Connect an inverter to the mains

[DD86]

Might be a question for Crossy.

Is it in principle possible to connect an inverter directly to the mains and If not what do I need to make such a connection?

There are probably more snakes in the grass then I can think of and how do emergency power generators do it?

Thanks

[carlyai]

On 3/24/2025 at 7:17 PM, DD86 said:

Might be a question for Crossy.

Is it in principle possible to connect an inverter directly to the mains and If not what do I need to make such a connection?

There are probably more snakes in the grass then I can think of and how do emergency power generators do it?

Thanks

[Crossy]

Generally, for both your inverter and a generator you will need a transfer-switch, it is vital that you never have both your inverter/generator and the mains connected together (unless the inverter is a grid-tie type).

 

If you have a hybrid inverter like @carlyai shows it has its own internal transfer arrangements, but an external switch is always handy as a backup if you need to service the inverter.

 

@DD86 What is your actual goal in life for this setup?

[DD86]

Thanks for replies.

Wouldn't actually call it a goal in life. I am 'researching' an alternative solution to a UPS. Household UPS only provides like 15 mins of operation time on the PC. When I added a couple more identicall batteries to my UPS it worked for a short while till the UPS stopped switching.

One simple solution for me would be to have my critical equipment connected seperately to an inverter, running all the time or only plugged in when needed.

 

[Crossy]

How much power (Watts/VA) do you need and for how long?

 

There are certainly UPS units that can take external batteries but they tend to be the more expensive variety.

 

Or, you could use a solar hybrid inverter without solar panels and just a battery pack. The world is your oyster then 🙂

[DD86]

Yes, a 2000 Watt inverter on batteries would suffice; in fact double of what I need.

During last earth quake we had a power outage for 2 hours.

 

[Crossy]

Something like this https://www.lazada.co.th/products/powmr-2kw-12v-20-150vdc-pv-bulit-140a-mppt-solar-charger-controller-2-i5482290007-s23263269028.html coupled to a 12V 250Ah ish LiFePO4 battery would be more than wonderful (maybe 2 x 100Ah would be cheaper).

 

 

[DD86]

Good idea when my application fails. Thank you.

[steven100]

just buy a UPS on Shopee ,,,,  no messing around and it arrives at your door,  plug it in... that's it, why bother with making your own.   ???

[sometimewoodworker]

4 hours ago, DD86 said:

Thanks for replies.

Wouldn't actually call it a goal in life. I am 'researching' an alternative solution to a UPS. Household UPS only provides like 15 mins of operation time on the PC. When I added a couple more identicall batteries to my UPS it worked for a short while till the UPS stopped switching.

One simple solution for me would be to have my critical equipment connected seperately to an inverter, running all the time or only plugged in when needed.

 

That is what we do. The main router and security CCV cameras are powered by our UPS. We have 24v 50Ah batteries powering it, it is trivial to add more capacity (though not cheap), we have had power cuts lasting more than 4 hours, I did not check the level of charge but suspect 8 hours with the current system will be no problem. We had baby UPS’s before but this small industrial one is likely bulletproof.

 

 

NB the guarantee is 24 month’s on site. I have used this once at about 20 months on a previous CyberPower UPS 

[ExpatOilWorker]

2 hours ago, Crossy said:

Something like this https://www.lazada.co.th/products/powmr-2kw-12v-20-150vdc-pv-bulit-140a-mppt-solar-charger-controller-2-i5482290007-s23263269028.html coupled to a 12V 250Ah ish LiFePO4 battery would be more than wonderful (maybe 2 x 100Ah would be cheaper).

 

 

Very interesting solution. 

What is the difference between the two mains out?

The 6.2 kW unit is rated foe 48V DC batteries and listed together with a 12.8V battery.  Are you suppose to connect 4 batteries in series?

That will give 51.2V, close enough to 48V?

Wouldn't it be better to connect a 5 kWh LVTOPSUN battery, it isn't that much more expensive. 

