Hi Sarac,
Yes, by having the UPS unit 'on-line' during use the feed to
charge the batteries would have to be sufficient
to run the load.
The extra current to keep the batteries charged however,
would not be much more, maybe not even noticeable.
I agree that converting to low voltage,
then back to 230 volts,
is wasteful, but its not as bad as double.
By having the UPS unit only nearby, not 'on-line' you would
be using it as a standby supply, a supply that you could use
if the mains were to fail.
The usual method in this case is to mount a socket(s) on the
UPS which is the output from the UPS. Then in the event of a
main supply failure, the appliance can be unplugged from the
main supply and plugged into the supply from the inverter,
in this way it ceases to be an "Uninterruptable Power Supply"
and becomes a standby supply.
It is tempting to think a set of relay contacts could easily
be arranged to switch the supply, and do this automatically,
but the electrical supply companies are usually very strict
about this sort of thing, and such a switching unit would
usually require written permission from them before they
allow its use, this would involve it being examined and
passed as fit for use by their representatives.
Such permission is rare for 230 volt systems.
I do not know of a 'fail-safe' system that would satisfy
the examiners, there may be one, but i dont know it.
In my experience every suggestion will get a rebuttal.
It is easy to think that a simple set of relay contacts
would do this, but the examiners will say that contacts
could stick, we may have people working along the line.
You may suggest an arrangement where two relays are used,
so the contacts are not shared, and 'sticking' would not
transfer any supply backwards.
An answer could be: what if this coil fails in use?
The examiners are not easy to satisfy.
The only system of transferring supply that i know of
which is acceptable (at 230 volts) for a domestic user
is where a different socket has the independant supply,
and there is no possibility of 'cross-connection'
Apart from the restrictions imposed by the electricity
authorities there are also practical difficulties to
be addressed when switching AC supplies, the problems
of voltage and sychronising.
The problem of voltage is not too difficult, but the
problem of synchronising is quite difficult to get
workable, although its not difficult to understand.
The additional problem of making the voltage and the
sychronisation automatic for the purposes of switching
is not something that even an experienced electrical
engineer would undertake to build from scratch without
considerable thought.
Another problem arises if the main supply resumes after
a short interval, the synchronising and voltage would
have to be matched before the standby supply could
switch the load back to the main supply.
It is not impossible,
but it is not easy,
and it is not a method that i have ever seen (for 230v)
and it would require permission to use,
which in my opinion would not be obtainable. (from
the electricity authorities)
However, if you use your own generator(s) or for some
reason you are independant of the main electricity
suppliers, then you can presumably do as you wish,
you would still have to meet the statuary household
safety regulations for domestic supplies,
earthing, insulation, etc.
Just give it a socket of its own.
Thats the easy way.
Or use it as a UPS, under load and charge.
Let us know how you get on, and which way you choose
to go, lots of us have battery run power supplies.
Regards, John