Community Resilience Hub Microgrid Sizing Calculator

Introduction

This calculator helps planners estimate how a community resilience hub could stay powered when the electric grid is down. A resilience hub is usually a trusted public building such as a library, school, recreation center, faith facility, or community center that can support residents during emergencies. During a long outage, that building may need to provide cooling or heating in selected rooms, refrigeration for medicine, device charging, communications, lighting, elevator access, and a safe place for people to gather. Because those services are essential, the power system has to be sized around the loads that truly matter most.

The tool focuses on a practical hybrid microgrid made up of solar photovoltaic generation, battery storage, and a backup generator. Instead of assuming that one technology does everything, it shows how these resources work together. Solar can reduce fuel use during daylight hours, batteries can bridge short gaps and smooth operations, and generators can carry the site through long cloudy periods or overnight demand. By entering planning-level assumptions, you can quickly compare different system mixes and see whether your proposed design is likely to meet a target outage duration.

This page is intended for early-stage planning, education, grant development, and scenario testing. It is useful when a team is still deciding how much of the building must remain open, how much fuel can realistically be stored, or whether a larger battery could reduce generator dependence. It is not a substitute for engineering design, code review, interconnection studies, or detailed hourly simulation. Still, it can help a project team move from a vague resilience goal to a more concrete discussion about kilowatts, kilowatt-hours, runtime, and cost.

What is a community resilience hub microgrid?

A community resilience hub microgrid is a local power system that can disconnect from the utility grid and continue operating independently. In normal conditions, the building may use grid power as usual. During an outage, the microgrid shifts into islanded operation and serves a defined set of critical loads. Those loads are usually much smaller than the building's full connected load because the goal is not to run every outlet and every room. The goal is to preserve the services that protect health, safety, communication, and accessibility.

Most resilience hub microgrids combine three core elements. Solar photovoltaic panels generate electricity when sunlight is available. Battery storage captures energy for later use and can provide fast response when clouds pass or loads change. A backup generator supplies dependable power during long outages, especially when solar production is low or the battery has been depleted. The best mix depends on local weather, fuel logistics, budget, noise concerns, emissions goals, and the level of service the community expects from the hub.

Because resilience hubs often serve vulnerable populations, planning should be conservative. A system that looks adequate on paper may still fall short if critical loads were underestimated, if fuel deliveries are delayed, or if severe weather reduces solar output. That is why this calculator emphasizes both energy and power. A site may have enough total energy over several days, but still fail if the battery or generator cannot meet the building's required kilowatts at a given moment.

How to use

Start by entering the building's Critical Load Demand in kilowatts. This is the approximate peak power needed for the equipment and spaces that must remain active during an outage. It should reflect a realistic emergency operating mode, not full normal occupancy. Next, enter Typical Critical Energy per Day in kilowatt-hours. This captures how much energy those critical loads consume over a full day. If you do not have a direct measurement, you can estimate it from equipment schedules, submeter data, or a reduced version of whole-building utility use.

Then enter the Target Outage Coverage in hours. This is the duration you want the hub to survive without losing critical services. The Load Shedding & Flexibility field lets you model emergency operating strategies such as closing nonessential rooms, widening thermostat setpoints, reducing lighting levels, or shifting some activities to daylight hours. A higher flexibility percentage lowers the effective critical load and can significantly improve coverage.

After that, fill in the solar, battery, and generator assumptions. For solar, enter installed capacity, capacity factor, and usable sunlight hours. For the battery, enter total storage, usable fraction, round-trip efficiency, and maximum discharge power. For the generator, enter rated power, fuel burn rate, and fuel on hand. The cost fields let you compare planning-level economics across scenarios. When you click the simulation button, the calculator estimates how much of the outage can be covered and how much each resource contributes.

It is often best to run several scenarios rather than relying on one answer. Try a generator-heavy case, a balanced hybrid case, and a solar-plus-storage case. Keep the critical load assumptions constant while changing the resource mix. That approach makes tradeoffs easier to explain to facility managers, emergency planners, finance staff, and grant reviewers.

Formula

The calculator uses simplified planning equations to estimate outage support. The first relationship is the basic connection between power, time, and energy:

where E is energy in kilowatt-hours, P is power in kilowatts, and t is time in hours. The tool adjusts the critical load for load shedding, then estimates the total outage energy requirement. In narrative terms, if you can reduce nonessential demand during an emergency, the same microgrid can support the site for longer.

Solar production is estimated from installed capacity and average performance assumptions. The page preserves the planning formula already used here:

In the script, solar contribution is also bounded so the result does not over-credit energy beyond the outage need. This is a simplified representation of real operations, but it is useful for comparing options quickly.

Battery support is based on nominal storage, usable fraction, and efficiency:

The calculator also checks battery discharge power. That matters because a battery may contain enough total energy but still be unable to deliver the required kilowatts if the inverter is undersized.

Generator runtime is estimated from stored fuel and burn rate:

That runtime is then converted into available generator energy by multiplying by generator power. The script combines solar, battery, and generator contributions, compares the total delivered energy with the required outage energy, and reports supported hours, shortfall, and suggested additional battery or fuel.

The page also includes a more explicit outage-energy relationship for load shedding:

Mathematically, the outage energy demand is $E=P\times t\times (1-f)$, where $P$ is the critical load in kilowatts, $t$ is target outage hours, and $f$ is the fraction of load that can be shed.

