Can you connect two UPS in series?

Can two UPS be connected in series

We had one of those unusual applications through last week. A customer had a load of 1000W that he needed to run for an hour. Simples – using our UPS calculator we can get one of our rack/tower VFI3000B units with 2 battery packs and problem solved. Or not. Problem is there is a maximum weight limit of 40kg and a dual battery solution weighed in at 120kg. At that load level, our KR3000RM-LI Lithium unit -weighing in at only 16kg – will deliver over 30minutes, hence the question: “Can you connect two UPS in series?”

Having given this some thought I was struggling to come up with reasons why not. Sure, there may be better ways, for example additional battery packs, but this isn’t an option on the Lithium units. A quick Google search came up with the “Top4 reasons you should never… 1” which intrigued. However those folk were arguing about using non-sinewave units and I couldn’t agree more with that. Our Lithium units are online double conversion, true sinewave with power factor corrected inputs. The output of these units is going to be a lot better than from the mains, and the input current waveform is a smooth as you can get (baby’s posterior and that) so there should be no reason at all not to do this.

Having brainstormed this for some time with the engineers we could only come up with 4 reasons that you need to consider. That is, aside from the fact the UPS’ must be sinewave output and power factor corrected inputs2.

can you connect two ups in series

1. It’s inefficient.

Can’t argue against that. UPS 1 is not only powering the load it is powering UPS2.

For a 1000W load with say 10% losses, UPS2 consumes 1100W and UPS1 will now be compounded eg. 1210W. An extra 110W in losses as opposed to a single unit with a battery pack.

As for runtime, as the load that UPS1 has to deal with on battery is higher than UPS2 it won’t last as long. Taking good quality online double conversion lead acid systems with 6 internal batteries. One UPS will give 13minutes runtime at 1000W. Add to this an additional battery pack and you will get 32minutes runtime.

In a series arrangement, the load on the first UPS will rise to (roughly) 1100W. This has now reduced the runtime of UPS1 to 11minutes. UPS2 will remain at 13minutes giving 24minutes runtime. Not as good as adding the battery pack.

However, in our case adding a battery pack wasn’t an option, so we are able to knock this objection away.

2. No wrap around bypass

In all our online double conversion units, there is a built-in bypass (this can be enabled or disabled in most models). What this does is bypass the internal UPS circuitry if there is an overload or a fault. If there is an overload, both UPS’ would detect this immediately and revert to bypass. Due to this there may be the possibility of lost load. This is because reverting to bypass is not instantaneous and there is a momentary drop in power during this transition. The UPS cannot revert to battery operation when in bypass and if UPS2 has no input then the output is also lost.

Good point well made Henry. However this is a fault tolerance issue and only occurs in the event of a fault. As such it can be noted and a decision made to introduce additional fault avoidance measures or to live with it.

3. Compounded Earth Leakage

The earth leakage on UPS systems (except for our isolated VFI-TX range) is cumulative with the load. That means if you have a load with 1mA leakage, with the UPS of 1mA as well you now have 2mA. Add to that another UPS then all of a sudden you have 3mA. This is a problem for pluggable devices as the limit for such is 3mA. However, the safety standards only require you to take some additional measures, such as supplementary bonding, or by opting for industrial connections.

So another factor to consider but not a deal-breaker.

4. More expensive

True, two UPS (of the same ilk) will be more expensive than a UPS with a battery pack (Even with additional batteries in most cases).

But this is fairly obvious.

Conclusion: Can you connect two UPS in series?

Absolutely. Given the caveat that the UPS’ should be pure sinewave and power factor corrected input there is no reason at all why you can’t.

In our application this was the only solution. Getting the weight down to 40kg for a 1000W load for 1 hour just isn’t possible with lead acid battery packs. Connecting two KR3000RM-Li units in series as above worked a treat. In our tests UPS1 went onto battery and UPS2 was unaffected. UPS1 eventually ran out of battery steam after 29 minutes and UPS2 went onto battery. The only thing disconcerting was the multitude of alarms going off – UPS1 emits a constant tone to warn you its just died and UPS2 starts its backup warning alarm. Having a smaller load UPS2 lasted 34minutes, thus giving the one hours autonomy that we required and weighing in at only 32kg!

