Community Resilience Hub Microgrid Sizing Calculator
This calculator helps you size a microgrid for a community resilience hub such as a library, recreation center, school, or emergency shelter. By entering your critical loads, solar, battery, and generator parameters, you can explore how long the hub can ride through grid outages and what different configurations might cost over time.
Use it as a planning and education tool for grant proposals, resilience plans, and early-stage design discussions. It is not a substitute for stamped engineering drawings or detailed power system modeling, but it can quickly show how design choices affect outage coverage, fuel logistics, and lifecycle costs.
What is a community resilience hub microgrid?
A community resilience hub is a building or campus that can safely support residents during emergencies: charging phones, storing medicines, providing heating or cooling, and offering a safe place to gather. Common examples include public libraries, community centers, schools, and faith-based facilities that serve as shelters during storms, heat waves, wildfires, or other disasters.
A microgrid for a resilience hub is a small power system that can disconnect from the main grid and operate independently. It typically combines three elements:
- Solar photovoltaic (PV) to generate electricity during the day.
- Battery energy storage to store electricity and smooth out solar variability.
- Backup generator (diesel, natural gas, or propane) to cover long outages or periods of low solar production.
The goal is to keep a defined set of critical loads running for as long as possible: emergency lighting, refrigeration for food and medicines, communications, limited plug loads, and sometimes heating or cooling in selected zones that meet accessibility and health requirements.
Key inputs and simplified formulas
The calculator uses your inputs to estimate how long the microgrid can support critical loads and what the main cost components look like. Below are the core concepts behind several important fields.
Critical power and energy
- Critical Load Demand (kW) is the approximate maximum power needed for essential equipment when the hub is operating in outage mode.
- Typical Critical Energy per Day (kWh) is the total daily energy consumption for those critical loads. You can estimate it from past utility bills by scaling down to the fraction of the building that must stay on during an outage.
If the critical load were constant over the day, then energy and power would be related by:
where E is energy (kWh), P is power (kW), and t is time (hours). Real hubs have variable loads, so this tool works with daily averages and peak estimates rather than minute‑by‑minute detail.
Solar PV production
Solar energy available per day is approximated from your installed kW and local solar conditions:
- Installed Solar PV Capacity (kW) – nameplate DC or AC capacity (use a consistent convention when comparing options).
- Solar Capacity Factor (%) – average output as a percent of nameplate over the whole year (often 15–25% for fixed-tilt systems, depending on location).
- Usable Sunlight Hours per Day – an intuitive way to represent when solar is producing enough to contribute meaningfully during an outage window.
A simplified daily solar energy estimate is:
where PPV is installed solar (kW), Hsun is usable sunlight hours per day, and CF is capacity factor (%).
Battery storage
- Battery Energy Storage (kWh) – total nominal energy capacity.
- Battery Usable Fraction (%) – how much of that energy can be discharged without shortening battery life (e.g., 80–90%).
- Battery Round-Trip Efficiency (%) – energy lost when charging and discharging.
- Battery Max Discharge Power (kW) – the highest sustained output the battery inverter can provide.
The effective usable energy from the battery is roughly:
where U is usable fraction (%) and η is round-trip efficiency (%).
Generator and fuel
- Backup Generator Power (kW) – rated continuous power.
- Generator Fuel Consumption (gallons/hour) – average burn rate at typical outage loading.
- Fuel On Hand (gallons) – how much fuel you can store or expect to receive during an event.
A simple estimate of generator-only runtime is:
where Fstored is fuel on hand (gallons) and rfuel is consumption rate (gallons/hour).
How to interpret the results
After you click the simulation button, the calculator estimates how long the combined solar, battery, and generator system can support your critical loads for a target outage duration, and what each component contributes to that coverage.
- Outage coverage – compare the simulated coverage with your Target Outage Coverage (hours). If coverage is lower than your target, you may need more storage, more generation, more fuel, or deeper load shedding.
