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Guys, I may be wrong, OT and several other unmentionable things, but in the discussion on the battery capacity, the time required to charge it and the energy requirements for a full charge are being treated a bit simplistically here...
There is a conversion from the AC mains to some level of DC (depending on the voltage required) required to charge the batteries. Depending on the way this conversion is derived there is a small to substantial energy wastage here. Even a switch mode power supply is really about 80% or so efficient.
The battery charging requires charging at prescribed rates to help maintain the battery life, otherwise it would go bad quickly.
There are ohmic losses in the battery/batteries, that is not taken into account when the charging energy is quantified.
So in general, the total energy required to charge up a 4kWh battery would take much more than 4kWh.
To come back to topic, I pay ~$0.06 to $0.09 per kWh on my grid electricity bill, which is charged in slabs - first slab is charged less, and if usage increases, the next slab is charged more (assuming the person would not mind paying more as he seems to be rich, based on the energy usage pattern). An additional $6 flat charge is applicable
We also require a backup community DG set for grid power outages (frequent). There is a minimum monthly charge of $8, adjusted @$0.17/kWh on the usage from the DG set.
So, depending on how much electricity I use (starting with 0 use = $14 flat charge) I pay ~$30 to $60 (in summer, owing to AC use) per month.
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