For an SDG&E-area solar-plus-battery PPA, ask how the forecast changes stored energy as the battery ages. A model that repeats first-year battery performance throughout the agreement needs an explanation, even when it already accounts for solar-panel aging.
A suitable no-upfront PPA can help eligible homeowners access solar and storage while preserving cash. Understanding the battery assumptions helps connect that access to realistic long-term household costs and backup expectations.
Separate age from use in the forecast
The System Advisor Model battery-life documentation distinguishes calendar degradation, associated with age, from cycle degradation, associated with charging and discharging. It also treats battery degradation separately from overall system degradation.
Ask the provider which model and equipment data support its assumed capacity over time. The documentation describes modeling methods; it does not predict your battery’s lifespan or require your provider to use that software.
Request these items in the proposal review:
- Initial usable storage and the capacity assumed in later years.
- How the model accounts for aging and charge/discharge use.
- Whether replacement is assumed, and in which years.
- The resulting changes in household battery use and utility purchases.
These are suggested questions, not a claim that every proposal uses the same model. If the forecast assumes a replacement, confirm the agreement’s actual service obligation and cost allocation. A modeled replacement is not itself a contractual promise.
Translate capacity into the evening plan
For a hypothetical comparison only, suppose the designer models 10 kWh of usable capacity initially and 8 kWh in a later year. That is a 2 kWh reduction before accounting for reserve, operating limits and losses. These are invented planning inputs, not specifications or a typical degradation rate.
Ask the designer to rerun the daily energy flows with that later-year capacity. Do not simply multiply the 2 kWh difference by a retail price and call it lost savings: the result depends on whether that energy would have served the home, been exported, or remained unused.
SDG&E’s Solar Billing Plan guidance explains time-dependent import and export values and the role of storage during evening demand. It identifies 4–9 p.m. as the residential on-peak window and directs community-choice customers to their generation provider for applicable pricing. Confirm your own tariff and generation provider; existing NEM accounts need their own analysis.
Keep the warranty beside the model
Our battery-capacity warranty guide explains why a retention threshold and an expected performance forecast are different. Ask for the warranty applicable to the exact offered model and the PPA provider’s separate maintenance and replacement commitments.
For outages, general battery guidance explains that available charge, connected loads and solar recharge limit duration. Ask for later-year backup estimates as well as first-year estimates, with supported circuits and output limits identified. Ordinary grid-tied solar alone generally does not provide outage power; backup requires compatible equipment and suitable conditions for solar recharge.
Compare the whole payment picture
The CPUC solar guide explains provider ownership and ongoing payments for solar electricity under a PPA. Review the price, escalator, term, battery fees, maintenance and home-sale provisions, together with remaining utility charges. No-upfront eligibility depends on the actual offer and property; electricity is not free and savings are not guaranteed.
Bring the battery capacity assumptions, yearly cost comparison and proposed agreement to the West Coast Alternatives homeowner journey. A clear explanation of the battery’s later years helps you compare offers built around your SDG&E account and household priorities.