"If a 7 amp/h battery shows 12.00 volts exactly, after 5 hours
with a 5 Watt resistor, what voltage should a 10 amp/h battery show?
Both batteries are of the same type and starting from
the same voltage with surface charge removed.
Udo"

Something seems mixed up here. Wattage is determined by multiplying the
current times the voltage. Excatly 5 watts would only be dissipated during
a brief period, since the voltage and the current will be changing during
the discharge cycle. I am wondering if the 5 watt rating of the resistor is
misleading you.

What is the resistor value in ohms? The manufacturer should have a curve
showing the voltage at various discharge rates vs. time.

Colin