For a home in SDG&E territory, a useful solar-plus-battery PPA comparison should use expected energy production, not assume the panels deliver their nameplate power whenever the sun shines. Panel temperature is one reason those figures differ. Ask how the proposal accounts for it before comparing projected payments and remaining utility costs.
A qualifying no-upfront PPA can make solar and storage accessible without an upfront equipment purchase. A realistic production forecast helps show how that arrangement could fit your household. Sources below were checked November 2, 2026.
Know which temperature the rating uses
The System Advisor Model’s PVWatts documentation defines array nameplate capacity at standard test conditions, including a solar-cell temperature of 25°C. That is the cell temperature, not simply the outdoor temperature reported by a weather app.
The same documentation distinguishes roof mounting from open racks. Airflow around an open rack helps cool the modules; residential roof mounting has more limited airflow behind them. For a given amount of sunlight, cooler cells can produce more power. A roof-mounted proposal should therefore use an appropriate mounting assumption rather than an unrelated ground-mount default.
This does not mean homeowners should alter panel clearances. The designer must select a compliant installation and model that design accurately.
Use a temperature example carefully
Sandia’s PV Performance Modeling Collaborative explains the relationship among sunlight, cell temperature and a module’s power-temperature coefficient.
For illustration only, assume a 400-watt module, a coefficient of minus 0.35% per °C, cell temperature of 45°C, and sunlight equal to the reference test irradiance. The 20°C difference gives a 7% temperature-related reduction: 400 × 0.93 = 372 watts, before other losses.
That simplified calculation is not an annual production forecast, an inverter-output prediction or a measured result for your roof. It also does not mean a 45°C weather forecast produces the same cell temperature. Ask the proposal preparer to use the selected equipment and site conditions in the full model.
Request the assumptions behind the quote
Our suggested review questions are:
- Which panel model or module category does the forecast use?
- Does the mounting assumption match the proposed roof installation?
- Where are temperature effects included, so they are not counted twice?
- Does the monthly production forecast change if the equipment changes?
- How does the revised forecast affect the PPA and utility-cost comparison?
Keep the answers with the equipment schedule. Our weather-data guide covers the separate question of which weather record underlies the estimate.
Connect production to your SDG&E account
SDG&E’s Solar Billing Plan page explains that surplus exports earn credits based on when energy reaches the grid. It directs community-choice customers to their generation provider for import and export pricing. Confirm your actual tariff and provider rather than applying Solar Billing Plan assumptions to every existing solar account.
Ask for a comparison that includes household consumption, battery operation, exports, continuing utility charges and contractual PPA payments. A reduction in estimated solar production is not automatically the same percentage change in total household costs.
The CPUC solar guide explains provider ownership and ongoing payments for generated electricity. Review offer qualification, rate, escalator, term, battery fees, maintenance and transfer provisions. No upfront purchase does not mean free electricity or guaranteed savings.
Keep backup expectations separate
PG&E’s general battery guidance explains that available charge, supported circuits and household demand limit backup duration. A production forecast does not guarantee outage runtime. Ordinary grid-tied solar alone generally does not provide outage power; recharging storage during an interruption needs compatible equipment and suitable sunlight.
Bring the equipment schedule, production assumptions and proposed payments to the West Coast Alternatives homeowner journey for a comparison grounded in your SDG&E-served home.