Wilfred Harte, 65, of Walworth, south-east London, was said to be angry because his step-daughter, her husband and their two children were always watching television and running up a large electricity bill.
Neil Saunders, for the prosecution, said Mr Harte snapped when they came to stay and stayed up watching television until 5am. He added that it was a miracle no one was killed.
Mr Harte denies arson with intent to endanger life, but admits a lesser charge.
The trial continues today.