Resilience Hub Backup Power Coverage Calculator
Size battery storage, solar production, and generator reserves to keep a resilience hub open for days during extended grid outages while meeting community service targets.
Building Confidence in Community Resilience Hubs
Resilience hubs are trusted spaces—often libraries, recreation centers, churches, or cultural institutions—that open their doors wider when climate disasters or grid outages hit. Keeping lights on, refrigeration running, charging stations active, and HVAC systems humming is essential for community care. Many hubs rely on a patchwork of batteries, rooftop solar, portable generators, and fuel cells to maintain operations. Yet staff and volunteers are frequently left with questions: How long will our storage last? Can we meet our service goals for neighbors seeking cooling, medical device charging, or warm meals? Should we invest in more batteries, fuel, or energy efficiency? This calculator translates those questions into tangible numbers to guide planning sessions, grant proposals, and tabletop exercises.
After entering your hub’s critical load in kilowatts, the battery storage capacity in kilowatt-hours, expected daily solar output during a grid outage, and any generator runtime you can depend on, the tool estimates total energy available. It also factors in open hours, people served per hour, and a planning horizon for the number of outage days you want to be ready for. Finally, a derating percentage accounts for inefficiencies like inverter losses, temperature impacts, and battery depth-of-discharge limits. The result panel summarizes whether the current assets can cover the full planning horizon, how many people you can serve during that time, and what size of energy shortfall or surplus to expect. Because it runs entirely in the browser with inline JavaScript, the calculator works offline once loaded—perfect for workshops where Wi-Fi may be spotty.
Energy Balance Formula
At the heart of the calculation sits an energy balance equation comparing available stored and generated energy to the total demand over the planning horizon. In MathML form, the energy required is:
, where is the critical load in kilowatts and is the outage horizon in days. The calculator compares this requirement to the sum of usable battery energy, expected solar production during the outage, and generator runtime converted into kilowatt-hours. Because real systems cannot discharge fully without losses, the script applies the derating percentage to available energy before making the comparison.
Mathematically, the available energy is modeled as , where is battery energy, is solar contribution, is generator energy, and is the derating fraction. The result divides available energy by the critical load to estimate total coverage hours, then contrasts those hours with the target outage window.
Worked Example: Cooling Center in Hurricane Country
Imagine a coastal community center that doubles as a resilience hub during hurricanes. The building’s critical load, including HVAC, refrigeration, lighting, and communications equipment, totals 18 kW. Thanks to a recent grant, the hub has a 160 kWh battery bank, 30 kW of rooftop solar that produces roughly 85 kWh per day during cloudy storm conditions, and a propane generator with fuel for 24 hours of runtime. Staff plan to keep the hub open 16 hours per day, serving 45 people per hour with cooling, medical device charging, and meal distribution. They want to be ready for a four-day outage and estimate system inefficiencies at 15 percent.
Feeding those values into the calculator reveals that total energy available, after derating, is approximately 329 kWh. The energy required for a four-day outage is 1,728 kWh (18 kW × 24 hours × 4 days). The result panel shows a significant shortfall of roughly 1,399 kWh, meaning the hub could cover only about 0.8 days of full operations with existing assets. On the service side, the plan would serve 2,880 visits over four days (45 people × 16 hours × 4 days), but energy limits mean the hub must either reduce open hours, scale back load, or add generation.
Scenario Comparison Table
The table below shows how different investments shift the coverage outlook for the same hub.
| Scenario | Battery Capacity | Solar Production | Generator Runtime | Coverage | Shortfall |
|---|---|---|---|---|---|
| Current Assets | 160 kWh | 85 kWh/day | 24 h | 0.8 days | -1,399 kWh |
| Battery Upgrade | 320 kWh | 85 kWh/day | 24 h | 1.5 days | -1,059 kWh |
| Add Solar Canopy | 160 kWh | 160 kWh/day | 24 h | 1.1 days | -1,223 kWh |
| Hybrid Strategy | 320 kWh | 160 kWh/day | 48 h | 3.0 days | -546 kWh |
Upgrading batteries alone provides a modest improvement, while combining storage expansion with additional solar and a longer generator runtime dramatically increases coverage. However, even the hybrid strategy still falls short of the four-day target, signaling a need for either load shedding, deeper efficiency retrofits, or partnerships with nearby facilities to share the burden. Planning teams can reference the Community Solar Subscriber Allocation Balancer to evaluate whether a solar garden subscription could offset more of the load, or the Heat Pump Water Heater Load Shifting Savings Calculator to understand how electrification measures might change baseline consumption.
Service Delivery Table
Energy planning must align with social services. The following table compares people served under different operating schedules for the same hub.
| Open Hours per Day | People per Hour | Days of Operation | Total Visits | Energy Adjustment Needed |
|---|---|---|---|---|
| 16 | 45 | 4 | 2,880 | Large |
| 12 | 45 | 4 | 2,160 | Moderate |
| 10 | 35 | 4 | 1,400 | Low |
| 8 | 35 | 3 | 840 | Minimal |
Reducing hours or managing throughput lowers energy demand but also limits community support. The calculator helps teams articulate these trade-offs transparently so decision-makers can align on priorities. Combine it with staffing planners like the Language Nest Staffing and Immersion Hours Calculator when training multilingual volunteers for hub operations.
Limitations and Assumptions
The model assumes critical load remains constant throughout the outage, even though HVAC demand fluctuates with weather and human occupancy. Solar output is treated as a fixed daily value, while real storms create unpredictable generation. Generator runtime is converted directly to kilowatt-hours using critical load, yet many generators have ramp limits that reduce efficiency at partial loads. The derating factor is a rough catch-all for these complexities. Additionally, the calculator does not account for demand response events or the ability to island certain circuits. Use the results as a conversation starter, not a final engineering design. Always consult licensed electricians and code officials before procuring new equipment.
Despite these limitations, the calculator provides immediate insight into whether your hub needs more storage, energy efficiency upgrades, or partnerships with organizations modeled by the Mutual Aid Fund Runway Calculator to sustain operations. Update the inputs after every drill or outage to keep the plan current, and document lessons learned for future community leaders.