Using a Space Heater from lithium Batteries?
Your math is wrong. The 7.6 Amp current is the continuous draw for as long as your heater is running.
The 828 watts is what is affected by time.
For easy calculations: Your heater ( my example )is using 800 watts of power. Over a four hour period you have use
3200 watts or 3.2 Kilowatts. Power used is based voltage (120 VAC) x Current (6.6 Amps) x Time (4 hours). Over 24 hours you'll probably use 800 watts x 24 hours x .8 (duty cycle) for a total of 15.4 KwHr of energy . Duty cycle is the ratio between the time plugged in versus how much it actually is in use.
Your battery has to be able to supply that 800 watts of power from your 12 volt system. Assuming no loss in converting 12vdc to 120VAC , the battery has to be able to supple 66 amps to run the heater.
You should be able to run your run the unit thru the inverter from your batteries.
A 250 AH battery is designed to supple the 250 Amps for a given period (usually 8 hours) before it reaches the lowest manufacture recommended voltage.
As Ray said if you are away from shore power and running on batteries . The propane furnace is a better option.