Lithium Battery UPS System


1This is from out of Barrington, RI, USA. Don’t know much about them except they come top in the Google search when searching for an answer to this “Can you connect two UPS in series” question. They say:

  1. You will void the factory warranty of both UPS systems
  2. You will not increase the capacity of your power protection         
  3. You will not increase run time
  4. You will jeopardize the proper operation of the data center EPO

It is true that you won’t get any additional power handling capacity in this configuration, but why should a manufacturer declare your warranty invalid for connecting to a superior power source, or having a power factor corrected load connected? I’ve just shown you can increase runtime and for properly configured EPO ports on both UPS why would data centre EPO be compromised?

2Power Factor Corrected input means the current waveform is in phase with the voltage waveform and in addition implies the current waveform is sinusoidal. Such a load is ideal for all sinewave UPS systems. Non PFC corrected UPS may take current in chunks, rendering the power factor outside the rating of many UPS. This is why I’ve included this as a requirement when daisy-chaining UPS.

Lithium-Ion UPS vs VRLA UPS

Lithium UPS

Our Lithium Ion UPS range is an impressive series of UPSs with internal Lithium Ion batteries, that make the units efficient, lightweight and more environment friendly. They also reduce the whole life costs of the UPSs. We have conducted some tests to show you how the Lithium-Ion UPS compares to the VRLA UPS in terms of runtime.

Each unit is connected to 1800W load. The Lithium UPS battery capacity is 48V 9.9Ah = 475VAh. The VRLA UPS battery capacity is 72V 9Ah = 648VAh. Although the Lithium UPS has only 75% of VRLA UPSs battery capacity, the runtime results are outstanding! See the video below:

We provide 5-year warranty on the Lithium-Ion UPS systems including the batteries.
Lithium-Ion UPS only from Power Inspired. Learn more at and register your interest.

Lithium-Ion UPS

Top 5 interesting facts about UPS

Facts about Uninterruptible Power Supply

You probably know how Uninterruptible Power Supplies work and what their purpose is, but we have the top 5 interesting facts about UPSs that you probably didn’t know!


UPS facts - UPS inventor

The history of Uninterruptible Power Supplies is somewhat a mystery – nobody can name one person or one date when UPSs systems were invented. There were many theorists and scientists involved in research and studies, however nobody  can claim to be the inventor of UPS.



UPS facts - ups patent
source: google/patents

This rather amusing acronym stands for “Apparatus for Maintaining an Unfailing and Uninterrupted Supply of Electrical Energy”, which is a first ever UPS patented in 1934 by John J. Hanley. Mr. Hanley mentions in the document that the invention is to be used with fire alarms and other safety systems to protect properties and lives. It is amazing how far UPSs have come since then and how the variety of applications widened since 1930s. The AMUUSEE was an ancestor of UPSs as we know them now. We are certainly grateful that UPSs are now called Uninterruptible Power Supply rather than the tongue twister that is used to be back in the day.

You can read the whole patent document at google/patents.


UPS facts - largest ups in the world

The largest Uninterruptible Power Supply system in the world is a 46-megawatt system in Alaska.  The system is called the “Battery Electric Storage System (BESS)” and is located in Fairbanks, Alaska where it powers the entire town! It can provide up to 15 minutes runtime at 26 megawatts, providing enough backup power until the generator comes on. The runtime is achieved by four battery strings, each containing 344 series of connected battery modules. That is one truly amazing UPS.


UPS facts - batteryA UPS is nothing without batteries. Did you know that majority of faults within UPSs are related to batteries? Good batteries matter and so to ensure your UPS provides maximum reliability, you need to organise for appropriate maintenance. Power Inspired Uninterruptible Power Supply systems come with VRLA batteries with 3-5 year design. This time is however approximate and depends upon many factors, including the environment in which the UPS is stored. You can see how to prevent premature battery failure in our previous blog.



UPS facts - TX Series Isolated UPS SystemsPower Inspired’s TX series provides unique safe UPS technology. The units contain an isolation transformer which protects from electric shock and ensures the UPS is isolated and safe. TX series units range between 1KVA – 10KVA, they have a small footprint and the design is highly modern and minimalistic. The larger units (3KVA, 6KVA and 10KVA) also come with wheels for ease of installation. TX series UPS are designed to provide highest degrees of power protection for laboratory, industrial and medical applications. More information about TX series can be found here.

Transfer times in UPS – what are they and how will they affect my equipment?

Transfer time in UPS

UPS transfer time

“What is transfer time in UPS?”

The definition of transfer time, sometimes also called switchover time, says it is the amount of time a UPS will take to switch from utility to battery supply during a mains failure, or from battery to mains when normal power is restored. What this means is that when the main power supply fails, the UPS will need to switch to a battery mode to provide sufficient power and ensure smooth running of the attached equipment. The transfer time duration differs, depending upon the UPS system attached. It should, however, always be shorter than your equipment’s hold up time. Hold up time is the amount of time your equipment is able to maintain consistent output voltage during a mains power shortage.

