In reply to pebbles:
If you're technically minded, and space is a premium, you can probably do better than anything you can buy. If you use a battery, an inverter, and a mains power lead to charge a laptop, you are converting from low voltage DC to mains AC then back to low voltage DC. It would be more efficient to use a DC-DC converter. The only problem is that different laptops use different voltages, so the system isn't universal.