All you have to do, is verify the wattage on the devices you wanto to power with the inverter, then add them up to get the total wattage. Your inverter should be @least be able to provide half more. For example, if your laptop uses 75W and the other device uses 20W, then a 200W inverter would be a good choice. If you wanto go further as how long it will last with the car off w/o killing your battery, divide that total wattage (75+20)by 110 and that will give you the total amperage/hour it will consume. It's about 1amp/hour; that translates into 10 hours.