Howwe tested them

WE started by making sure that the maximum output matched the sockets on each unit, plus we looked for at least one 2.1-Amp USB connection to cater for tablets. We also wanted leads that were of a useful length, as well as a way of attaching the units securely.

Power indicators were also useful and we liked to see battery charge level monitors – handy for making sure power-hungry devices like cool boxes don’t sap your car battery too much when the engine is off. The final factor was price from online sources.