OK right hour battery has a capacity say 64kWh that’s the size of your “tank” like saying 50 litres in petrol terms.
What you can fit in it will depend on the charge level from your usage. So say the gauge is at 50% charge. That means roughly speaking you need to put 32kW (50% of 64 being 32) into the battery to take it back to 100% OK?
Now you get your unit price for electricity which is kWh it’ll be on your online account, bill etc & you multiply this by the previous figure in this case 32.
So say your electricity is 14.59p per kWh you’d do the following sum
0.1459 x 32 = £4.67 (rounded up)
That’d be the most you’d pay obviously less if you stopped charging before 100%
If you’re on economy 7 or another low unit overnight tariff then your overnight rate can often be 5p per kWh so
0.05 x 32 = £1.60
So quite a difference & well worth shopping your energy supplier around.
Obviously it gets more complex if you have solar panels & it’s not quite as accurate as that as there are losses due to heat etc but don’t worry about that just stick with the above
Unit price x kW needed = cost