Add energy, not advertised wattages
For each device, multiply watts × daily hours × quantity. A 10 W lamp used for five hours consumes 50 Wh; a 60 W laptop for three hours uses 180 Wh. Together they use 230 Wh a day. Measure a compressor fridge across 24 hours because its motor cycles on and off.
Convert demand to nominal capacity
Multiply daily Wh by backup days and divide by usable fraction and conversion efficiency. Illustrative example: 600 Wh/day × two days ÷ (0.8 × 0.85) = 1,765 nominal Wh. Adjust the usable fraction and losses to the actual equipment rather than treating them as universal constants.
Check power as well as energy
Even enough Wh may not run an appliance if the inverter cannot handle its continuous watts or starting surge. Include the charge controller, protection, charging and wiring in your budget. Try our calculator, then compare storage options.
Frequently asked questions
Is a 100 Ah battery enough for my RV?
It depends on voltage, daily use, backup time and losses. At 12.8 V, 100 Ah is 1,280 nominal Wh; delivered energy differs.