Battery & Inverter
Size Calculators
Free, accurate tools for US homeowners, solar installers, and electricians. Know your battery runtime, bank size, and inverter requirements — in seconds.
Choose Your Calculator
Each tool solves a specific sizing question — pick the one that matches your planning stage.
Battery Backup Duration
Enter your battery Ah rating, voltage, and connected load to instantly calculate backup runtime — accounting for inverter efficiency and depth of discharge. Perfect for hurricane prep, off-grid cabins, and home backup.
- Lead-acid & lithium (LFP) support
- Inverter efficiency factor
- DoD-adjusted runtime
Battery Sizing Calculator
Calculate the exact Ah capacity required for your load and desired backup hours. Includes depth-of-discharge and temperature derating factors — critical for US climate zones from Alaska to Florida.
- Daily kWh input supported
- Multi-day backup planning
- Temperature derating
Inverter Sizing Calculator
Add your appliances and get a precise inverter wattage recommendation — including startup surge capacity for motors, fridges, and AC units. Supports 120V and 240V US loads.
- Surge watt calculation
- 120V / 240V US standards
- Generator & solar hybrid mode
How These Calculators Work
No sign-up, no paywall. Each tool runs standard electrical engineering formulas, adjusted for US grid voltage and common battery chemistries.
Choose your calculator
Pick from Battery Backup Duration, Battery Sizing, or Inverter Sizing — each designed for a specific planning question.
Enter your load details
Input appliance wattages, battery specs (Ah, voltage, chemistry), and required backup hours. US 120V/240V standards are pre-loaded.
Get instant results
See runtime, required bank size, or inverter wattage — with formula breakdowns so you understand exactly how the number was reached.
Built for US Power Backup Needs
From coastal hurricane zones to solar-rich sunbelt states, these tools account for US grid standards and common battery setups.
Hurricane & Storm Prep
Size a backup battery bank to keep your fridge, lights, and medical equipment running through multi-day outages.
Rooftop Solar Systems
Pair the right battery bank with your solar array to store daytime generation and cover nighttime loads.
Off-Grid Cabins & RVs
Calculate exact battery capacity and inverter size needed for reliable off-grid power, whether 12V, 24V, or 48V.
Whole-Home Backup
Find the minimum inverter wattage that handles your critical loads — HVAC, well pump, refrigerator — including surge current.
Understanding Battery & Inverter Calculations
Choosing the right inverter and battery bank is the foundation of any reliable backup power system. An undersized inverter trips under load; an undersized battery runs out before the storm passes.
Battery capacity: the Ah × Wh relationship
Battery capacity is measured in ampere-hours (Ah), but your load runs on watts. Multiply Ah × voltage to get watt-hours (Wh). A 100Ah 48V battery holds 4,800Wh. At 50% DoD for lead-acid, you have 2,400Wh usable — a 600W load runs for 4 hours.
Inverter sizing: running watts vs. surge watts
Inverter sizing starts with total running watts. But motors — fridges, well pumps, AC units — draw 2–7× their running wattage at startup. Your inverter must handle that peak surge. Always check surge capacity, not just continuous rating.
Backup Hours = (Ah × Volts × DoD) ÷ Load Watts
Battery Ah Needed = (Load Watts × Hours) ÷ (Volts × DoD × Efficiency)
Inverter Size = Sum of Running Watts × 1.25 (safety factor)Frequently Asked Questions
Common questions from US homeowners about battery backup and inverter sizing.