Battery Sizing Calculator: How to Size Any Backup System

The single most common mistake in backup storage is sizing the battery to a feeling instead of a number. Too small and it dies at 2 a.m.; too large and you paid for capacity you will never cycle. This guide gives you a five-step method that works for a city apartment, a villa or a small factory — the same math, just different inputs.

Capacity = Daily kWh × Backup Hours ÷ Depth of Discharge. Everything else is just filling in those three numbers honestly.

Step 1: List Your Loads

Write down every device you want the battery to cover during an outage or off-grid stretch. For each, note its rated power in watts and how many hours per day it runs on battery.

Step 2: Daily kWh

The formula is simple: kWh = Watts × Hours ÷ 1,000. Add every row. That subtotal is the energy your battery must deliver per day of backup operation. If you are pairing with solar, this is also the amount your panels need to replace.

Step 3: Backup Hours

Decide how long you need to run on battery alone. A weekend cabin might need 48 hours; a city flat during a grid event might only need 6–8 hours overnight. Backup hours is the multiplier that turns a daily figure into a storage requirement.

Step 4: Depth of Discharge (DoD)

You can never use 100% of a battery without killing it. LiFePO₄ safely delivers 80–90% DoD; lead-acid should stay at 50%. A higher usable DoD means you need a smaller nominal pack for the same job. Always size against usable capacity, not nameplate.

Step 5: The Capacity Formula

Put it together:

Required capacity (kWh) = Daily kWh × (Backup Hours ÷ 24) ÷ DoD

Using our 10.7 kWh/day example, 8 hours of backup and a 0.9 DoD: 10.7 × (8 ÷ 24) ÷ 0.9 ≈ 3.96 kWh usable — so you would round up to a 5 kWh unit.

Worked Examples: 5 / 10 / 20 kWh

Pick the Right Product Tier

Once the number is on paper, match it to hardware rather than the other way around:

• SH-Home covers 5–16 kWh residential installs — the sweet spot for villas and family homes.
• SH-Stack starts where a single cabinet stops, scaling from 20 kWh upward for whole-property or small-commercial needs.
• A SH-Go portable handles the 1–3 kWh tier for lights, routers and medical devices during short outages.

Bottom Line

Sizing is not guesswork. List the loads, sum the daily kWh, pick honest backup hours, respect the DoD, and the capacity falls out of one line of arithmetic. Get that line right and the rest of the project — inverter, solar array, payback — becomes straightforward.

SH Energy supplies LiFePO₄ systems from 5 kWh to multi-hundred-kWh stacks. Message us on WhatsApp with your load list and we will size it for you.