 

[DD86]

21 hours ago, steven100 said:

just buy a UPS on Shopee ,,,,  no messing around and it arrives at your door,  plug it in... that's it, why bother with making your own.   ???

Operating time.

[DD86]

19 hours ago, sometimewoodworker said:

That is what we do. The main router and security CCV cameras are powered by our UPS. We have 24v 50Ah batteries powering it, it is trivial to add more capacity (though not cheap), we have had power cuts lasting more than 4 hours, I did not check the level of charge but suspect 8 hours with the current system will be no problem. We had baby UPS’s before but this small industrial one is likely bulletproof.

 

 

NB the guarantee is 24 month’s on site. I have used this once at about 20 months on a previous CyberPower UPS 

Please give some more details, Names, website...thanks

 

[steven100]

10 minutes ago, DD86 said:

Operating time.

disagree .... just buy one...why mess around to make  ?   many things the same... I learnt years ago,  china is cheaper 

[DD86]

6 minutes ago, steven100 said:

disagree .... just buy one...why mess around to make  ?   many things the same... I learnt years ago,  china is cheaper 

A regular house hold UPS is only meant for two reasons. Surge protection and short term power supply (typical 10 to 15 mins) to be able to safe your data.

Just buy one?? I did twice and not happy with them. What's good about them is the U, uninterruptable.

[BritManToo]

21 hours ago, steven100 said:

just buy a UPS on Shopee ,,,,  no messing around and it arrives at your door,  plug it in... that's it, why bother with making your own.   ???

Because most UPS only last for 2-3 minutes at most.

My hybrid inverter lasts for 16hrs from a full battery with no sun or grid, and runs my entire home.

[steven100]

5 minutes ago, BritManToo said:

Because most UPS only last for 2-3 minutes at most.

My hybrid inverter lasts for 16hrs from a full battery with no sun or grid, and runs my entire home.

well,  sounds to me like there is a problem with the design

Buy another good one....  but why on earth mess around to save 2000 baht ??      some are crazy ?

[BritManToo]

6 minutes ago, steven100 said:

well,  sounds to me like there is a problem with the design

Buy another good one....  but why on earth mess around to save 2000 baht ??      some are crazy ?

There are no good ones!

And you can't add extra batteries because they overheat and fail if running for more than that 2-3 minutes.

[steven100]

27 minutes ago, BritManToo said:

There are no good ones!

And you can't add extra batteries because they overheat and fail if running for more than that 2-3 minutes.

ok ...  I see what u mean

 

[DD86]

36 minutes ago, BritManToo said:

There are no good ones!

And you can't add extra batteries because they overheat and fail if running for more than that 2-3 minutes.

BritMan, tell me why this hapens. I doubled and tripled my batteries (identical ones) and the outcome was that the device didn't work (switch) at all with a single battery. No overheating of batteries here.

What is wrong with two batteries in parallel, I don't get it.

 

[steven100]

9 minutes ago, DD86 said:

BritMan, tell me why this hapens. I doubled and tripled my batteries (identical ones) and the outcome was that the device didn't work (switch) at all with a single battery. No overheating of batteries here.

What is wrong with two batteries in parallel, I don't get it.

 

I think Crossy will know the answer ......   imo

[Crossy]

This issues with adding capacity to existing UPS units are mainly: -

 

The electronics (including heatsinks) are designed to run for as long as the standard batteries permit without cooking (value engineering). You can mitigate by adding ventilation and fans and/or improving the (minimal) heatsinks.

 

And.

 

The inbuilt charger was designed to charge the designed battery capacity in a reasonable time and may never actually be able to charge your extra packs (bigger capacity, longer charge time and larger leakage). Add an external charger to at least get your external batteries to a point where the internal charger can maintain.

 

It is certainly possible to expand run-time with external batteries, but it's never as simple as just nailing on more battery capacity.

 

For extended run time at reasonable power levels, it may be more economical to get one of the baby "silent" generators and retain your UPSs to keep things alive whilst you start the generator. We were doing this for several years before we became a solar household.