- Resource contributions – you can run scenarios that emphasize solar, battery, or generator and see which mix gives robust coverage with acceptable costs, emissions, and logistics.
- Economic metrics – the solar, battery, generator, and fuel cost inputs allow high-level lifecycle comparisons over your Analysis Horizon (years) and Discount Rate (%). Treat results as planning-level, not precise financial forecasts.
For grant and program applications, you can report approximate kW and kWh requirements, expected outage coverage, and how the system supports specific services like refrigeration, device charging, and accessible heating/cooling zones.
Worked example: library resilience hub
Imagine a mid-sized library that serves as a cooling and charging center during summer outages. Staff identify about 85 kW of critical load (lighting in key areas, some HVAC, Wi‑Fi, computers, refrigeration for medicines, and elevator service) and roughly 1,900 kWh of critical energy per day in outage mode.
They are considering the following design, similar to the default values in the form:
- Critical Load Demand: 85 kW
- Critical Energy per Day: 1,900 kWh
- Target Outage Coverage: 72 hours (3 days)
- Load Shedding & Flexibility: 15%
- Installed Solar PV Capacity: 150 kW, 18% capacity factor, 4.8 usable sunlight hours
- Battery: 600 kWh, 85% usable fraction, 92% round-trip efficiency, 250 kW discharge
- Generator: 120 kW, 8 gallons/hour fuel consumption, 600 gallons on site
With these values, the battery alone could carry a fraction of a day if fully charged, while the generator plus stored fuel could carry several days even in poor solar conditions. Solar reduces fuel use and extends coverage, particularly in multi-day heat waves where restocking fuel is challenging.
By adjusting the battery size upward (for example from 600 to 900 kWh) and slightly reducing generator capacity, planners can see whether they still achieve 72 hours of coverage with less noise and emissions but higher upfront capital cost. They can also model more aggressive load shedding (e.g., 25–30%) by tightening which areas remain open during an event.
Comparing design strategies
The table below summarizes typical tradeoffs among three common strategies: generator-heavy, balanced, and solar-plus-storage focused microgrids.
| Strategy | Typical characteristics | Strengths | Challenges |
|---|---|---|---|
| Generator-heavy | Smaller solar and battery; large generator and fuel storage. | Lower upfront capital cost; simple control; reliable if fuel deliveries are secure. | High fuel use and emissions; noise; dependence on supply chains that may be disrupted. |
| Balanced mix | Moderate solar and battery; medium generator sized for peak critical load. | Good resilience in varied conditions; reduced fuel use; flexible operation. | More complex design and controls; moderate upfront cost. |
| Solar + storage focused | Large PV and battery; smaller generator mainly for rare extended events. | Lowest fuel use and emissions; quiet; strong performance in frequent shorter outages. | Higher capital cost; must carefully size for worst-case multi-day clouds and seasonal variations. |
Use the calculator to approximate each strategy for your site by changing solar, battery, and generator inputs while holding your critical load assumptions constant.
Assumptions and limitations
This tool uses simplified methods that are appropriate for early-stage planning, not for final design. Key assumptions and limitations include:
- Averaged loads – critical loads are represented as daily energy (kWh) and a single peak demand (kW). Hourly or sub‑hourly variations, motor starting currents, and power quality issues are not modeled.
- Outage profile – the tool assumes outages as aggregated hours per event or per year, rather than a fully specified sequence of events with changing weather.
- Simplified solar resource – solar generation is estimated from capacity factor and usable sun hours, not from detailed irradiance data or shading analysis.
- Battery behavior – state of charge, degradation, and detailed control strategies are approximated. The usable fraction input is intended to reflect recommended depth of discharge for the chosen technology.
- Generator performance – fuel consumption is treated as an average gallons/hour; in practice, fuel use varies with loading, maintenance condition, and ambient temperature.
