I agree with Phil but if you meant that in the field there are numbers with the unit in a string format, you will need to adjust it to make it a number.
CD_UNIT = '500 W', '500 kW'
You would need to convert it to the same unit in script:
VALUE = 500, 500000 / UNIT = 'W'
Then, you would be able to sum it with your expression
Hope this helps,