and connect them to a battery with the same operating voltage, usually 12 V, or less often 24 V.

The wires should have a large cross-section (usually 6 mm² or more) and be as short as possible. As it happens, I got my hands on a UPS that someone had already modified to work with an external battery, while keeping the original battery usable too.


People build some pretty substantial setups with high-power UPS units.


A quick note before I forget: if you have more than one battery, don’t just connect them any which way—connect them diagonally so the load is shared evenly between the batteries. This is important!


But back to the matter at hand—we’ll talk about batteries later.
If the UPS wasn’t designed from the outset to work with an external battery (why else would we be modifying it?), you’re 100% going to run into a cooling issue.
As an example, I’ll show you the transformer and heatsinks from a 1000 VA APC UPS that wasn’t designed for an external battery.
The heatsinks are the aluminum blocks on the right in the photo.


The transformer is the thing in the middle of the case.


For comparison, here are the heatsinks and transformer from a Novatek-Electro UPS with exactly the same power rating.


It also has a fan.


So, based on all of the above, we can draw two conclusions:
1. The unit needs cooling
2. It’s best not to run the UPS at full power, at least not for extended periods.

I think it’s easy to guess how to cool it: use a fan. Better still, increase the heatsink area—for example, by attaching heatsinks from an old computer power supply to the stock ones. But keep in mind that the heatsinks must not be connected to each other or to the case!
By the way, if you reduce the load, you may not need extra cooling for the heatsinks, because the heat they dissipate drops with the square of the load reduction. Roughly speaking, if you halve the load, the heatsinks will dissipate four times less heat!

You can connect the fan either directly to the battery or power it from the main transformer. Since the latter isn’t so easy for an inexperienced user, let’s look at the first option.
A typical UPS has a 12 V battery, less often 24 V, and even less often 36 or 48 V. Higher-voltage models are extremely rare in the consumer segment. Although I do have a 1000 VA UPS at home that runs on 60 V.

It’s best to connect the fan through a temperature controller board, which will control it based on temperature changes. You can attach the sensor to the heatsink of one of the power transistors. But bear in mind that since all of this is connected directly to the battery, it will slowly drain it while switched off if you put the UPS away after using it. So it’s best to have a switch that cuts power to the board.


There’s also a less “intelligent” option, but it’s cheaper, even simpler, and doesn’t need a switch.
Yes, a regular thermal switch. They’re available with different temperature ratings; I’d go with something around 45–50 °C. No need to worry about a switch: when you turn off the UPS and it cools down, the fan will shut off completely.
Get one with normally open contacts (NO), so they close when heated. By the way, I’ve also reviewed this kind of setup—link.


We’ve more or less covered cooling and wiring for now. I don’t think there are any major difficulties here, and you can always ask in the comments.
You can deal with the buzzer using at least some electrical tape—or a soldering iron. But the UPS shutting itself off under a very light load is trickier: you’ll need to look up information for each model individually, though fortunately this issue doesn’t come up very often.

That leaves one last problem: low charging current.
There are several ways to deal with this, but first let me explain what the problem is.
Let’s say we have a UPS with a 250 W load connected to it. It’s powered by a car battery, and the runtime is about two hours (yes, it really is that bad). Let's assume we're happy with the runtime.
So the power was out for two hours, the battery is almost completely discharged, then the power comes back and it starts charging—say, at 1 A rather than the measly 0.4 A that sometimes happens. Now the question is: how long will it take to fully charge?

The simplest calculations show that charging a 55 Ah battery fully at 1 A will take at least 55 hours. And what if the power goes out twice a day? By the way, I should say right away that lead-acid batteries must not be charged at too high a current, but that's a topic for another article.

You can increase the charging current using the UPS itself, in principle, but circuit designs vary, so a universal guide is unlikely to be possible.
The second method is simple, but requires spending some money: use an external charger connected to the battery and the mains, upstream of the UPS, of course, not downstream :)
For example, here's a setup with an inverter. I think the idea is clear, but as always, there are a couple of caveats:
1. If you won't be using the setup for a long time, it's best to disconnect the charger from the battery so the battery doesn't discharge through the charger's output circuitry.
2. Standard car battery chargers are designed for cyclic charging, so their output is often noticeably higher than the 13.75–13.85 V recommended for a UPS. And that's fine if the charger has automatic shutoff when charging is complete; otherwise, it will simply boil the battery.


So what should you do?
There are a few options here too. For example, buy a charger like this and adjust it from 12.6 V to 13.8 V, while reducing the output current from 3 A to 2.5 A to improve reliability. I reviewed it, and as usual, the review includes the schematic and board: link.


