I use a car battery and an inverter myself. In the UK, Maplins keep a range, as noted above. A car battery will do up to 12 hours, maybe more, depending on how much you plug into it.

Simple calculation: 60W means about 5 amps from a 12v battery.
A fully charged battery will provide 5 amps for only about 12 hours, probably less because the inverters have a low-voltage cutout, intended to stop you from completely flattening your car battery.

I also have a couple of 'power units' with 7 amp-hour batteries. These last less than 2 hours.

I have just bought a UPS which should have a similar capacity. This contains a built-in battery and inverter, and can be used at home in its intended role as well.

In order to save power, I have my working files on a USB memory stick, which means that the hard drive is not used too often.

Another possibility - Makro are currently selling 650W generators for £45 plus VAT. These will run for 8 hours on a 5L fill.