"For reference a standard 200 Amp H car battery is around 2 KWh though I'd not use them"
Indeed. The safe level of discharge for a lead acid battery is around 25-30%; as you go beyond that life starts to get shorter. A nominal 200AH battery is good for 600WH in static use.
This is one reason why you need such a big battery for a car; another of course is the cranking current. But people are using lithium batteries in cars with around 12-14AH capacity, which is about the real capacity of a 60AH lead acid battery.
The biggest problem with the adoption of electricity for any kind of off grid use as that all current batteries are bad in at least one significant way.