A more reliable solution might be to buy a 14.6 V charger and then lower its output voltage, for example, this one


If these adjustments seem complicated or confusing, there's another option: simply buy a 24 V power supply (for a 12 V battery) with suitable power, plus a converter. Plug the power supply into the mains, connect its output to the converter input, and connect the converter output to the battery. You'll need a multimeter to set the voltage to 13.8 V and the current to, say, 5 A. Set the voltage with the battery disconnected, and set the current with a discharged battery connected.
Naturally, the converter needs adjustable current.
For example, here's a power supply: link; a review of a similar one; or this option: linkor this one. If you're not sure about the quality, buy a Mean Well, for example, LRS-100-24
Converter: link, review, or this one: link, review.




I think we've covered UPSs for now. In any case, if anything is unclear, it's best to ask in the comments so you don't end up regretting the time and money you've spent.

Let's move on to inverters.
Broadly speaking, things are very similar here, except that inexpensive inverters—and, in fact, the vast majority of them—don't have a circuit for switching to battery power when the mains fails. You'll have to do it manually by unplugging and replugging the mains plug, or, in more advanced setups, using a switch.
There was an image above; I'll show it again. Here, an ATS handles switching between mains and inverter. I've explained what it is and what it looks like before.


But in principle, you can also solve this with a simpler device, such as a phase selector relay.


In some situations, and if you have enough money to spend, it's easier to buy an inverter for solar panels. First, they often provide a sine-wave output; second, they're designed for continuous operation and high battery charging currents; and third, you can add solar panels to the system later.


These inverters are usually larger, and with them it also makes sense to integrate the system permanently into your home's electrical wiring. That's exactly the kind of system we put together at a friend's house.


I think the general idea is clear, so I'd like to spend a little more time on connecting the thing everyone needs: gas boilers. As I wrote right at the beginning, we’ve already run into problems with these devices when powered by UPS units.
First, let’s look at running them from a cheap inverter with a square-wave output, or from a basic, inexpensive UPS.

On the left is what you get from the mains; on the right are examples of an approximated sine wave, or, more simply, a square wave. As you can see, the waveform is very different from the one shown on the left.


In reality, the mains waveform isn’t such a beautiful sine wave; its peaks are usually clipped a little. That’s tolerable, though I wouldn’t call it good either.


And that’s where the problem lies: asynchronous motors, such as those used in pumps, refrigerators, fans, range hoods, and so on, work perfectly well when the current changes smoothly. But they start to run hotter when the waveform is closer to a square wave, and can get even hotter if it contains pulses at frequencies higher than 50 Hz as well.
That’s why there’s a certain combination at play: the UPS and the boiler motor. If one person happens to have a UPS with a square-wave output that’s at least clean, and their boiler motor is built with plenty of headroom, everything may work fine. For someone else, it could be quite the opposite.
The solution is to power this kind of equipment from converters and UPS units with a pure sine-wave output. There are ways to “clean up” a square wave, but they don’t always work well, and it’s easier to use the right inverter.

The second problem, also related to the current waveform, is that a modern boiler’s controller may simply not like it and refuse to work, displaying an error. The newer and more “premium” the boiler, the more likely this is.

The third problem, which people often don’t even think about while looking sadly at their newly purchased equipment and a boiler that won’t work, is connecting the UPS correctly.

Our experience has shown that simply supplying 220 V to the boiler isn’t always enough for it to work properly; you also need to make sure the neutral isn’t disconnected.
The thing is, basic UPS units usually switch only one wire. It doesn’t matter which one; the important thing is that only one is switched. And sometimes certain boilers refuse to work when connected to a UPS.
After looking into it, we found that these boilers work properly if the live wire is switched while the neutral remains connected.
It’s fairly easy to check: either use a multimeter to measure resistance—the connection will read close to zero—or use the “trial and error” method. Fortunately, there are only four possible ways to plug the two plugs into the two sockets.


To wrap up this section, I want to tell you about another category of UPS: low-voltage units.
If you have low-voltage loads, it’s much more efficient to power them without converting from 12 V to 220 V and then back from 220 V to 12 V or 5 V, for example. Typical examples include routers, lighting, video cameras and DVRs, security devices, and even some electric gate models.
There are industrial versions, but more than 8 years ago I also explained in detail how to modify a regular 12 V power supply so it could work as a UPS.


There are also branded models, such as Meanwell. I’ve already published a review of one of these UPS units.
Unfortunately, they all share one drawback: when running on battery, the output voltage can fluctuate within a range of about 10.5–14 V, although that doesn’t bother a lot of equipment.


I also reviewed some even more interesting UPS units: they use a single lithium battery and provide 9/12 V and 5 V outputs. They’re inexpensive, take up little space, and have regulated output voltage. Unfortunately, their load current is limited to just 1 A.


That’s all for today. I think I’ve already gone on long enough. But please forgive me, friends: there really is a lot of information, and even more I’d like to tell you. I couldn’t figure out how to fit it all into one article, so I’m splitting it into parts. The next one will most likely be about batteries—their types, selection, calculations, connections, and so on.