Conditional multplication in excel

I am building a forecast sheet for a new project and I am stuck on how to conditional multiplication in excel.

what I am trying to achieve is:

A1 = 10 | A2 = 10 | A3 = 5

A1 * A2 (until a total of 1000) then change to A1 * A3

It needs to use A2 up to the 1000 mark then from 1000 on use A3 so I can use the standard if If >1000 then use A3 formula.

this is an electricity forecast as I have a set rate (A2) till '1000' units is consumed then I am given rate (A3) and I wanted to know if this can be automated so I can create different models without needing to manually rework it every time?

Thanks and I hope that makes sense :)

Range `D10:E21` contains the formula `=MIN(D\$8-SUM(D\$9:D9),\$B10-SUM(\$C10:C10))` Range `D23:E23` contains the formula `=D7*SUM(D10:D22)`