Line interactive UPS systems, such as our VIX or VIS series, have transfer time typically between 2-6 milliseconds. For regular computer based systems, where hold up time is approx. 5 milliseconds, line interactive UPS systems are usually sufficient; however some computer systems, as well as other critical sensitive equipment, are more sensitive and require shorter transfer time. Hence in this case you should always choose UPS with zero transfer time like our VFI series.
If your equipment is critical and doesn’t tolerate even slightest power distortion, we recommend choosing online double conversion UPS technology with zero transfer time to ensure your equipment has the highest degree of protection.


Here’s a quick look up of transfer times for Power Inspired UPS systems:


Product UPS technology Typical transfer time
VIX3065 Line interactive UPS Typically 2-6 milliseconds
VIX1000N Line interactive UPS Typically 2-6 milliseconds
VIX2150 Line interactive UPS Typically 2-6 milliseconds
VIX2000N Line interactive UPS Typically 2-6 milliseconds
VIS1000B Line interactive UPS with sinewave inverter Typically 2-6 milliseconds
VIS2000B Line interactive UPS with sinewave inverter Typically 2-6 milliseconds
VFI1500B Online double conversion UPS Line to battery 0 milliseconds
Line to bypass Approx. 4 milliseconds
VFI3000B Online double conversion UPS Line to battery 0 milliseconds
Line to bypass Approx. 4 milliseconds
VFI3000BL Online double conversion UPS Line to battery* 0 milliseconds
Line to bypass Approx. 4 milliseconds
VFI6000BL Online double conversion UPS Line to battery* 0 milliseconds
Line to bypass Approx. 4 milliseconds
VFI10KBL Online double conversion UPS Line to battery* 0 milliseconds
Line to bypass Approx. 4 milliseconds
VFI1000T Online double conversion UPS Line to battery 0 milliseconds
Line to bypass Approx. 4 milliseconds
VFI3000T Online double conversion UPS Line to battery 0 milliseconds
Line to bypass Approx. 4 milliseconds
VFI10KT Online double conversion UPS Line to battery 0 milliseconds
Line to bypass Approx. 4 milliseconds
TX1K Online double conversion UPS with isolation transformer Line to battery 0 milliseconds
Inverter to bypass 4 milliseconds
Inverter to ECO Less than 10 milliseconds
TX3K Online double conversion UPS with isolation transformer Line to battery 0 milliseconds
Inverter to bypass 4 milliseconds
Inverter to ECO Less than 10 milliseconds
TX6K Online double conversion UPS with isolation transformer Line to battery 0 milliseconds
Inverter to bypass 4 milliseconds
Inverter to ECO Less than 10 milliseconds
TX10K Online double conversion UPS with isolation transformer Line to battery 0 milliseconds
Inverter to bypass 4 milliseconds
Inverter to ECO Less than 10 milliseconds

*unit doesn’t contain internal batteries, requires external battery pack/cabinet

Transfer times are dependent on which stage the power interruption occurs in. That’s why the transfer times stated in the above table are approximate.
As previously mentioned, transfer times also measure the amount of time it takes for the UPS to switch back to mains. The transfer back to mains power is always controlled with minimal interruption as this transfer is planned. As opposed to an unplanned mains failure which happens suddenly and hence a variation in the actual time taken.

transfer time

We have conducted a transfer time measurement using an oscilloscope (photograph above). For purpose of this exercise, we have used a standard line interactive UPS system and stimulated a power cut. The oscilloscope managed to capture the transfer time which on this occasion lasted 15 milliseconds, due to the original sine wave being interrupted at the peak of the cycle.

 “How does transfer time affect my equipment?”

That’s simple – if your equipments tolerance is below UPS transfer time, the UPS will not provide power in sufficient time in order to keep your equipment running.
Let’s say you have highly sensitive laboratory equipment with hold up time of 2 milliseconds. Line interactive UPS will not be sufficient in this case as it will not switch to battery mode quick enough. You will need to invest in an online double conversion UPS or Isolated online double conversion UPS in order to avoid any downtime. On the other hand if your equipment is a very basic computer workstation with approximate transfer time of 10 milliseconds, you can use the line interactive UPS system with peace of mind that your equipment is protected.

Transfer time is definitely one of the things you need to keep in mind while searching for suitable UPS. More factors affecting your choice of UPS technology are covered in this article.

