Even still, assuming they were 90% efficient, is it worth the extra 10% energy consumption just so you can run your computer for 1 hour at night when the solar energy provider ups their rate? You still need to consume the additional energy in the day to charge the battery.
For me to label it efficient, I'd be looking for 95%+, but 99% would be preferable.
Remember, this efficiency declines as the battery ages. You only get a few years of efficient use before it declines significantly. Then you have a disposal problem. A typical UPS uses lead-acid batteries, the same type used in cars. They're not environmentally friendly.
One of the main issues with nuclear is that it isn't cost effective to turn on/off to meet demand changes in day and night. Nuclear power stations usually only generate energy for the baseload. If we were to cut energy consumption down at night (by increasing costs), the baseload would also drop, and leave an even larger gap to meet for peak, which must be met by other means like solar or fossil fuels.
If anything, we want to shift some energy consumption to off-peak times like early hours, so that the baseload is higher and can be met by cheaper nuclear, along with other solutions like tidal. If this happens, the gap between peak and baseload during the day/evening will be smaller, and can be met with solutions like (concentrating) solar which are more environmentally friendly than photovoltaics + batteries.
