Case Study: Electrifying an ADU with Solar and Battery Backup – Real Costs and Payback

Accessory Dwelling Units (ADUs) are booming across California and especially in the Bay Area. With new state incentives, zoning relaxations, and the urgent push for clean energy, many homeowners are rethinking how their backyard units are powered.

At Greenberg Construction, we recently completed an all-electric ADU project integrated with solar panels and a battery backup system. The goal? Energy independence, long-term cost savings, and future-proofing.

This case study breaks down the real costs, timelines, and payback of electrifying an ADU—and offers a playbook for any Bay Area homeowner looking to do the same.

The Client Vision: Off-Grid Ready and Net-Zero Living

Our client a Palo Alto-based family had three main priorities for their new ADU:

1. Full electrification (no gas line).
2. Solar panels to reduce dependency on PG&E.
3. Battery storage to support time-of-use savings and backup power.

They also wanted an aesthetically designed unit that met Title 24 energy standards and could serve both as a guest house and as a rental unit in the future.

Planning and Design: Building Smart from the Ground Up

The first step was conducting an energy needs analysis. Based on projected ADU usage (average daily kWh, HVAC load, appliance consumption), we designed a solar array and battery system to match demand.

Key specs included:

• 800 sq. ft. ADU
• 6.5 kW rooftop solar system (18 panels)
• 1 Tesla Powerwall (13.5 kWh capacity)
• All-electric appliances (induction cooktop, heat pump HVAC, electric water heater)

During architectural planning, we oriented the roof to maximize southern solar exposure and minimized shading from nearby trees. The electrical layout was optimized to reduce installation costs by grouping loads and shortening conduit runs.

Construction & Installation Timeline

Here’s how the project unfolded:

• Week 1–4: Foundation and framing
• Week 5–7: Electrical and mechanical rough-in
• Week 8: Roof and solar racking setup
• Week 9: Solar panel and battery installation
• Week 10–12: Final finishes and Title 24 inspections

From permits to power-on, the ADU was fully operational in just under 3 months.

Cost Breakdown: Where the Money Went

Here’s the real cost data from the project:

Item	Cost
ADU Structure & Finishing	$240,000
Solar System (6.5 kW)	$17,000
Tesla Powerwall Installation	$10,500
Electrical Upgrade (200A panel)	$2,500
Permits, Title 24, Inspections	$3,500
Total	$273,500

Federal and state incentives helped offset this:

• 30% Federal Solar Tax Credit (ITC): $8,250
• California Self-Generation Incentive Program (SGIP): $3,500 for battery

Final Net Cost: $261,750

Energy Savings & Payback

The ADU’s monthly energy usage was projected at 450–500 kWh. With net metering and time-of-use savings, the solar + battery system offsets nearly 100% of the energy cost.

At $0.35/kWh average PG&E rates, that’s $160–$180/month in savings, or $2,000+ annually.

Payback Timeframe:

• Solar + Battery Total Cost: $27,500
• Annual Savings: ~$2,000+
• Payback: 13–14 years
• Add rental income potential: payback drops to <7 years

Key Benefits Beyond the Payback

While energy savings are great, the benefits went far beyond dollars:

1. Blackout Protection: The Tesla Powerwall provided seamless backup during a 6-hour PG&E outage—keeping lights, fridge, and Wi-Fi running.
2. Increased Property Value: Studies show homes with solar sell for 4–5% more. Add an ADU and that value multiplies.
3. Faster Permits: Electrified ADUs often qualify for expedited permitting under California’s green building codes.
4. Climate Impact: Offsetting nearly 6 tons of CO₂ per year, this project contributes to long-term sustainability.

Challenges and Lessons Learned

• Panel Sizing: Oversizing solar arrays for future EV charging is wise. We accounted for this by reserving inverter capacity.
• Battery Siting: Locating the battery indoors delayed inspection—placing it in the garage simplified access and compliance.
• PG&E Delays: Net metering approvals took 3 weeks; homeowners should plan for this.

Is Solar + Battery Worth It for Your ADU?

If you live in the Bay Area, the answer is increasingly yes.

With skyrocketing utility rates, more frequent blackouts, and attractive rebates, electrifying your ADU is no longer a niche idea—it’s a smart investment.

Whether you’re building a backyard rental, multigenerational unit, or work-from-home studio, consider powering it with the sun and securing it with a battery.

Want to Plan Your Own Electrified ADU?

At Greenberg Construction, we handle the full design-build process—from solar feasibility to architectural design to battery storage compliance. Let’s make your ADU efficient, sustainable, and future-ready.