Isn't it usually ~100Ah@6V/~50Ah@12V? For example according to the chart on
this page, the T-105 will last for 115minutes@75A. Since it's a 6V battery that means it can put out 75A(6V)=450W for a little under two hours. If the current draw is doubled even less power will be extracted, so I don't think it could run a 1kW BH for an hour, but it should be able to run a 400-500W BH for an hour.