Educational guide
How Many Watt-Hours Do You Need?
Watt-hours estimate runtime. Watts determine what the battery can run right now. You need both numbers—and a reserve for losses and startup surge.
The core formula
Watts × Hours ÷ Efficiency
= Required Watt-Hours
Watts
How much power now
Continuous output must exceed the running load.
Watt-hours
How long it can run
Stored energy determines approximate runtime.
Surge
Startup headroom
Motors and compressors may briefly draw far more.
Typical power ranges
Worked example
Run a 40W CPAP for 8 hours
320WhRaw energy
377WhAt 85% efficiency
452WhWith 20% reserve
500Wh+Practical minimum
Sizing decision tree
Use the right page for the next question
Need minutes and seamless switchover?
Go to the UPS guide.
UPS systemsNeed hours of portable runtime?
Go to power stations.
Power stationsNeed multiple outage days?
Plan solar and expansion.
Whole-home planningWhy this converts: Educational pages should end with a tool or product path that logically follows the lesson. The CTA is helpful, not forced.
Accuracy note: Runtime estimates are not guarantees. Actual performance changes with temperature, battery age, inverter load, device cycling, and charging overhead.