calaculation of colums

HELP after spending all weekend trying to work out how to do the following:

wanted to do an Excel spred sheet for monthly KWh usage
Doc2.docx

for example column C was Jan's reading (1561811)
column D Feb's readins ( 1587661)
so in column E I put =D3-C3 which gave me the correct monthly usage for Feb's (25850),
but with feb reading in D3 and no readings in G3 which will be input around the end of the month I'm getting the Mar reading as a minus -1587661
I have used the same calculation G3=F3-D3
how do i keep G3 blank until the reading for March is input?