Before comparing savings estimates for an SDG&E-area solar-plus-battery PPA, ask the provider to model the backup reserve you expect to use. A forecast built around frequent battery discharge may not describe a household that plans to retain more stored energy for interruptions.
A suitable qualifying no-upfront PPA can make solar and storage accessible without an upfront equipment purchase. Comparing reserve scenarios helps you choose a proposal that reflects both household costs and backup priorities.
Ask for two forecasts with one changed input
Our suggested comparison is straightforward: request the same equipment, solar production, household usage, utility tariff and PPA terms, modeled at two reserve settings appropriate for the proposed system. Ask the provider to identify every other input that changes.
Tesla’s Backup Reserve guidance explains the underlying tradeoff for Powerwall: energy held for backup leaves a smaller portion available for Self-Powered or Savings mode. It also says a 100% reserve prevents those modes from using the battery and reduces their potential economic benefits. Other products may manage reserves differently; use the offered equipment’s documentation.
This is a request for a forecast comparison, not a recommendation to lower your reserve. Your household may reasonably place more value on retaining energy for an interruption.
Translate a reserve difference carefully
For an illustrative calculation only, assume a hypothetical battery has 10 kWh of usable capacity and its reserve percentages map directly to that capacity. A 20% reserve represents 2 kWh; a 50% reserve represents 5 kWh. The difference is 3 kWh of capacity allocated to backup rather than ordinary daily discharge.
That is not automatically 3 kWh of lost savings every day. Ask the designer to model whether the battery would have charged, whether household demand would have used that energy, and how exports, losses and grid purchases change. These are invented comparison inputs, not product specifications or recommended settings.
The Department of Energy’s storage explanation distinguishes energy capacity from power output and notes that storing and retrieving electricity involves losses. Avoid treating a capacity difference as guaranteed delivered electricity.
Use your actual SDG&E account assumptions
SDG&E’s Solar Billing Plan guidance explains that import and export values depend on timing and identifies the residential 4–9 p.m. on-peak period. It directs community-choice customers to their generation provider for applicable pricing. Confirm your tariff and generation provider; an existing NEM account needs its own analysis.
Ask each forecast to show annual utility purchases, export credits, PPA payments and any battery fees. Our usage-time alignment guide explains another input to hold consistent when comparing offers.
Keep the agreement beside the savings model
Changing the modeled reserve does not itself change the signed payment terms. The CPUC solar guide explains provider ownership and ongoing payments for solar electricity. Verify no-upfront eligibility, rate, escalator, term, battery ownership and fees, maintenance, replacement and transfer provisions, plus remaining utility charges. No upfront purchase does not mean free electricity or guaranteed savings.
Ask who can adjust reserve settings and whether a proposed battery program imposes operating requirements. Request an outage estimate separately: supported circuits, output limits, available charge and losses constrain runtime. Ordinary grid-tied solar alone generally does not supply outage power; compatible backup equipment and suitable sunlight are needed for solar recharge during an interruption.
Bring both cost scenarios and your backup priorities to the West Coast Alternatives homeowner journey for a proposal discussion grounded in how you intend to use the system.