Free Home Backup Power calculator with clear step-by-step results.
Sizes a home backup bank from the load you actually need to keep alive rather than whole-house demand. Energy delivered is grossed up for depth-of-discharge limits and inverter losses to give installed capacity, and the inverter is sized on the greater of continuous load with headroom or half the motor surge.
Installed capacity
Bank kWh = load W x hours / (depth of discharge x inverter efficiency) / 1000
Inverter rating
Rating = greater of continuous load x 1.25 and surge / 2
Backup power systems involve significant stored energy and interact with the grid supply. Installation, transfer switching and earthing must comply with local electrical code and be carried out by a licensed electrician.
Typically a fridge or freezer, some lighting, networking equipment, phone charging and any medical device. Space heating, electric cooking and tumble dryers are almost never economic to back up.
Quality inverters tolerate roughly twice their continuous rating for a few seconds, which covers compressor and pump starting. Cheaper units have less headroom and need the full surge figure.