[time-nuts] Power Back-up

[Bruce Griffiths]

The inverter efficiency drops below 90% for loads of less than 30% of
the rating (as shown by graph on the linked page).
However the battery discharge characteristics also have to be taken into
account.
The capacity of the battery used increases as the load current
decreases, the net effect being to shift the system peak efficiency to
lower loads than 30% of rating.

Bruce

Hal Murray wrote:
>> Keep in mind that the batteries are not the only thing on small cheap
>> Domestic UPS's that are rated for minutes. Same goes with their
>> electronics, Heatsink etc. 
>>     
>
> If you are interested in times measured in hours, you will be running a 
> typical small UPS at well below full load.
>
> For example, if full load is 5 minutes, 2 hours will be 5%.
>
> That's assuming time*load is linear.  From what I've lower loads load are 
> more efficient. For example:
>   Typical Backup Time at Half Load 15.9 minutes (250 Watts)
>   Typical Backup Time at Full Load  4.6 minutes (500 Watts)
>
> That's for an APC SUA750 from:
> http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=SUA75
> 0&total_watts=50
>
> Somebody pointed out that low loads are inefficient.  That doesn't match the 
> numbers I'm seeing,  Maybe I'm not looking at low enough loads.  From their 
> chart:
>
> Watts * minutes
>   50 * 103  = 5150
> 100 *  50   = 5000
> 250 *  15.9 = 3975
> 500 *   4.6 = 2300
>
> I wonder where the knee is.  20w? 10w?
>
> That's also from the data sheet rather than measured.  It probably assumes a 
> new battery.
>
>
>