In absolute terms it's nothing though. An average household uses 4000 kWh of electricity in the UK (less than half of the average us household), so saving the 5kWh on 4 phones is a 0.5% saving, which is roughly equivalent to running your home boiler for 10 minutes per year. Turning off your thermostat for one evening would have the equivalent impact of not using a wireless charger for a century.