We sell 300w, 700w and 1000w Inverters which is more than enough for standard equipment used with a 12 volt battery.
You need to determine the wattage (w) of Inverter you require. For an example if you use a laptop at 70w for one hour, and one 8w lamp, for 4 hours a day, with a leisure 85ah battery. It would look like this 1x8w (lamp) x 4 (hours) + 70w (laptop) this = 102 watts, divide by 12 (volts) = 8.5 ah (ampere hours) per day. Divide your 85ah battery by the ah used and this results at 10 days use for 4 hours a day for the lamp and with 1 hour laptop use. Now this is a rough estimate guide only, and providing you use a fully charged battery. But remember, very important, never completely discharge a leisure battery as it will be difficult to fully charge the battery again. You should only use 70% of its capacity before recharge.
You can use this wattage calculator as a guide click here