Just to reiterate, but the wattage rating is irrelevant (almost). The figure you need is the average energy used over a known period of time i.e. kIlo-Watt-hours per so many days. Then you can work out how many watts it will consume per day. Convert this to how many Amp-hours per day. Then work out how many days you can run this befor discharging your battery to 50% , 135Ah in your case. Then cut back on this figure as you need battery for other appliances too.