Usable energy
Current state of charge minus protected reserve, conversion losses and unavailable capacity.
Resilience
Battery percentage is not a resilience plan.
A useful score should answer three questions: what can keep running, for how long and how confident the estimate is. Everything else is an input or a simplification.
Short answer
Report endurance and uncertainty, not only a reassuring number.
Two homes at 80% battery charge can have completely different resilience. One may run refrigeration, networking and lights for many hours. Another may try to power electric heating, an oven and an EV charger, exhausting the same stored energy quickly.
A credible resilience result therefore needs the load plan, backup hardware and assumptions beside the estimate.
Score inputs
Six dimensions determine whether stored energy is actually useful.
Essential load
The devices that must run, their average demand and short power surges.
Backup path
Whether the inverter, switching, wiring and selected circuits can operate safely without the grid.
Future supply
Expected solar production, weather and the uncertainty around both.
Power limits
Whether simultaneous devices fit inside inverter, battery and circuit limits even when enough energy remains.
Operational readiness
Recent tests, working communications, current settings and a clear manual fallback.
Worked example
The load list changes the answer more than the battery icon.
Imagine a 10 kWh usable battery at 80% state of charge with 10% protected as minimum reserve. That leaves roughly 7 kWh before allowing for additional system losses.
0.35 kW essential loadabout 20 hours by simple division
1.00 kW mixed loadabout 7 hours
2.00 kW heavy loadabout 3.5 hours
These are arithmetic examples, not runtime promises. Real endurance changes with battery limits, inverter overhead, temperature, load cycling, surge demand and future solar. A planning margin should reduce the published estimate.
Hardware reality
Solar panels alone do not guarantee power during an outage.
Grid-connected PV commonly shuts down when the grid fails. The U.S. Department of Energy notes that backup operation needs a properly configured inverter and storage system. In practice, switching equipment, wiring and the circuits selected for backup also matter.
This is why a resilience score must distinguish energy available from loads that can actually be supplied. A full battery behind hardware that cannot island the home is not whole-home outage protection.
Electrical design and life-safety loads belong with a qualified installer. Software can improve planning, but it cannot certify an unsuitable backup installation.
Action, not anxiety
The score is useful only when it changes the plan.
- 1Before a risk window
Raise reserve when justified, charge from the grid if permitted and necessary, and verify the backup path.
- 2During an outage
Protect essential circuits, stagger high-power devices and update expected endurance from actual load.
- 3When solar returns
Use production first for essentials, then decide whether additional loads fit the forecast and battery plan.
- 4After restoration
Review what ran, where the estimate was wrong and whether reserve or load priorities need adjustment.
The conclusion is simple: resilience is a time estimate with conditions, not a battery percentage.
FAQ
Resilience score questions.
What is a home energy resilience score?
It is an estimate of how prepared a home is to keep selected essential loads operating through an outage or other energy disruption. A useful result includes expected duration, assumptions and confidence, not only a 0-100 number.
Is resilience the same as battery state of charge?
No. State of charge is only one input. Outage duration also depends on usable battery capacity, essential load, inverter limits, islanding capability, weather, future solar and forecast uncertainty.
Will rooftop solar work during a grid outage?
Not automatically. Many grid-connected PV systems shut down during an outage. Backup operation requires a suitably designed system, including compatible inverter, controls, wiring and usually storage.
Can a resilience score change before a storm?
Yes. Expected weather, solar production, battery charge and household needs change the estimate. A management system can also raise reserve or reduce nonessential demand before a known risk window.
Should medical equipment be planned from a website score?
No. Critical medical and life-safety needs require a professional continuity plan, suitable equipment and tested backup arrangements. A consumer resilience estimate is not a safety certification.
Sources
Primary resilience references.
- U.S. Department of Energy: Solar and Resilience Basics
Explains why ordinary grid-connected solar may shut down in an outage and why compatible inverter, storage and controls matter.
- U.S. Department of Energy: Distributed Energy Resources for Resilience
Describes resilience planning and REopt analysis of how long critical loads can be sustained during an outage.
- NREL REopt: outage duration depends on charge, critical load and solar
A primary REopt case study showing that survivable outage duration changes with battery state of charge, critical load and solar resource.
- NREL: Solar Resilience text version
Explains critical-load subpanels, essential loads and the controls required for solar-plus-storage backup operation.
From guide to your home
See whether GridPassport fits the equipment you already own.
Check compatibility first. If your setup is a fit, the founding-members programme is the next step toward coordinated home power automation.