from decimal import Decimal

'%.2E' % Decimal('40800000000.00000000000000')

# returns '4.08E+10'

In your '40800000000.00000000000000' there are many more significant zeros that have the same meaning as any other digit. That's why you have to tell explicitly where you want to stop.

If you want to remove all trailing zeros automatically, you can try:

def format_e(n):
    a = '%E' % n
    return a.split('E')[0].rstrip('0').rstrip('.') + 'E' + a.split('E')[1]

format_e(Decimal('40800000000.00000000000000'))
# '4.08E+10'

format_e(Decimal('40000000000.00000000000000'))
# '4E+10'

format_e(Decimal('40812300000.00000000000000'))
# '4.08123E+10'

Unfortunately, you can not change this default behavior since you can not override the str methods.

However, you can wrap the float, and use the __format__ method:

class MyNumber:
    def __init__(self, val):
        self.val = val

    def __format__(self,format_spec):
        ss = ('{0:'+format_spec+'}').format(self.val)
        if ( 'E' in ss):
            mantissa, exp = ss.split('E')            
            return mantissa + 'E'+ exp[0] + '0' + exp[1:]
        return ss


     print( '{0:17.8E}'.format( MyNumber(0.0665745511651039)))