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Non-techie lay-user question to experts...

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I have a 500W IPS (deep cycle? 120AH-150AH lead-acid battery based power back-up) connected to the Mains with IPS mode output being 230 VAC Nominal in Square Wave/or modified square wave (Transfer time: 10 ms). The IPS also has a regulated UPS mode, i.e. a 180-260 VAC regulated range of input, supposedly for computers. The IPS provides 2 hour back-up if fully used. Currently I use this as a back-up power-source for lights and fans.

I also use a 400W offline UPS (+AVR) connected to the Mains, for my computer, with a Square Wave output. This provides only 10 minutes back-up.

Now I want to plug-in my UPS to one of the outlets of the IPS instead of the Mains, because this way the computer will get a longer back-up uninterrupted (2 hr instead of 10 mins). But the UPS manufacturer's service-personnel are discouraging me from doing this. They say, there will be a mismatch between the two and the UPS will crash. I think they are talking about the mismatch between the IPS's output wave form, i.e. Square wave, and the UPS's input : Sine Wave. But I tried it nevertheless (albeit for less than a minute) and it worked -- but the UPS was making quite a bit of noise and I shut it down. The UPS manufacturer's service-personnel said this is a bad sign.

NOW MY QUESTION IS, is there a safe way to use the IPS and the UPS in conjunction for the purpose I want to use them in conjunction ?

Secondly, I can alternatively connect the computer directly to the IPS since it claims a "UPS mode" (180V-260V). But I am really not confident about this because it's a new brand and it's not known how it works with computers. Besides, the IPS is connected to different types of equipments/loads, i.e. celling fan an energy saving and fluroscent tube lights -- which may cause spikes while transfering from Mains to battery and which may in turn cause damage to the computer ? So, I think it's better to have my old and trusted UPS in between the computer and the IPS.

By the way, frequent power-outages/load-shedding is a common feature of life in the summer where I live (at least at present). And I need to work with computers for long stretches without such interruption.

So what do you think ? Is there any inexpensive solution to this, i.e. other than buying a new "pure sine wave" IPS or high-end UPS with hours of back-up ?
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