Backup power sizing is not about buying the largest device you can tolerate. It is about matching power to the outage you are actually planning for.
Start with Outage Priority List . Then size around the loads that made the cut.
Step 1: List the loads
For each load, write:
- watts
- hours needed
- whether it has startup surge
- whether it is safety-critical
- whether it can be cycled or delayed
Use measured data where possible. A refrigerator is easier to plan when you know its daily kWh and surge behavior.
Step 2: Calculate energy
watts x hours = watt-hours
Add the watt-hours. Then add margin for inverter losses, battery reserve, cold weather, aging, and uncertainty.
Step 3: Check power output
Energy capacity tells you how long the system can run. Power output tells you what can run at the same time.
If the system has 2 kWh of usable capacity but only modest AC output, it may charge phones and run a router beautifully while failing to start a compressor load. Capacity and output are separate decisions.
Step 4: Decide recharge strategy
Backup systems need a way back to full:
- wall charging before the storm
- solar charging during daylight
- vehicle charging if supported
- generator charging where safe and appropriate
- grid recharge after power returns
Solar recharge sounds simple, but the input limit matters. A battery that can accept only a small solar input may recharge slowly even with more panels available.
Decision section
| Outage target | Typical direction |
|---|---|
| Phones and lights | Small battery system |
| Router and laptop | Portable power station or UPS-style plan |
| Refrigerator overnight | Larger portable station or critical-load backup |
| Sump pump | Surge-capable system and careful safety planning |
| HVAC or whole home | Permanent battery, generator, or integrated design |
Continue with Battery Runtime Calculator for the actual math.
