If you have a 400Ah battery bank, the honest answer is always: it depends on your daily usage. The capacity label is only part of the story.
A simple way to think about it is to start with usable energy, not headline capacity: Ah × V × usable SOC window. That gets you much closer to real life.
A simple way to estimate runtime
- Convert to watt-hours: 400Ah at 12V is 4,800Wh nominal.
- Use a realistic usable amount, not 100% of that figure.
- Estimate your real daily usage, including inverter and standby losses.
- Divide usable energy by daily use to estimate days of runtime.
Common mistakes
- Ignoring always-on loads like fridges, routers, alarms, and chargers.
- Forgetting winter performance and battery age reduce usable energy.
- Assuming appliance label watts equals battery draw for AC loads.
If your estimate feels too good to be true, add margin. In off-grid life, conservative plans are usually the ones that feel best day to day.
Use the Off-Grid Planner to run this with your own setup.