Generator Sizing Calculator

Purpose and model boundary

Computes real and apparent running load, unrolls six motor-start groups, compares manual, priority-shed, automatic, and listed-EMS routes, checks split-phase leg loading and a site-derated generator candidate, and interpolates an editable manufacturer fuel curve.

Do not treat this result as an electrical design, code-compliance determination, permit approval, utility commitment, equipment approval, safety certification, cost quote, or endorsement by a cited source. Service data, panel ratings, conductor properties, equipment loads, route lengths, manufacturer instructions, utility requirements, locally adopted codes, and approval thresholds remain the user's responsibility.

Inputs and units

• transfer route: a listed route or text value.
• priority cutoff: entered in priority tier.
• running reserve margin: entered in fraction.
• leg imbalance check threshold: entered in fraction of larger leg.
• ems maximum running power: entered in kW.
• ems maximum running apparent power: entered in kVA.
• ems maximum starting apparent power: entered in kVA.
• candidate running power: entered in kW.
• candidate running apparent power: entered in kVA.
• candidate surge power: entered in kW.
• candidate surge apparent power: entered in kVA.
• site capacity factor: entered in fraction of rating.
• fuel unit: a listed route or text value.
• fuel burn at zero: entered in selected unit/hour.
• fuel burn at 25: entered in selected unit/hour.
• fuel burn at 50: entered in selected unit/hour.
• fuel burn at 75: entered in selected unit/hour.
• fuel burn at 100: entered in selected unit/hour.
• outage duration: entered in hours.
• candidate source note: a listed route or text value.

The load grid contains 12 fixed data rows plus one header row with these columns: Available, Load label, Manual selected, Priority (priority rank), Connection, Running input basis, Running value (selected running basis), Running power factor (fraction), Efficiency (fraction), Starting input basis, Starting value (selected starting basis), Starting power factor (fraction), Start group (start group), Duty factor in window (fraction), Scheduled hours per day (hours/day), Source note. Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

For active load i, running real power is P_i (W) and running apparent power is:

S_i = P_i/PF_i

PF_i is running power factor (fraction). The selected row input basis first converts current, real power, apparent power, or horsepower to these quantities. The workbook totals:

P_run = sum(P_i)

S_run = sum(S_i)

over the active transfer scenario.

For start group g, simultaneous-start demand is:

S_start,g = S_run - sum_g(S_i) + sum_g(S_start,i)

with the analogous real-power expression. The required peak is the largest group result and never less than running demand. Entered reserve is applied to the required running and peak values. Candidate usable running and surge capacity is each entered nameplate rating multiplied by the single entered site capacity factor, applied alike to the real-power and apparent-power ratings; split-phase per-leg current is apparent leg load divided by 120 V, and usable per-leg current is derated candidate kVA times 1,000 divided by 240 V.

The fuel curve is not read at the instantaneous running demand. Each active row also contributes a duty-weighted average real power, and the workbook totals:

P_avg = sum(P_i d_i h_i / 24)

d_i is row i's duty factor inside its availability window (fraction, 0 to 1) and h_i is its scheduled hours per day (hours/day, 0 to 24). Their product divided by 24 is the row's average on-fraction across a whole day, so P_avg is average real power (W) over the day. The load fraction is:

lambda = P_avg / P_usable,running

P_usable,running is usable running real power (W): the candidate running power rating (kW) times 1,000 times the site capacity factor (fraction). Both terms are real power, so lambda is a duty-weighted daily average of real power against real-power capacity, not a ratio of the apparent-power demand S_run.

Hourly fuel is linearly interpolated between the two adjacent entered fuel-curve points that bracket lambda. Outage fuel is:

F_outage = f_hour(lambda) t_outage

f_hour is fuel per hour in the selected unit and t_outage is hours.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• required running power (kW): Active real running load or EMS real-power setpoint, whichever applies, plus entered reserve.
• required running apparent power (kVA): Active apparent running load or EMS apparent-power setpoint, whichever applies, plus entered reserve.
• required peak power (kW): Larger of reserved steady kW and all unrolled start-group kW events.
• required peak apparent power (kVA): Larger of reserved steady kVA, all unrolled start-group kVA events, and any visible EMS start setpoint.
• peak leg a current (A): Largest leg A current across final steady load and all unrolled start groups, using starting current for the active group and running current for earlier groups.
• peak leg b current (A): Largest leg B current across final steady load and all unrolled start groups, using starting current for the active group and running current for earlier groups.
• candidate running margin (kW or kVA binding margin): Smaller site-derated running kW or kVA margin after required running load.
• candidate surge margin (kW or kVA binding margin): Smaller site-derated surge kW or kVA margin after required peak load.
• candidate surge leg margin (A): Site-derated surge kVA divided by 240 V less the larger modeled start-event leg current; a negative result flags winding or leg surge insufficiency.
• estimated fuel rate (selected unit/hour): Piecewise-linear interpolation of the entered manufacturer fuel curve at average load fraction, capped at the 0 to 100 percent range.
• estimated outage fuel (selected fuel unit): Interpolated hourly fuel rate times entered outage duration; safe storage and supply are not determined.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• The system is a 120/240 V single-phase split-phase source. LRA times voltage is starting volt-amperes, not watts; starting power factor remains explicit.

• Rows sharing a start-group number start simultaneously. Higher-numbered groups start after lower groups are running; group zero represents non-surge load available before each event.

• Scheduled hours per day defines the daily availability window; duty factor is the average on-fraction inside that window. Their product divided by 24 weights the fuel-load estimate but does not reduce running or starting capacity requirements.

• The transient per-leg check allocates the site-derated surge apparent-power rating evenly across the 120/240 V winding as surge kVA divided by 240 V. Manufacturer-specific winding, breaker, or unbalanced surge limits must replace this planning proxy when available.

• The EMS route is used only for a listed, verified system and sizes to at least its entered maximum connected running kW, running kVA, and starting kVA setpoints.

• Generator ratings, site derate, and all fuel-curve values are user-entered manufacturer data; shipped values are synthetic illustrations.

• This is not a generator selection approval, installation design, transfer-equipment design, fuel-system design, emissions/noise review, grounding/bonding determination, short-circuit study, or code-compliance calculation.

• The model does not predict voltage/frequency dip, alternator subtransient response, nonlinear/harmonic loads, motor acceleration time, soft starters/VFDs, cycling coincidence, altitude/temperature derates beyond the entered factor, or fuel availability under site conditions.

• A qualified electrician/engineer, manufacturer, utility, fuel supplier, and authority having jurisdiction must verify the adopted code, generator and transfer ratings, load management, neutral/grounding, clearances, ventilation, fuel storage/supply, permits, and installation.

• Never backfeed premises wiring. Portable generators must remain outside at least 20 feet from the home with exhaust directed away, and working CO alarms are required on every level and outside sleeping areas.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 140 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• NFPA public record for optional standby system capacity - Official standards-development record distinguishing manual load selection, full automatic transfer load, and listed energy-management maximum connected load. Official NFPA public record; no code table or installation prescription is embedded.
• NFPA public record for transfer/interconnection equipment - Official source for the requirement that transfer/interconnection equipment prevent inadvertent source interconnection. Official NFPA public record; no code table or installation prescription is copied or reproduced.
• CPSC Carbon Monoxide Information Center - Official federal safety source for outdoor-only generator operation at least 20 feet from the home, exhaust direction, and CO alarms. Official U.S. Consumer Product Safety Commission public guidance; no safety figure or installation specification is copied or reproduced.
• OSHA Portable Generator Safety - Official federal warning against directly attaching a generator to a structure without properly installed transfer equipment. Official U.S. Occupational Safety and Health Administration public guidance; no safety figure or installation specification is copied or reproduced.
• FEMA P-1019 Emergency Power Systems for Critical Facilities - Official federal explanation that transfer equipment prevents generator backfeed into utility systems. Official U.S. Federal Emergency Management Agency public guidance; no design table is reproduced.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, forecast, or external dataset is embedded. The calculator independently implements general engineering arithmetic from cited public and official sources; all defaults, labels, conductor/load rows, equipment assumptions, diagrams, and test values are synthetic illustrations. No NFPA load table, motor table, appliance wattage table, manufacturer candidate catalog, or proprietary fuel curve is reproduced; all shipped examples are synthetic and conspicuously editable.