{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"generator-sizing"},"result":{"entity_type":"calculator","id":"generator-sizing","calculator_id":"generator-sizing","canonical_url":"https://tttkmbb.com/energy/generator-sizing","name":"Generator Sizing Calculator","title":"Generator Sizing Calculator – kW and kVA from Running Watts, Largest Motor Starting Surge and Safety Margin","category":"energy","category_name":"Home Energy & Electrical","tool_name":"estimate_generator_size","featured_mcp_tool":false,"description":"Estimates the generator size needed for a set of loads from the total running watts, the starting surge of the largest motor (peak = running total + starting − running of that motor), a safety margin and the power factor, giving the required kW (rounded up to 0.5 kW) and kVA plus the running current at 120 V and 230 V.","use_when":"You are choosing a portable or standby generator for a home, site or event and know the running and starting watts of the equipment.","do_not_use_when":"You need branch-circuit or breaker checks (use circuit-load), inverter or battery runtime (use battery-runtime), or three-phase and transfer-switch design; the sequential-start model assumes only one motor starts at a time.","inputs":[{"name":"running_watts_total","label":"Total running watts","type":"number","unit":"W","required":true,"max":10000000,"exclusive_min":0,"description":"Sum of the running (continuous) wattage of everything powered at the same time, including the largest motor's running watts.","example":3000},{"name":"largest_motor_running_watts","label":"Largest motor running watts","type":"number","unit":"W","required":false,"default":0,"min":0,"max":10000000,"description":"Running watts of the single motor-driven load with the highest starting surge (refrigerator, pump, air conditioner); 0 if none.","example":800},{"name":"largest_motor_starting_watts","label":"Largest motor starting watts","type":"number","unit":"W","required":false,"default":0,"min":0,"max":10000000,"description":"Starting (surge, LRA) watts of that motor, typically 2–3 × its running watts; must be at least its running watts.","example":2200},{"name":"safety_margin_percent","label":"Safety margin","type":"number","unit":"%","required":false,"default":20,"min":0,"max":100,"description":"Reserve added to the peak load so the generator runs below full output (20–25 % typical).","example":20},{"name":"power_factor","label":"Power factor","type":"number","required":false,"default":0.8,"max":1,"exclusive_min":0,"description":"Power factor used to convert kW to kVA and watts to amps; generators are rated at 0.8 lagging.","example":0.8}],"outputs":[{"name":"peak_watts","label":"Peak load","type":"number","unit":"W","decimals":0,"description":"running_watts_total + (largest_motor_starting_watts − largest_motor_running_watts)."},{"name":"required_watts_with_margin","label":"Required output with margin","type":"number","unit":"W","decimals":0,"description":"peak_watts × (1 + safety_margin_percent / 100)."},{"name":"recommended_kw","label":"Recommended generator size","type":"number","unit":"kW","decimals":1,"description":"required_watts_with_margin rounded up to the next 0.5 kW."},{"name":"recommended_kva","label":"Recommended size (kVA)","type":"number","unit":"kVA","decimals":2,"description":"recommended_kw / power_factor."},{"name":"running_amps_120v","label":"Running current at 120 V","type":"number","unit":"A","decimals":2,"description":"running_watts_total / (120 × power_factor)."},{"name":"running_amps_230v","label":"Running current at 230 V","type":"number","unit":"A","decimals":2,"description":"running_watts_total / (230 × power_factor)."},{"name":"load_percent_of_recommended","label":"Running load of recommended size","type":"number","unit":"%","decimals":1,"description":"running_watts_total / (recommended_kw × 1000) × 100; 50–80 % is the efficient operating band."}],"input_schema":{"type":"object","properties":{"running_watts_total":{"description":"Sum of the running (continuous) wattage of everything powered at the same time, including the largest motor's running watts. Unit: W.","type":"number","maximum":10000000,"exclusiveMinimum":0,"examples":[3000],"x-unit":"W"},"largest_motor_running_watts":{"description":"Running watts of the single motor-driven load with the highest starting surge (refrigerator, pump, air conditioner); 0 if none. Unit: W.","type":"number","minimum":0,"maximum":10000000,"default":0,"examples":[800],"x-unit":"W"},"largest_motor_starting_watts":{"description":"Starting (surge, LRA) watts of that motor, typically 2–3 × its running watts; must be at least its running watts. Unit: W.","type":"number","minimum":0,"maximum":10000000,"default":0,"examples":[2200],"x-unit":"W"},"safety_margin_percent":{"description":"Reserve added to the peak load so the generator runs below full output (20–25 % typical). Unit: %.","type":"number","minimum":0,"maximum":100,"default":20,"examples":[20],"x-unit":"%"},"power_factor":{"description":"Power factor used to convert kW to kVA and watts to amps; generators are rated at 0.8 lagging.","type":"number","maximum":1,"exclusiveMinimum":0,"default":0.8,"examples":[0.8]}},"additionalProperties":false,"required":["running_watts_total"]},"output_schema":{"type":"object","properties":{"peak_watts":{"description":"running_watts_total + (largest_motor_starting_watts − largest_motor_running_watts). Unit: W.","type":"number","x-unit":"W"},"required_watts_with_margin":{"description":"peak_watts × (1 + safety_margin_percent / 100). Unit: W.","type":"number","x-unit":"W"},"recommended_kw":{"description":"required_watts_with_margin rounded up to the next 0.5 kW. Unit: kW.","type":"number","x-unit":"kW"},"recommended_kva":{"description":"recommended_kw / power_factor. Unit: kVA.","type":"number","x-unit":"kVA"},"running_amps_120v":{"description":"running_watts_total / (120 × power_factor). Unit: A.","type":"number","x-unit":"A"},"running_amps_230v":{"description":"running_watts_total / (230 × power_factor). Unit: A.","type":"number","x-unit":"A"},"load_percent_of_recommended":{"description":"running_watts_total / (recommended_kw × 1000) × 100; 50–80 % is the efficient operating band. Unit: %.","type":"number","x-unit":"%"}}},"formula":"peak_watts = running_watts_total + (largest_motor_starting_watts − largest_motor_running_watts); required_watts_with_margin = peak_watts × (1 + safety_margin_percent / 100); recommended_kw = ceil(required_watts_with_margin / 500) × 0.5; recommended_kva = recommended_kw / power_factor; running_amps = running_watts_total / (V × power_factor)","method":"Sequential-start method: the generator must carry all running loads plus the extra surge of the largest motor when it starts (motors draw 2–6 × running power for a fraction of a second). Generators are rated in kVA at 0.8 power factor; running them at 50–80 % of rated load gives the longest life and lowest fuel use per kWh.","sources":[{"name":"Wikipedia – Engine-generator","url":"https://en.wikipedia.org/wiki/Engine-generator","type":"reference","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Power factor","url":"https://en.wikipedia.org/wiki/Power_factor","type":"reference","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Inrush current","url":"https://en.wikipedia.org/wiki/Inrush_current","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"3,000 W running, largest motor 800 W running / 2,200 W starting","inputs":{"running_watts_total":3000,"largest_motor_running_watts":800,"largest_motor_starting_watts":2200},"expected":{"peak_watts":4400,"required_watts_with_margin":5280,"recommended_kw":5.5,"recommended_kva":6.88,"running_amps_120v":31.25,"running_amps_230v":16.3},"url":"https://tttkmbb.com/api/v1/calculate/generator-sizing?running_watts_total=3000&largest_motor_running_watts=800&largest_motor_starting_watts=2200"},{"name":"6,500 W running, well pump 1,500 / 4,500 W, 25 % margin, PF 1","inputs":{"running_watts_total":6500,"largest_motor_running_watts":1500,"largest_motor_starting_watts":4500,"safety_margin_percent":25,"power_factor":1},"expected":{"peak_watts":9500,"required_watts_with_margin":11875,"recommended_kw":12,"recommended_kva":12,"running_amps_120v":54.17,"running_amps_230v":28.26},"url":"https://tttkmbb.com/api/v1/calculate/generator-sizing?running_watts_total=6500&largest_motor_running_watts=1500&largest_motor_starting_watts=4500&safety_margin_percent=25&power_factor=1"}],"faq":[{"q":"Where do I find starting watts?","a":"On the appliance nameplate as LRA (locked-rotor amps × voltage) or in the manufacturer's data; if unknown, use 3 × running watts for compressors and pumps, 2 × for fans and power tools."},{"q":"Why kVA as well as kW?","a":"Generators are rated in apparent power (kVA); the real power they can deliver is kVA × power factor, conventionally 0.8, so a 6.9 kVA set delivers 5.5 kW."},{"q":"Should I size for all motors starting together?","a":"Normally no: start loads one at a time, largest first. If several motors can start simultaneously (e.g. two compressors on one thermostat), add their surges to the peak."}],"tags":["generator size","generator sizing calculator","starting watts","kva to kw","portable generator watts","standby generator"],"related":[{"calculator_id":"circuit-load","reason":"Breaker check for the circuits the generator feeds."},{"calculator_id":"electric-power","reason":"Watts, volts and amps of individual loads."},{"calculator_id":"battery-runtime","reason":"Runtime of a battery or inverter for the same loads."}],"links":{"html":"https://tttkmbb.com/energy/generator-sizing","markdown":"https://tttkmbb.com/energy/generator-sizing.md","json":"https://tttkmbb.com/energy/generator-sizing.json","api":"https://tttkmbb.com/api/v1/calculate/generator-sizing","schema":"https://tttkmbb.com/api/v1/calculators/generator-sizing","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:44:26Z"}