If you have vehicles that struggle to start, or you just need a handy source of electricity while away from home, the Clarke JSM400 could be money well spent. Weighing 1.37kg and small enough to fit in a coat pocket or glovebox, it’s able to start a diesel engine of up to 4.0-litres and a 6.0-litre petrol engine with a flat battery.  Its 21Ah capacity means it can be used around 15 times from a single charge. Not only that, but it’s equipped with 5-volt USB sockets and an adaptor cable for USB-C, Micro USB and iPhone Lightning connections, so you can keep devices topped up, making it not only handy to use, but a potential lifesaver in an emergency situation.