It all depends.

If it is a battery that provides the back-up, then it is a reasonably simple
calculation;  take the total capacity in watt-hours of the battery, and divide
it by the average load, again in watts;  the result is the number of hours
cover that you get.  It is probably prudent to divide by a further factor of
say 2, to guard against all sorts of variations.

If it is a motor generator, powered by a diesel (or petrol) engine, things are
not quite so easy.  Small diesel/petrol engines are not very efficient, and
neither are small alternators.  Overall, only something like 20% of the energy
in the fuel tank will end up as useful electrical power.  So, work out the
total energy in your fuel tank;  divide that by 5; and divide that by the
average load.

If you are really paranoid, you will run your system (fridge or whatever) off
the motor alternator, and use the mains as a back-up.  It sounds daft, but the
probability of the mains failing at the same time as your alternator is quite
low, especially as you can switch to the mains while you maintain your motor
alternator.

Mike Wells





Quoting Roger Gardiner <[log in to unmask]> on Tue 13 Nov 2007 05:59:23 PM
GMT:

> How long do UPSs work for?
>
>
> Roger
>
> -----Original Message-----
> From: James McGlew <[log in to unmask]>
> To: [log in to unmask]
> Sent: Tue, 13 Nov 2007 5.17pm
> Subject: Re: Vaccines in fridge during power cut!!
>
>
>
> How about an uninterupptable power supply, same as you use for a computer?
> I have a cheap one for this PC, which should allow save/quit,  which was £50,
> but for a few hundred quid it should keep things ticking over ok.
>
> James
>
> I wrote this.
> Be excellent to each other.
>
> > Date: Tue, 13 Nov 2007 11:28:13 +0000
> > From: [log in to unmask]
> > Subject: Re: Vaccines in fridge during power cut!!
> > To: [log in to unmask]
> >
> > Paul
> > Buy a generator like us folks living out in the arsehole of nowhere tend
> > to do. You will need some fancy switching to ensure that absence of
> > current on the mains side of the switch results in turning over the
> > circuits to the generator side and starting up the generator
> > automatically. A fridge consumes v little power and one of those cheap
> > wee generators you can buy in places like Homebase would probably do but
> > the wiring up of the fridge circuit back to it may cost as much.
> > Alternative would be a 5kW job with a bit less switching needed.
> > Declan
> >
> > Oh yes and home insurance usually covers loss of contents but not sure
> > about practice policies.
>
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