Whole house or essential loads? Generator decides.
Whole-house ATS covers everything but requires a large generator; essential-loads ATS covers selected circuits with a smaller generator. The transfer switch choice flows from the generator decision.
By Steven Lockhart · Reviewed by Alex Towery, C-10 #989290
Updated 2026-06-16
Whole-house transfer switch: routes all panel circuits through ATS, requires generator sized for whole-home peak load (typically 22-30 kW for Marin homes). Essential-loads transfer switch (also called critical loads sub-panel): routes only selected essential circuits through ATS, allows smaller generator (10-22 kW). Whole-house costs more in generator scope ($18,000-$28,000 vs $10,000-$15,000) but covers everything during outages. Essential-loads costs less but limits backup operation to pre-selected loads. Most Marin homes choose essential-loads.
01How the two configurations differ.
Whole-house configuration: the ATS sits between the utility meter and the main panel. When grid fails, ATS switches the entire main panel to generator power. Every circuit in the home works (subject to generator capacity limits). The generator must be sized for peak whole-home load — typically 22-30 kW for a 200A Marin home, sometimes 30-48 kW for larger homes or homes with significant electrification. Generator costs scale with size: $18,000-$28,000 for whole-house Marin install including ATS.
Essential-loads configuration: the ATS feeds a separate critical loads sub-panel containing pre-selected essential circuits. The main panel still gets grid power during outages but goes dark when grid is down; the critical loads sub-panel gets generator power. Generator sized for essential loads only — typically 10-22 kW depending on what's included. Generator costs $10,000-$15,000 for typical essential-loads Marin install including ATS. The functional limit: any circuit not on the critical loads panel doesn't work during outages.
02How to choose between them.
Choose whole-house when
Budget supports premium backup ($25,000-$32,000+ total install), home has high-value electronics or critical equipment throughout (not just in essential locations), homeowner doesn't want to think about what's covered during outages, larger home with significant load. Often paired with natural gas service for unlimited generator runtime.
Choose essential-loads when
Budget targets $12,000-$18,000 range, home's essential loads are well-defined and stable, willing to accept some circuits being dark during outages, generator size limits favor smaller installation (lot constraints, fuel availability). Most common Marin configuration.
Hybrid with smart panel
Smart panels (Span, Lumin) enable a hybrid: whole-house circuits but with software-managed load shedding during generator operation. Generator can be smaller because not all loads run simultaneously. Adds $3,000-$5,000 smart panel cost but reduces generator size requirement and adds modern monitoring.
03Common Marin configurations.
Most common (60% of installs): essential-loads with 14-22 kW generator covering refrigeration, lighting, HVAC blower (gas furnaces), networking, garage door, select outlets. Total install $13,000-$18,000. Covers homeowner essential needs during outages while keeping cost reasonable. Generator sized to allow modest expansion of essential loads in future without replacement.
Less common but increasingly chosen (25% of installs): whole-house with 22-30 kW generator. Total install $22,000-$30,000. Covers everything including HVAC, electric water heater, EV charging at moderate rates, full kitchen including range. Strong fit for homes prioritizing complete backup continuity. Last 15% of installs are smaller scope (sometimes manual transfer switch with portable generator) or larger commercial-grade systems on larger Marin properties.
Common questions.
Can my generator handle whole-house if I get the right size?
Yes — properly sized generators can handle whole-home loads. The sizing analysis: identify peak simultaneous load (which appliances actually run at once), apply continuous-duty derating (generator output rated continuous is less than peak), include starting load surge for motors (HVAC compressor, well pump, refrigerator compressor). Typical Marin 200A service home: 22-30 kW generator sufficient. Larger homes or fully electrified (EV + heat pump + induction range + electric water heater) may need 30-48 kW.
Can I expand essential loads later?
Yes — adding circuits to existing critical loads sub-panel is straightforward as long as the generator has remaining capacity. If essential loads expansion exceeds generator capacity, generator upgrade is also required. Typical expansion: started with 14 kW generator and 6 essential circuits, expanded to 8 circuits within the same generator capacity by load math. Adding heavy loads (AC, electric water heater) usually requires generator upsize.
What if I want both solar+battery and standby generator?
Many premium Marin installs include both. The battery handles short outages (minutes to hours) silently; the generator activates for extended outages (hours to days) or when battery depletes. Configuration: battery system has integrated transfer switching (Tesla Gateway, Enphase IQ Combiner); separate ATS coordinates generator with grid and battery system. Total scope: $40,000-$70,000+ for premium solar+battery+generator integration. Premium peace of mind for premium homes.
Does whole-house transfer require panel upgrade?
Often yes, especially for older Marin homes. Whole-house transfer switch interfaces directly with main panel and may require panel upgrade if existing panel is dated (Federal Pacific, Zinsco, 100A service). Essential-loads configurations have less impact on the main panel since the critical loads sub-panel is separate. Whole-house projects on older homes typically scope panel upgrade as integrated part of the project.
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.
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