For an SDG&E homeowner comparing solar-plus-battery proposals, “two batteries” can describe different capabilities. One design may add stored energy for a longer evening. Another may also add inverter power so more appliances can operate together.
A clear equipment list helps you choose the capability your household needs and understand what a qualifying no-upfront PPA includes. This guide uses official sources checked September 11, 2026.
Ask what each additional unit actually adds
Request the model number and function of every proposed unit. Ask the designer to show the complete system's usable energy in kilowatt-hours, continuous output in kilowatts and any separate starting-load limits.
As a specific example, Tesla's Powerwall 3 expansion guide says an Expansion unit supplies additional storage and does not contain an inverter. Its installation requires compatible Powerwall 3 equipment and the manufacturer's approved configuration.
Tesla also states in its U.S. system application note that each Expansion unit adds 13.5 kWh of energy without increasing system power. This illustrates why counting enclosures alone is insufficient. It does not establish which equipment a West Coast Alternatives proposal includes, or that every manufacturer's expansion works the same way.
Decide whether you need more hours or more simultaneous loads
Describe the problem you want the design to solve. Is it keeping existing essentials powered longer, or adding another appliance while those essentials are running?
For a hypothetical illustration, suppose a system has 10 kWh available to its loads and a 5 kW continuous output limit. Increasing available energy to 20 kWh while retaining that output limit would extend an idealized 1 kW load from 10 hours to 20 hours. It would still not support a sustained 6 kW load.
That arithmetic assumes constant demand and ignores reserve, losses and changing conditions; it is not a product specification or runtime promise. Real estimates must include those factors and appliance startup requirements. Our battery capacity and runtime guide explains the underlying units.
Check the circuits and the recharge plan
PG&E's general battery guidance explains that backup commonly serves selected circuits and that demand, reserve and solar availability affect duration. Those equipment principles are useful for SDG&E homes too; the proposed system still needs its own design review.
Ask which circuits receive backup and whether larger loads need automatic controls. More storage does not itself connect an additional circuit. Request overnight and daytime estimates using the same household priorities.
Also ask how the larger battery supply will recharge. An expansion does not add solar panels, and additional storage is useful only to the extent it can be charged. Outage replenishment requires usable sunlight and compatible backup equipment.
Include SDG&E's project review
SDG&E's installation center directs new solar or battery projects through its interconnection application process. For an expansion, have the contractor confirm the required application or modification, local permits and authorization before operation.
Ask the contractor to document any effect on your existing solar billing arrangement. Equipment compatibility alone does not establish utility approval or preservation of an existing tariff.
Make the PPA equipment schedule specific
A qualifying no-upfront solar-plus-battery PPA can preserve cash while providing generation and storage. The provider owns the solar system and charges for its generated electricity, as the CPUC's consumer guide explains.
Confirm the included units, battery ownership, installation costs, backup controls and maintenance or replacement coverage. Review ongoing payments, rates, any escalator, term and home-sale provisions. A later expansion should have its own written scope and pricing; it is not automatically included in an existing agreement.
Compare all provider payments and remaining SDG&E charges. Explore the homeowner journey with your bills and essential-load list to discuss a system sized for the capability you value. No upfront purchase does not mean free electricity; savings are not guaranteed and backup energy is finite.