- Cost modeling – solar, battery, generator, and fuel costs are captured at a high level. The calculator does not include all potential expenses such as interconnection upgrades, structural work, ADA improvements, permitting, or ongoing O&M contracts.
- Non-binding results – outputs are for educational and planning purposes only. Any system that will serve as critical emergency infrastructure should be designed and reviewed by qualified engineers and code officials using detailed, site‑specific data.
Within these constraints, the calculator can still provide valuable insight into which combinations of solar, storage, and generators are most promising for your resilience hub and where more detailed analysis would be worthwhile.
Next steps and complementary tools
After exploring scenarios here, you may want to:
- Refine your critical load list by working with facility managers and emergency planners.
- Review local solar resource data and building constraints with a solar developer or engineer.
- Coordinate with emergency management, public health, and access and functional needs planners to ensure that power priorities align with community needs.
- Use more detailed modeling tools or work with a consultant for time‑series simulations and grid interconnection studies.
If you have other calculators or educational pages on solar sizing, battery storage, or outage cost estimation, consider linking them near this section to support deeper exploration and more complete resilience planning.
Why Community Resilience Hubs Need Tailored Microgrid Planning
Public libraries, recreation centers, and congregational halls are increasingly tapped as resilience hubs—spaces where neighbors can charge devices, receive medical support, and shelter during heat waves or winter storms. Unlike typical commercial buildings, these facilities serve as lifelines when municipal services are stressed. They must power refrigeration for medication, provide Wi-Fi for emergency updates, run HVAC systems to maintain safe temperatures, and keep essential lighting and security equipment active. Traditional backup generator calculators ignore the interplay between solar, storage, and load shedding that makes community resilience hubs so effective. This calculator bridges that gap by combining critical load estimates with solar production, battery behavior, and fuel logistics.
The model is intentionally transparent. Rather than presenting a single yes-or-no answer, it highlights how each asset contributes to outage coverage and where gaps remain. Facility managers can adjust load flexibility assumptions, tinker with solar size, and see whether fuel deliveries must be staged for week-long outages. The tool also surfaces lifecycle costs, helping grant writers quantify the annualized expense of resilience relative to the value of service provided to the community.
How the Microgrid Coverage Model Operates
The simulation begins by converting the facility’s critical load into an outage energy requirement. Critical load in kilowatts multiplied by the target outage duration produces kilowatt-hours. Load shedding reduces the total by the percentage entered, representing thermostatic setbacks, reduced lighting, or relocating nonessential equipment. Mathematically, the outage energy demand is , where is the critical load in kilowatts, is target outage hours, and is the fraction of load that can be shed. For cross-checking, you can compare the result to your typical daily energy use; if daily critical energy exceeds the outage calculation, the calculator flags the larger value to avoid unrealistic optimism.
Solar contribution is derived from the array size, capacity factor, and usable sunlight hours. Capacity factor translates the DC nameplate into average hourly output, while sunlight hours represent the window during which solar power aligns with the building’s occupied schedule. The tool estimates total kilowatt-hours produced during the outage and caps the value at the critical load to avoid overstating midday surplus. Battery support comes next: the storage bank’s usable capacity multiplies by round-trip efficiency to determine net discharge energy. The calculator also checks whether the inverter and battery discharge ratings can cover the critical load; if not, it signals that power electronics upgrades are required.
Generators provide the final layer of defense. Fuel on hand divided by hourly burn rate yields run hours, which multiply by generator power to determine kilowatt-hours available. If the generator is oversized relative to load, the calculator still respects fuel limits. Any shortfall after solar, batteries, and generators is reported in both energy and hours so teams can decide whether to procure additional mobile batteries, secure fuel contracts, or lengthen outages they are willing to tolerate.
