Alang, I have had the 1800 for a number of years, the internal batteries finally packed it in, I removed the internal batteries and replaced with a couple deep cycle 12 volt units, bad news is it is no longer portable, good news is it used to power my tank for 45 min. tested it over the last couple days and got 17 hours and still had 10% reserve.
and unlike the generator I do not need to be home to switch things over, when the power comes back on it automatically recharges the batteries for next time.