An SDG&E homeowner comparing solar-plus-battery PPAs should ask what weather data supports each production estimate. A model based on a typical meteorological year can be useful for long-term planning, but it does not predict the actual weather next year or promise a particular monthly bill.
A qualifying no-upfront PPA can provide access to solar and storage without an outright equipment purchase. Understanding the forecast helps you evaluate that offer's expected benefits alongside its ongoing payments. Sources were checked October 30, 2026.
What does a typical weather year represent?
The national laboratory's System Advisor Model documentation explains that a typical-year file selects representative months from a multi-year weather record. The resulting year describes longer-term conditions; its months need not come from the same historical year. Single-year data instead describes a particular year's weather.
That distinction matters when a proposal labels a chart with calendar months. A modeled July is not a weather forecast for your next July. Ask the designer to identify the data type and explain what uncertainty remains.
For example, PVWatts V8 documentation describes its use of 2020 TMY data for covered locations and notes that model and weather-data updates can change estimates relative to the earlier version. This does not establish which software or settings your provider used; request that information from the proposal's author.
Compare proposals on documented assumptions
Our suggested forecast checklist is:
- Weather-data source, location and whether it is typical-year or single-year data.
- Modeling software and version.
- Panel layout, orientation, shading and equipment assumptions.
- Expected annual and monthly production, with the limitations explained.
- Treatment of battery operation and household electricity use.
For a hypothetical comparison, suppose two providers propose the same equipment but report different annual production. Before interpreting the larger number as a better system, ask whether they used comparable weather data and site assumptions. No output level or difference is predicted here.
A forecast can support a thoughtful decision without eliminating uncertainty. Keep it separate from any contractual production guarantee, whose measurement period, exclusions and remedy must be read in the agreement.
Connect modeled production to the SDG&E bill
SDG&E's Solar Billing Plan values imports and exports according to time, with export credits based on the hour energy reaches the grid. An annual generation total alone therefore does not determine the remaining bill.
Have the provider model your actual tariff and generation supplier. For San Diego Community Power or Clean Energy Alliance customers, the CCA's generation pricing also matters. Existing NEM customers need their own billing treatment rather than an assumed Solar Billing Plan calculation.
Request the combined cost of PPA payments, any separate battery charges and remaining utility charges under the stated assumptions. Our utility-price assumptions guide covers another variable in that comparison: projected future rate changes.
Keep backup expectations separate from an annual forecast
A typical-year solar estimate cannot establish the battery's charge at the start of a specific outage. General battery guidance explains that stored energy, supported loads and solar conditions affect duration. Confirm supported circuits, reserve settings and equipment limits. Ordinary grid-tied solar alone generally cannot power the home during an outage; compatible backup equipment is necessary, and recharging depends on conditions.
The CPUC solar guide explains PPA provider ownership and ongoing electricity payments. Review qualification, rate, escalator, term, battery ownership and fees, maintenance and home-sale terms. No upfront purchase does not mean free electricity or guaranteed savings.
Bring the proposal and its forecast assumptions to a West Coast Alternatives consultation for a clearer discussion of the equipment, backup scope and payment plan.