How to cope when there’s a Power Cut

Power Cut

Not many people know what to do when they have a problem with their electricity. Some people just sulk and bear-it and there are a fortunate few people that don’t suffer from the glitch as the power cut didn’t last more that a couple of minutes. Luckily, when it happens we can ‘google it’, (the power cut will effect the broadband therefore wifi is off, whoohoo for a smart phones that’s fully charged!).

What’s a power cut?

Your experience might be your PC died so you press the start button to restart it and stroll over to the kettle believing that’ll sort it out. ‘Strange’ – you notice the kettle isn’t coming on now. ‘Oh-wait’ -the penny has dropped. There’s been a power cut.

When you have no electricity from any wall-socket in a building that’s a power cut. It’s caused by bad weather, a fault or planned maintenance work gone wrong. Your Electricity Supplier would give you 28 days notice from receipt of your last bill before cutting your electricity supply off for a unpaid bill.


If something in another part of the building is still operating from the mains power then go and check your fuse box/circuit breakers. In which case something has caused these little safety mechanisms to trip, (in my house it’s a burnt out raisin in the toaster).  This is not a power cut but try to find out what caused it to happen and try to prevent it in future.

Who do I call?

If you can tell that the same thing has happened to the neighbours- e.g. no lights on in their house when there should be, then it’s a problem that your Local Distribution Company (LDC) needs to know about. It is the LDC that is responsible for wires & cables that supply the electricity from the National Grid to your home, business or a public building. So you need to contact the LCD, originally known as DNO before privatisation, in your area. Switching a light on will give you a easy visible sign to know when power returns. Bear in mind the cordless phone won’t work so a mobile phone is necessary. The DNO will advise you on what to expect.

There is a map to help you determine who you need to call in your area, provided by the Energy Networks Association (ena) who are the trade organisation for your District Network Operator (DNO). View the map of the DNO’s on the ena website.

Telephone contact details

The following information will help you contact the right organisation in the event of a power cut in your area, there is a quick postcode look up here at the ena website:

Area Company Emergency No. Website Twitter account
North Scotland SSE Power Distribution 0800 300 999 @hydroPD
Central and Southern Scotland SP Energy Networks 0800 092 9290

(or 0330 1010 222 from a mobile phone) @SPEnergyNetwork
North East England Northern Powergrid 0800 668 877 @Northpowergrid
North West England Electricity North West 0800 195 4141 @ElectricityNW
Yorkshire Northern Powergrid 0800 375 675 @Northpowergrid


North Wales &

North Shropshire

SP Energy Networks 0800 001 5400

(or 0330 1010 400 from a mobile phone) @SPEnergyNetwork
East Midlands,

West Midlands,

South Wales &

South West England

Western Power Distribution 0800 6783 105 @wpduk
Eastern England UK Power Networks 0800 783 8838

(or 0333 202 2021 from a mobile phone) @UKPowerNetworks
Southern England SSE Power Distribution 0800 072 7282 @southernelecPD
London UK Power Networks 0800 028 0247

(or 0333 202 2022 from a mobile phone) @UKPowerNetworks
South East England UK Power Networks 0800 783 8866

(or 0333 202 2023 from a mobile phone) @UKPowerNetworks
Northern Ireland Northern Ireland Electricity 0345 764 3643 @NIElectricity

Fingers-crossed power cuts don’t inconvenience you too much but they are part of life.

This information is provided in good faith and produced using the best information available at the time it was compiled. We cannot guarantee the complete accuracy of this content and commercial decisions should not be based on it. Power Inspired accepts no responsibility or liability arising from the use of this information.

UPS vs Tubular Motor Cross Reference Guide

How to select correct UPS for door applications

How to select correct UPS for door applications

We’ve created this guide to assist in selecting the correct UPS for a range of typical tubular motors. From the Nm (torque) reading on your motor, select the corresponding UPS from the list below. If your torque isn’t specified then use the next highest.

Motor Torque
Power Rating
Recommended UPS
12 168 VIS1000B
20 178 VIS1000B
50 310 VIS1000B
60 310 VIS1000B
120 450 VIS1000B
150 500 VIS2000B
180 520 VIS2000B
230 870 VIS2000B
330 980 VIS2000B
500 1350 VFI3000T

Note this table is based on Eurodrive tubular motors from published information. Where conflicts have been identified we’ve used the worst case. There is allowances made for start up currents of a factor of 1.4.

Remember UPS can have some failings in door applications in that they switch to battery immediately following a power cut. This can drain the battery. To avoid this happening you will need to use the Power Inspired GATEWAY product.