Commercial generator sizing: load study drives it.
Commercial generator sizing requires load analysis matching equipment power needs to generator capacity. Right-size for actual essential loads; oversizing wastes meaningful cost.
By Steven Lockhart · Reviewed by Alex Towery, C-10 #989290
Updated 2026-06-16
Commercial generator sizing for Marin businesses: small commercial essential loads (POS, lighting, key refrigeration) 14-22 kW; standard commercial essential loads (above plus HVAC blower, basic operations) 22-30 kW; full commercial operation 30-60 kW; larger commercial or industrial 60-150 kW+. Sizing analysis: identify essential loads, calculate peak simultaneous demand with starting-current margin, apply NEC continuous-duty derating, select generator size. Wrong sizing creates operational issues (undersized) or wastes cost (oversized).
01Sizing methodology.
Step 1: identify essential loads. Equipment that must operate during outages — refrigeration, POS, key lighting, sometimes HVAC, sometimes specific business equipment. Decisions vary by business type and operational priorities.
Step 2: calculate peak simultaneous demand. Equipment nameplate × usage diversity factor. Restaurant kitchen at peak: most cooking equipment, all refrigeration, hood ventilation running simultaneously. Office: HVAC + lighting + workstations + select equipment. Add starting-current margin for motor loads (refrigeration compressors, HVAC). Apply NEC continuous-duty derating where applicable. Round up to standard generator size.
02Common Marin commercial sizing scenarios.
Restaurant (essential loads): 14-22 kW
Refrigeration + POS + emergency lighting + select kitchen. Doesn't cover full kitchen operation but protects critical inventory and revenue equipment.
Office (full operational continuity): 22-40 kW
HVAC + lighting + workstations + servers + key equipment. Enables business operation continuation during outages.
Larger commercial (whole-building): 60-150 kW
Multi-tenant commercial buildings, larger restaurants, healthcare facilities. Substantial scope requiring detailed engineering.
03Avoiding sizing mistakes.
Oversizing is the most common error. Adding 50% capacity 'just in case' costs $5,000-$15,000+ in equipment and install with no operational benefit unless that capacity is actually used. Undersizing creates worse problems (generator overload, can't run essential loads, breaker trips during outages) — but oversizing wastes money.
Right-sizing requires honest analysis of actual essential loads, not aspirational coverage. Most businesses identify essential loads through critical thinking: what stops the business if it's not operational during the outage? What protects inventory or revenue? What's life safety? Everything else is non-essential and shouldn't drive generator sizing.
Common questions.
What size generator do I need?
Marin small commercial essential loads: 14-22 kW typically. Standard operational continuity: 22-40 kW. Full whole-building or larger commercial: 60-150 kW. Specific sizing requires load study based on your actual essential equipment. We provide sizing analysis during quote phase.
Can I upgrade size later?
Generator upgrades involve substantial scope — typically requires generator replacement, sometimes service entrance changes, transfer switch upgrade. Cost similar to original installation. Right-size at original install with modest margin for changes rather than planning to upgrade later.
What about future business growth?
Modest growth margin (15-25%) above current essential loads is reasonable. Significant growth assumptions warrant scrutiny — businesses often overstate future expansion. Right-size for current need with reasonable margin; address significant growth via upgrade when actually needed.
How do you handle pricing — flat fee or hourly?
Both, depending on scope. Routine service calls and predictable scope (panel replacement, EV charger install, transfer switch install, fixture replacement) are flat-fee quoted up front. Diagnostic visits and emergency scope are hourly because we don't know what we'll find. Either way, you get a written quote before work starts on anything significant, and we don't proceed beyond the quote without your sign-off.
How quickly can you schedule?
Depends on scope and season. Emergency dispatch: same-day across Marin County (1-3 hours business hours, 1-4 hours after-hours). Same-day standard service: 3-6 hours from call when capacity allows; otherwise next business day. Scheduled service work: 1-3 weeks typical. Larger projects requiring permits and PG&E coordination: 4-12 weeks depending on scope. We're upfront about timeline at the quote stage.
What warranty do you offer on work?
Our standard residential warranty: 2 years on labor, manufacturer warranty on parts (varies by product — typically 1-10 years). Major installs (panels, generators, EV chargers) often have longer manufacturer warranties (5-10 years). If there's an issue with our work in the warranty period, we come back and fix it at no cost. We also stand behind work past warranty — repeat customers and referrals matter more than a transactional fix.
Are you licensed and insured?
Yes. California State License Board C-10 Electrical Contractor #989290, in good standing. Carry general liability insurance and workers' compensation insurance as required by California law. Bondable on projects requiring it. Happy to provide proof of insurance to clients or property managers on request. License lookup: cslb.ca.gov.
Talk to a licensed electrician in Marin.
Family-run, C-10 #989290, 20+ years in the trade. Honest scoping, fair pricing, work that lasts.