Resilience Hub Backup Power Coverage Calculator
A resilience hub is a trusted place where people can go during disasters or extended grid outages to stay safe, charge devices, get information, and sometimes access heating or cooling. The Resilience Hub Backup Power Coverage Calculator helps you estimate how long your hub can keep critical services running using battery storage, solar generation, and generators or fuel cells.
This tool is intended for community resilience planners, facility managers, emergency operations staff, and consultants who need planning-level estimates. It does not replace detailed engineering design, but it can clarify whether your current concept is closer to a few hours, a day, or several days of backup coverage.
How the backup power coverage estimate works
The calculator compares your total available energy supply to the energy required to operate critical loads over the outage planning horizon. At a high level, it looks at:
- Battery storage (kWh) as a flexible energy reservoir that can be charged before or during an outage.
- Solar production (kWh per day) as an average daily contribution during the outage.
- Generator or fuel cell runtime (hours) as additional hours when critical load can be met directly by a fuel-based source.
- Critical load (kW) as the average power needed to serve essential functions at the hub.
- Planned open hours per day as the time when the hub is actively serving people and drawing its full critical load.
- System derating (%) to account for round-trip losses, inverter efficiency, and real-world operating constraints.
Conceptually, the daily energy needed to run the hub during open hours is:
Where Pcritical is the critical load in kW, and hopen is the planned open hours per day.
Total effective energy supply over the planning horizon combines:
- Battery energy after derating losses.
- Solar energy across the outage horizon, also derated.
- Generator runtime energy, approximated as critical load (kW) multiplied by generator runtime hours.
An approximate relationship is:
Where:
- Ebatt is battery capacity in kWh.
- Esolar,day is average solar production per day in kWh.
- D is the outage planning horizon in days.
- η represents the derating factor (for example, 85% if derating is 15%).
- tgen is generator or fuel cell runtime in hours.
The tool then compares total effective energy Etotal to the total load energy required over the same planning horizon. It highlights whether your current resources are likely to cover the full period or fall short and how many people you can serve within the open hours you specify.
Key inputs and how to choose them
- Critical load demand (kW): Include only essential equipment such as emergency lighting, communications, basic plug loads, limited HVAC for safe occupancy, refrigeration for medications, and key IT/AV systems. Exclude non-essential uses like decorative lighting or full-building cooling. Use an average value over your open hours.
- Battery storage capacity (kWh): Use the usable capacity, not the nameplate rating if there are depth-of-discharge limits. For example, a 200 kWh battery that is operated between 20% and 90% state of charge has about 140 kWh usable.
- Average solar production per day during outage (kWh): Enter a conservative value. Consider seasonal solar resource, typical cloud cover during the disaster season of concern, and possible shading. Many planners start with 50–70% of the site’s typical sunny-day output.
- Generator or fuel cell runtime (hours): Estimate the total number of hours you can run the generator at the critical load level, based on available fuel and expected resupply. This is different from how long the generator can run continuously without maintenance.
- Planned open hours per day (hours): Set the number of hours per day the hub will serve the public. Longer hours increase community benefit but also increase energy use. Some hubs use reduced hours at night to preserve fuel and battery life.
- People served per hour: Estimate the average number of people who can realistically use the hub per hour during open times, given staffing, space, and safety limits. This might be higher during heatwaves and lower overnight.
- Outage planning horizon (days): Choose the number of days you want to be able to operate. Common planning horizons are 1 day (short event), 3–5 days (typical storm outage), or 7+ days (major disaster).
- System derating for inefficiencies (%): Reflects losses in inverters, wiring, battery round-trip efficiency, and real-world operating constraints. Typical planning values are 10–20% derating.
Interpreting the results
After entering your assumptions, the calculator estimates whether your backup power and generation can cover the critical load for the entire planning horizon, and how many people you can serve during that time.
Consider the following when reading the results:
- Coverage shortfall: If total available energy is less than the required load energy, the hub may not be able to stay open for the full time or may need to reduce loads or hours. Look at which inputs you can realistically adjust.
- Excess margin: If there is significant excess energy, you may have room to increase open hours, serve more people, or support additional critical equipment, subject to safety and staffing.
- People served: Multiply people served per hour by open hours and days to gauge total reach. This helps compare different hub concepts or prioritize investments.
