Well uh .... theoretically.
One problem with PbA batteries is that deep-discharging tends to kill them dead. It's not recommended to discharge to less than half the capacity, even once, even on those marketed as "deep cycle". So, in practice, you would get one hour at 40 watts, max, and even that is pushing it as far as long-term life of the battery goes.
Overall, given this that and the other, I would recommend NiMHs for DIY.