The blaze at St Michael's Hospice, in St Leonards, meant patients, staff and the fundraising operation had to be moved quickly to other temporary sites.
Irene Dibben, chairwoman of the hospice, said the consequences of the 11 July fire had been "quite major".
Rodney Smith, 67, of Quarry Hill, St Leonards, was charged with arson with intent to endanger life.
During the blaze 23 people had to be evacuated.
Two patients, Jill Moon, 62 and David Denness, 81, suffered smoke inhalation and died later in hospital.
Ms Dibben said: "The difficulty is that it took out all of the power, the boilers were lost, the servers for IT was lost and smoke damage was extensive.
"Although people might look at the building and think it was a limited fire, the consequences for us have been quite major."
A hospice damaged by fire in which two people died may take up to a year to reopen.