- Sensitivity to assumptions: Small changes to critical load or open hours can have large effects on coverage. It can be useful to run a few scenarios to see what combination of load, storage, and solar provides a robust plan.
Worked example: community center cooling and charging hub
Imagine a medium-sized community center that will serve as a cooling and device charging hub during summer heatwaves. The planning team uses the following values:
- Critical load demand: 18 kW (reduced HVAC, lights, outlets, networking)
- Battery storage capacity: 160 kWh usable
- Average solar production during outage: 85 kWh/day
- Generator runtime: 24 hours at the 18 kW critical load
- Planned open hours: 16 hours/day
- People served per hour: 45
- Outage planning horizon: 4 days
- System derating: 15%
The daily energy needed for critical load is approximately:
18 kW × 16 hours = 288 kWh per day.
Over 4 days, the total required energy is roughly 1,152 kWh. After applying derating, effective battery energy plus solar contributions and generator hours can be compared to this requirement. The calculator will show whether this mix of resources is sufficient for the full 4 days and, if not, how large the gap is.
If coverage is short, options might include lowering the critical load (for example, tightening thermostat setpoints or reducing plug loads), adding battery capacity, increasing solar array size, or planning for more generator fuel and runtime.
Scenario comparison: how design choices affect coverage
The table below illustrates how different planning strategies influence backup coverage. Values are illustrative, not exact outputs from the calculator.
| Scenario | Key characteristics | Typical planning horizon | Energy strategy | Trade-offs |
|---|---|---|---|---|
| Battery-only, short outage | Small hub, modest critical load, no on-site generation | 8–24 hours | Relies entirely on pre-charged batteries | Simple to operate, but limited duration and sensitive to load assumptions |
| Solar + battery, no generator | Community hub with rooftop PV and storage | 1–3 days (longer with good sun) | Battery covers nights; solar recharges during the day | Lower fuel dependence, but vulnerable to cloudy conditions and seasonal variation |
| Solar + battery + generator | Larger hub, diverse services, fuel storage on site | 3–7+ days | Generator supports peak load; solar and battery reduce fuel use | More resilient and flexible, but requires fuel logistics and maintenance |
| Reduced-load operation | Hub operates in an "energy-saving" mode | Varies | Lower critical load and shorter hours to stretch resources | Extends duration but may limit comfort or services |
Assumptions and limitations
This calculator makes simplifying assumptions to keep the tool easy to use. Treat results as planning guidance, not final design values.
- Constant average load: The tool assumes the critical load is roughly constant during open hours. In reality, loads can vary by time of day and by activity (for example, peaks when many devices charge at once).
- Average solar production: Solar inputs are averaged over the outage period and do not account for hourly variability, storms, smoke, or snow. Using conservative values is recommended.
- Generator runtime simplification: Runtime is based on operating at the critical load. Actual fuel use may differ at part-load operation, and maintenance needs may limit continuous use.
- Single derating factor: All system losses are lumped into one percentage. In practice, specific components (batteries, inverters, wiring, temperature effects) have different efficiencies.
- No detailed state-of-charge tracking: The tool does not simulate hour-by-hour battery charging and discharging, or solar curtailment. It is a high-level energy balance.
- Staffing and safety not modeled: The calculator focuses on energy and people throughput, not staffing levels, safety requirements, accessibility, or social services capabilities.
- Not a code-compliance or engineering tool: Use qualified engineers and emergency planners for final system sizing, code compliance, interconnection requirements, and safety reviews.
Because of these limitations, use the calculator to compare scenarios, highlight gaps, and start conversations with technical experts rather than to finalize equipment sizes or fuel storage quantities.
Using this tool in your planning process
To get the most value from the calculator:
- Run at least two or three scenarios with different assumptions about open hours, load levels, and resource sizes.
- Share results with your emergency management team, facility engineers, and community partners to align expectations.
- Document the assumptions you use so they can be revisited as your hub concept evolves.
- When results suggest that coverage is marginal, consider both technical upgrades (more storage, solar, or fuel) and operational strategies (staggered hours, load shedding).
By combining this high-level energy view with local knowledge and professional design support, you can build resilience hubs that reliably serve your community when they are needed most.
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.