Worked Example: 72-Hour Heat Dome Response
Consider a resilience hub that must support 85 kW of diversified load for three days. Load shedding strategies—shifting laundry to daytime and consolidating cooling zones—trim 15% of demand, resulting in an effective 72.25 kW critical load. Over 72 hours, that equates to 5,202 kWh of energy. The hub already consumes about 1,900 kWh per day during normal operations, so planners know the outage calculation is in the right ballpark. A 150 kW solar canopy operating at an 18% capacity factor generates roughly 648 kWh per day, or 1,944 kWh over three days. Because much of that output aligns with a 4.8-hour daylight window, solar covers a significant portion of daytime load and trickle charges the battery.
The battery bank stores 600 kWh, of which 85% is usable. Applying 92% round-trip efficiency leaves 469 kWh of discharge energy. That covers 6.5 hours of critical load on its own. A 120 kW generator burning 8 gallons per hour with 600 gallons of diesel can run for 75 hours, delivering 9,000 kWh—well above the remaining need. Because the generator’s output exceeds the post-solar, post-battery shortfall, fuel reserves are more than adequate. The calculator reports total supported hours of 72.0, meaning the hub meets its coverage target with modest slack. It also reveals that extending coverage to 96 hours would require either another 270 kWh of batteries or an additional 320 gallons of diesel at the current load profile.
Scenario Comparison Table
The following table shows how different asset mixes impact outage coverage. Use it as inspiration for board presentations or grant proposals. The numbers are illustrative rather than tied to your inputs.
| Scenario | Solar (kW) | Battery (kWh) | Generator Fuel (gallons) | Supported Hours |
|---|---|---|---|---|
| Minimal Generator Only | 0 | 0 | 600 | 58 |
| Balanced Hybrid | 150 | 600 | 600 | 72 |
| Extended Islanded Hub | 240 | 1,200 | 800 | 104 |
The table underscores how batteries stretch fuel supplies and how solar reduces both generator runtime and indoor air quality concerns. By presenting multiple configurations, planners can align resiliency goals with community expectations and funding realities.
Cost Modeling and Limitations
Capital costs are estimated using the unit costs you provide. Solar cost per kilowatt and battery cost per kilowatt-hour combine with generator purchase and transfer switch expenses to create a baseline investment. The calculator applies a capital recovery factor to spread that cost over the analysis horizon, then divides by expected outage hours to produce a levelized resilience cost—essentially the dollars spent per kilowatt-hour delivered during emergencies. Fuel expenses are prorated by the ratio of expected outage hours to the modeled event. If your community expects one major outage per year, set expected outage hours equal to the target outage duration. If smaller outages are common, use the historical annual duration so the model scales fuel use appropriately.
Like any planning tool, this calculator simplifies reality. It assumes solar output remains consistent with historical capacity factors, even though storms often dim sunlight. To account for cloudy conditions, you can reduce the capacity factor or increase the target outage duration to introduce a buffer. Battery degradation and generator maintenance are not explicitly modeled; consider pairing this tool with the Battery Second-Life Capacity Calculator to account for aging packs, or explore failure risks using the Microgrid Islanding Failure Risk Calculator. The model also assumes fuel deliveries are unavailable during the outage. If you have guaranteed resupply agreements, you can lower the stored fuel input to reflect just-in-time logistics.
Using the Results in Community Planning
Present the output to stakeholders as a blend of technical readiness and budget impact. Sustainability coordinators can highlight avoided generator runtime thanks to solar and batteries, while emergency managers focus on supported hours and remaining gaps. Share the levelized resilience cost with finance committees to compare the investment against alternate strategies such as temporary mobile generators or mutual aid agreements. Integrating the results into grant applications demonstrates due diligence and helps justify funding for inclusive resilience hubs that serve frontline communities.
Finally, treat the calculator as a living document. Update inputs after energy audits, post-storm lessons learned, or changes in occupancy. Cross reference normal operations using the Grid-Interactive Building Demand Flex Savings Calculator and revisit demand flexibility strategies with the Residential Demand Charge Mitigation Calculator. With consistent updates, your community resilience hub will stay ready for whatever the grid throws its way.