Solar + battery means essential loads panel.
Adding battery to solar isn't just a battery — it's an electrical reconfiguration. Critical loads panel, transfer logic, smart panel coordination, grid-tie compliance. Here's the scope.
By Steven Lockhart · Reviewed by Alex Towery, C-10 #989290
Updated 2026-06-16
Solar + battery installations require splitting your home's loads into two panels: an essential loads (critical loads) sub-panel that the battery can power during outages, and the main panel for everything else. Battery system handles transfer logic automatically. Typical electrical scope: 200A panel upgrade if needed, dedicated essential loads sub-panel install, circuit migration between panels, smart panel integration if using Span/Lumin. Total electrical: $4,000-$12,000 incremental beyond battery hardware cost.
01Why solar + battery needs an essential loads panel.
A battery sized for whole-home backup (typically 20-40+ kWh) costs $25,000-$50,000+ and is rare in residential applications. Most battery installations use 10-20 kWh systems that can power critical loads — refrigeration, lighting, internet, select outlets, sometimes HVAC — but not everything in the home simultaneously. To enable this 'critical loads only' operation during outages, the electrical system needs to separate critical from non-critical loads physically, on different panels.
The essential loads sub-panel is fed by the battery's backup output during outages. When grid is up, it draws from grid through normal interconnection. When grid is down, the battery's transfer logic switches the essential loads panel to battery power within milliseconds. The main panel (non-critical loads: pool pump, EV charger, electric range, dryer) doesn't get battery power during outages — those loads run only when grid is available.
02Critical decisions during planning.
Which circuits go on essential loads
Refrigerator/freezer, key lighting circuits, networking equipment, garage door opener (one), select outlets, gas furnace blower (if applicable), bedroom outlets, kitchen counter outlets. Typically 8-15 circuits depending on battery capacity and homeowner priorities.
Whole-home backup option (with smart panel)
Smart panels (Span, Lumin) enable a hybrid approach — all loads on one panel, with software-managed load shedding during outages. Higher upfront cost ($3,000-$5,000 for smart panel) but eliminates physical load splitting and allows dynamic prioritization.
Generator coordination if both present
Some homes have both battery and generator. Electrical design must coordinate transfer logic so they don't conflict (battery feeds essential loads during short outages; generator can supplement for extended outages, charging battery while running).
03Common Marin scenarios.
Scenario A — adding battery to existing solar: most common. Existing solar inverter (string or microinverter), adding Tesla Powerwall, Enphase IQ Battery, or similar. Electrical scope: essential loads sub-panel ($1,500-$3,000), circuit migration ($1,500-$3,000), battery installation electrical (typically handled by battery installer with our coordination for panel-side work). Total electrical: $3,000-$6,000.
Scenario B — full solar + battery new install: simultaneous solar and battery installation, often during major remodel or new construction. Electrical scope: panel upgrade if needed ($3,500-$6,000), essential loads sub-panel or smart panel ($2,000-$5,000), pre-wiring for inverter/battery locations, conduit and structured cabling, full interconnection coordination. Total electrical: $6,000-$12,000 depending on scope.
Common questions.
Can my existing panel support solar + battery?
Depends on panel capacity and configuration. Modern 200A panels in good condition often support both with addition of essential loads sub-panel. Older or smaller panels typically need upgrade. We assess during the planning phase — panel inspection, busbar evaluation, breaker space count, code compliance review. Documented findings inform the electrical scope.
How do I decide what goes on essential loads?
Trade-off between backup runtime and what stays operational during outages. With 13.5 kWh Powerwall (typical size), essential loads list of refrigeration + lighting + networking + gas furnace blower + select outlets runs ~24-36 hours. Adding AC, electric water heater, or EV charging significantly reduces runtime. We work with homeowner to balance the priorities during planning.
Does Tesla Powerwall handle the transfer logic?
Yes — Powerwall (and Enphase IQ Battery, Generac PWRcell, others) include integrated transfer logic. During grid outages, the system disconnects from grid in milliseconds and supplies the essential loads panel from battery. When grid returns, the system re-syncs and reconnects. The electrical scope is configuring the panel topology to support this — the battery hardware handles the switching itself.
Should I get smart panel or essential loads sub-panel?
Smart panel (Span, Lumin, Schneider Energy Center) costs more upfront ($3,000-$5,000 premium) but enables software-managed load shedding without physical load splitting. Essential loads sub-panel is cheaper and uses proven approach. For households with strong preference for dynamic load control and integration with EV/heat pump electrification, smart panel is favorable. For straightforward backup, essential loads sub-panel works fine.
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.