Income-Related ESA is a means tested benefit, so how much the claimant is entitled to will depend on their personal circumstances and their income and which 'phase' they are in ie assessment or main.
Their 'applicable amount' is compared to their assessable income and Income-Related ESA is paid to 'top up' any income to their 'applicable amount'. If they have no assessable income then the amount of Income-Related ESA awarded equals their 'applicable amount'.
Some people may have to wait 7 days before their claim starts, other's will have transitional protection.
And changes from April 2017 will mean that some new claimants will get less than those whose claimed before that date.