Critical Loads Planner
Load-planning note: Critical-load planning is about priorities, not just arithmetic. A small list of essential circuits is usually more predictable than trying to run a whole home from a finite battery.
Enter up to five must-run loads. Use nameplate ratings where possible.
Inputs
| Load | Watts | Hours/day | Start surge W |
|---|---|---|---|
Result
How to use this calculator
Use this planner before sizing an inverter, transfer switch, battery, or generator-backed system.
What the result means
The result estimates the watts and energy tied to the devices you select as essential.
What the result does not settle
It does not confirm panel wiring, transfer equipment, motor-starting surge, or whether a circuit can be moved safely.
Inputs that change the answer most
- Which loads are truly essential
- Running watts and starting surge
- Hours of use during an outage
- Medical, water, refrigeration, heating, and communications needs
- Load management for large appliances
Readable method
Energy for each load = watts × hours of use. Total energy is the sum of all selected loads; inverter size must also account for simultaneous and surge watts.
Before you act
Have circuit changes and transfer equipment handled by qualified professionals, and test the final backup plan under real loads before storm season.
How this is calculated
Daily backup kWh = Σ(watts × hours) ÷ 1000. Peak running watts = simultaneous must-run watts. Surge watts = peak running watts + largest motor-start adder.
Assumptions and formula
Use these inputs as planning assumptions, not as a final design, tax filing, permit package, or equipment approval.
- running watts for each selected load
- hours of use during an outage
- motor-starting surge
- which circuits are truly essential
- load-management behavior
Formula
Energy for each load = watts × hours. Total energy is the sum of selected loads; inverter size also needs simultaneous and surge watts.
How to Choose Critical Loads
A critical-loads plan defines what keeps running when the grid is down. The goal is not to run the whole house exactly as normal; it is to preserve safety, communications, refrigeration, basic lighting, and any health or water systems the household depends on. A smaller critical-loads panel can make battery backup more affordable and predictable.
Common critical loads
- Refrigerator or freezer
- Internet modem, router, and phone charging
- LED lighting in key rooms
- Medical equipment or accessibility devices
- Well pump, sump pump, boiler controls, or security system where needed
Loads to treat carefully
Central air conditioners, electric ranges, water heaters, dryers, EV chargers, and resistance heaters can overwhelm small inverters or drain batteries quickly. Some can be supported with larger systems and load controls, but they should not be assumed by default.
Frequently Asked Questions
Why use a critical-loads panel instead of whole-home backup?
It limits the backup system to circuits that matter most, reducing inverter size, battery cost, and the risk of accidental overload during an outage.
Should a well pump be included?
Often yes, but pumps can have high starting surges. Verify both running watts and locked-rotor or starting requirements before sizing the inverter.
Sources
Source notes
Use these as starting points when the page affects a purchase, design, tax, utility, or safety decision.