Watts
Watts tell you how much power a device needs while it is running.
Outage power planning
How much backup power do you actually need?
Pick what you want to keep running and for how long. We’ll estimate the battery capacity, inverter output, and startup power your plan calls for.
Calculate my backup power
No signup. Brand-neutral sizing for common 120 V plug-in loads.
01
Build your outage load
What needs to stay on?
Select only what you expect to use. You can change the default wattage for any item.
MVP boundary: use this for ordinary 120 V plug-in loads. Do not use it to size hardwired, 240 V, transfer-switch, or whole-home electrical systems.
02
Set the outage window
How long do you want to cover?
Refrigerators and freezers use a duty cycle. Short-use appliances are counted by their planned daily use.
Your result
What to size for
This is a planning estimate, not a guaranteed runtime.
Recommended minimum battery
—
—
Estimated battery energy needed—
Recommended inverter—
Recommended startup capability—
Reserve included20%
How the estimate was built
Your loads
Medical equipment: do not treat this estimate as a safety guarantee. Check the exact device requirements and backup instructions with the manufacturer, supplier, or care provider, and maintain an emergency plan that does not depend on this calculator alone.
Shop after sizing
Models that meet these minimums
Products appear only when stored manufacturer capacity and AC-output specifications clear the calculated thresholds. Verify ports/connectors, voltage, surge duration, and device-specific compatibility before purchase.
Affiliate disclosure: links are not affiliate-enabled in this MVP. If affiliate links are added later, qualifying purchases may earn a commission at no added cost to you. The calculation itself does not change.
The math, without the jargon
Four things matter.
Watt-hours
Watt-hours tell you how much stored energy the battery needs to supply over time.
Startup power
Motors and compressors can briefly draw more power when they start.
Losses + reserve
The estimate allows for conversion losses, AC-system overhead, and a 20% planning reserve.
Assumptions used by this calculator
AC delivery: 88% planning efficiency.
DC / USB delivery: 93% planning efficiency.
AC-system overhead: 15 W while the AC inverter is expected to remain enabled. Actual self-consumption varies by power station and load.
Battery reserve: 20% held back as planning margin, not as a universal battery-health requirement.
Continuous-output headroom: 20%, rounded up to a common inverter-size step.
Startup headroom: 10% above the modeled peak. The model assumes one major startup event at a time, not simultaneous compressor starts.
These are calculator assumptions, not manufacturer guarantees. At very low AC loads, a power station's own inverter draw can materially reduce runtime. Measured energy use and device-specific manufacturer guidance are better inputs when available.
Want a tighter estimate?
Use the wattage from the appliance label or manual. For refrigerators and freezers, a plug-in energy meter measured over 24 hours is better than nameplate wattage alone.
Hawaiʻi preparedness
Planning for hurricane season or a longer outage?
Use the same calculator with Hawaiʻi-specific preparedness guidance and official sources.