I’ve been trying to work out the same (i offered to help a friend spec an off-grid system to continuously power a 120w heater on an allotment during winter which is proving way more difficult than i anticipated!).
While not a comprehensive answer 2 things you need to take into consideration are how much you want to sacrifice the life of the battery, and temperature.
Have a read through here: https://batteryuniversity.com/learn/article/lead_based_batteries but some key info is a table half-way down that states:
| Depth of discharge | Starter battery | Deep-cycle battery |
| 100% | 12–15 cycles | 150–200 cycles |
| 50% | 100–120 cycles | 400–500 cycles |
| 30% | 130–150 cycles | 1,000 and more cycles |
and:
“The optimum operating temperature for a VRLA battery is 25°C (77°F); every 8°C (15°F) rise above this temperature threshold cuts battery life in half. ( See BU-806a: ”How Heat and Loading affect Battery Life”:BU-806a: How Heat and Loading affect Battery Life - Battery University ) Lead acid batteries are rated at a 5-hour (0.2C) and 20-hour (0.05C) discharge rate. The battery performs best when discharged slowly; the capacity readings are substantially higher at a slower discharge than at the 1C-rate.”
And i can’t remember where i pulled this from so you’ll have to verify it from your own sources but:
“The amp hours rating that batteries have is based on the temperature being 25 degrees Celsius and with every degree drop, there is a drop in the performance by 1%.
For instance, if you have an 110ah battery, but the temperature is 15 degrees Celsius, it will be performing like a 100ah battery.”