There is 2 factors at play there, firstly the rated power of the inverter supplying the AC power, usually the inverter onboard the EV is only good for a couple of kW so it would have difficulty just boiling an electric jug but a system designed for home backup will have a separate inverter thats good for 5 to 10 kW.
Then there is battery capacity, most medium sized EVs have a capacity of around 50 kWh with the larger ones can go over 100kWh. An average low use household uses about 20kWh per day or less where a higher consumption one with frequent Air Con use could go above 30kWh per day. So depending on the situation most EVs could easily power a whole home for a complete day if it needed to.
However the whole aim of V2G/H/X isn't to regularly power the whole home for long periods, it is to supplement home energy use during peak demand and/or supply some power to the network if the situation warrants it and the capacity is available.
