Home Backup Power 101: How to Size a Battery System

When the grid goes down, the right battery system keeps your essentials running - but "right" depends entirely on what you need to power and for how long. Here is the simple three-step method our installers use on every site survey.

Step 1: List Your Essential Loads

Walk through your home and note what must stay on during an outage. Typical draws:

  • Refrigerator: 100-200W running, 600-1,200Wh per day
  • Wi-Fi router and modem: 10-20W
  • Lights (LED, whole home): 100-300W
  • Sump pump: 800-1,500W when running
  • Furnace fan (gas heat): 300-600W
  • Well pump: 1,000-2,000W
  • Central AC: 3,000-5,000W - the big one

Step 2: Separate Watts from Watt-Hours

Watts (W) tell you whether a system can start and run a device. Watt-hours (Wh) tell you how long it can keep running. A system with 3,600W output but only 2kWh of storage will start your well pump - and be empty in about an hour of heavy use. For most homes we recommend sizing output first (add up everything that could run at once), then storage (multiply daily essential consumption by the number of outage days you want to ride through).

Step 3: Match a System Tier

  • Essentials only (fridge, lights, phones, router): a portable station like the EcoFlow DELTA 3 Plus (1kWh, expandable to 5kWh) covers short outages.
  • Essentials plus furnace, sump, or well pump: DELTA Pro Ultra class - 6kWh+ storage, 7,200W output, expandable as needs grow.
  • Whole home, including central AC: DELTA Pro Ultra X (12kW output, 12-180kWh) with a Smart Home Panel for automatic, circuit-level switchover.

Don't Forget the Transfer Path

Extension cords work for a fridge. Anything hardwired - furnace, well pump, whole-home coverage - needs a transfer switch or smart panel installed by a licensed electrician. That is where we come in: MTD designs the system, pulls the permits, and installs it.

Request a free system proposal and we will size it for your actual loads - no guesswork.

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