Run the Runtime Calculator
Translate watts and runtime into a starting capacity target before shopping.
Open calculatorWhat Size Battery Backup Do I Need?
Battery-backup sizing starts with the devices you want to power, their average running watts, any startup surge, and the number of hours you need to stay online. This guide keeps the process factual and planning-focused.
Step 1Add up the real running watts for every device you want to protect.
Step 2Multiply watts by target runtime hours to estimate usable watt-hours.
Step 3Add efficiency losses and reserve margin before you narrow down products.
Basic Sizing Formula
Required nominal capacity ≈ running watts × target hours ÷ system efficiency. For example, a 500W load for 4 hours needs about 2,000Wh of usable energy. With 85% efficiency, that points to roughly 2,353Wh of nominal battery capacity before extra reserve.
Questions That Change the Answer
- Does the load have startup surge, compressor cycles, or motor inrush?Surge headroom
- Do you need a UPS ride-through window or multi-hour emergency runtime?Use case
- Will solar, alternator, or AC wall charging replenish the battery during use?Recharge path
- Do you want one all-in-one unit or a modular battery plus inverter setup?System design
Use the Internal Tools
Compare Real Products
Use the side-by-side comparison tool once you know your likely watt-hour range.
Compare productsBrowse the Product Database
Filter the internal catalog by type, brand, and compatibility signals.
Browse database