The amount of 'vampire drain' will depend on the model but, if you leave it fully charged, we doubt it'll drain fully in three months. In the worst case scenario, we expect it'd lose around one per cent of charge a day, meaning it'll be almost entirely flat by the time you return - although most owners report much smaller reductions in charge while they're not using their cars. If it is flat, an overnight recharge should have it back up and running again. With fewer moving parts compared to a petrol or diesel model, EVs are, in theory, better suited to being left for long periods.