Example

Consider a mid-sized public library that doubles as a cooling center during summer emergencies. Staff identify 85 kW of critical load for selected lighting, communications, refrigeration, elevator service, security systems, and limited HVAC in occupied refuge areas. They estimate about 1,900 kWh of critical energy per day in outage mode. The community wants the site to remain functional for 72 hours because regional storms and heat events can interrupt service for several days.

Suppose the project team enters the following values: 15% load flexibility, 150 kW of solar, an 18% solar capacity factor, 4.8 usable sunlight hours per day, a 600 kWh battery with 85% usable fraction and 92% round-trip efficiency, and a 120 kW generator burning 8 gallons per hour with 600 gallons of fuel on site. Those assumptions represent a balanced hybrid design rather than a generator-only strategy.

With those inputs, the battery provides a meaningful but limited amount of standalone support. It is especially useful for transitions, short overnight gaps, and reducing generator runtime. Solar contributes energy across the outage window and can directly cover part of the daytime load. The generator then fills the remaining gap, provided that its power rating is high enough and enough fuel is stored. In this example, the system is likely to meet the 72-hour target with some margin, while also reducing fuel use compared with a generator-only design.

This kind of example is helpful because it shows why resilience planning is not only about buying the largest generator possible. A larger battery may reduce noise, emissions, and fuel dependence. More solar may stretch fuel reserves during multi-day events. More aggressive load shedding may allow the same budget to support a longer outage. The calculator is designed to make those tradeoffs visible.

How to interpret the results

After you run the simulation, the result panel summarizes the adjusted critical load, the total outage energy requirement, and the estimated contribution from solar, battery storage, and the generator. The most important line for many users is Supported outage duration. Compare that number with your target outage hours. If the supported duration is lower than the target, the system as entered is not large enough under the current assumptions.

The Solar contribution value shows how much energy the PV system can provide during the modeled outage. The Direct daylight coverage figure highlights the portion that can directly serve daytime operations. The Battery coverage line shows how much energy the battery can actually deliver after accounting for usable fraction, efficiency, and discharge limits. The Generator contribution line reflects the remaining energy the generator can provide before fuel runs out or the outage need is fully met.

If the result includes an Energy shortfall, the calculator also estimates additional battery capacity and additional fuel that could close the gap. These are rough planning suggestions, not final design recommendations. They are useful for asking practical questions such as whether it is easier to add another battery cabinet, increase fuel storage, or reduce the critical load list.

The cost outputs are also planning-level indicators. Capital cost combines the entered solar, battery, and generator costs. Levelized resilience cost spreads annualized capital and expected fuel use over expected outage energy served. This can help compare resilience strategies on a common basis, especially when discussing grants, public funding, or long-term operating tradeoffs.

Comparing design strategies

Different communities will arrive at different microgrid designs because resilience goals vary. Some sites prioritize the lowest upfront cost and accept higher fuel dependence. Others want quieter operation, lower emissions, and less reliance on fuel deliveries during disasters. A balanced design often performs well because it spreads risk across multiple resources.

Use the calculator to test each strategy with the same critical load assumptions. That makes it easier to explain why one option may be more resilient operationally even if another appears cheaper at first glance.

Limitation

This calculator is intentionally simplified. It does not perform a full hourly dispatch simulation, detailed weather modeling, generator part-load analysis, battery degradation forecast, or electrical protection study. Real resilience hub design also depends on transfer equipment, controls, code compliance, ventilation, structural capacity, acoustic treatment, and site-specific operating procedures. Those details matter and can change the final system size.

The solar estimate uses average assumptions rather than storm-specific irradiance. In reality, severe weather can reduce production exactly when the building needs power most. The battery model treats usable fraction and efficiency in a simplified way and does not represent temperature effects, aging, or reserve state-of-charge strategies. The generator model assumes an average fuel burn rate, even though actual consumption changes with loading and maintenance condition.

Load assumptions are another major source of uncertainty. If the critical load list is incomplete, the result may be too optimistic. If the list is too broad, the project may appear unaffordable when a more disciplined emergency operating plan would make it feasible. For that reason, the best use of this tool is as a screening calculator that helps teams identify promising configurations before moving into detailed engineering.

Additional planning guidance

Community resilience hubs need tailored microgrid planning because they serve as lifelines when municipal systems are stressed. Public libraries, recreation centers, and congregational halls may need to 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 often ignore the interplay between solar, storage, and load shedding. This calculator is more useful for resilience planning because it combines those elements in one place.

The model is intentionally transparent. Rather than presenting a simple yes-or-no answer, it highlights how each asset contributes to outage coverage and where gaps remain. Facility managers can adjust load flexibility assumptions, test larger or smaller solar arrays, and see whether fuel deliveries must be staged for week-long outages. The tool also surfaces lifecycle-style cost indicators, helping grant writers and public agencies discuss the annualized expense of resilience relative to the services provided to the community.

Generators provide the final layer of defense in many scenarios. Fuel on hand divided by hourly burn rate yields run hours, which multiply by generator power to determine available energy. 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 narrow the list of critical services.

The following illustrative comparison is not tied to your exact inputs, but it shows how different asset mixes can change outage support:

These examples underline an important planning lesson: batteries can stretch fuel supplies, and solar can reduce both generator runtime and local air quality impacts. By presenting multiple configurations, planners can align resilience goals with community expectations, funding realities, and operational constraints.

After exploring scenarios here, the next step is usually to refine the critical load list with facility staff and emergency planners, review local solar resource and roof or canopy constraints, and confirm whether the building can safely host the required equipment. Teams should also coordinate with public health, accessibility, and emergency management stakeholders so that the final power priorities match actual community needs. Once a preferred concept emerges, a qualified engineer should perform detailed time-series modeling and design review.