Solar and Battery Sizing in California

Size the solar array and battery as related—but separate—decisions

Solar-array sizing begins with annual and seasonal electricity use, roof production potential and the utility tariff. Battery sizing begins with a different question: what work must stored energy perform, and for how long? A house can have an appropriately sized solar array but a poorly matched battery if the storage decision is based only on annual kilowatt-hours or panel count.

Collect twelve months of bills when available, plus interval data from the utility portal. Record changes that the history does not show: a planned electric vehicle, heat pump, induction range, pool equipment, accessory dwelling unit or additional occupants. Mark which changes are firm and which are possibilities. A proposal should keep measured consumption, documented future loads and modeling assumptions in separate lines so that a homeowner can see what is driving each equipment choice.

Define the battery’s job before discussing capacity

Battery storage can serve several purposes: moving midday solar energy into the evening, reducing imports during higher-priced time-of-use periods, preserving energy for outages, or supporting a selected set of circuits when the grid is down. Those goals compete for the same stored energy. A high backup reserve can improve outage readiness but leave less capacity for daily bill management. A battery optimized for frequent rate shifting may begin an unexpected outage at a lower state of charge.

Write a one-sentence priority before reviewing equipment—for example, “keep refrigeration, internet, lights and one medical device operating through a typical overnight outage,” or “reduce 4–9 p.m. grid imports while retaining a 30% emergency reserve.” The final design may balance several goals, but it should not imply that one battery can maximize every goal simultaneously. Ask who sets the operating mode, who can change the reserve and whether seasonal or storm settings are automatic.

Keep power in kW separate from energy in kWh

Kilowatts measure power at a moment; kilowatt-hours measure energy over time. Both constrain backup. A battery may store enough energy for many hours of small loads yet lack the power to start or run several large appliances together. Motors and compressors can also have brief starting demands above their normal running power.

A simple illustration shows the distinction. Ten usable kWh supplying a steady one-kW load equals ten hours before losses, reserve settings and other limits. The same energy capacity does not mean the system can supply a ten-kW load for an hour unless its inverter and backup equipment permit that output. Ask the proposal to state usable kWh, continuous kW, short-duration or surge capability, supported voltage, and any load-start limitations. Product-family names and total nominal capacity alone are not enough.

Build a circuit-level load inventory

List the equipment you want available during an outage and record its running watts, expected hours of operation and starting characteristics. Refrigerators cycle rather than run continuously. Internet equipment and medical devices may operate all night. Lighting depends on how many fixtures are used. Air conditioning, electric resistance heat, well pumps, ovens, dryers and EV charging can consume far more power than essential electronics.

For each load, estimate energy as watts multiplied by hours, then divide by 1,000 to obtain kWh. Add a clearly identified margin for cycling, conversion losses and uncertainty rather than hiding it inside every line. Decide which loads can be paused or shed. Medical or life-safety needs require a reliability plan appropriate to the device and user; do not depend on a generic solar-battery estimate. The appropriately licensed project team must confirm panel capacity, wiring, transfer equipment and the circuits that the installed backup system will actually serve.

Use usable energy—not the largest number on the brochure

A battery specification may list total or nominal energy and a smaller usable-energy figure. The proposal should identify which value its backup estimate uses. Operating reserve, state-of-charge limits, temperature, equipment protection, conversion losses and aging can further reduce energy available to household loads. Round-trip efficiency describes energy recovered after charging and discharging; it is not a promise that the full nameplate capacity reaches the circuits during an outage.

Compare at least three scenarios: the intended daily reserve, a battery that begins an outage partly charged, and a longer outage with limited solar recharge. State the assumed household load in each case. A precise-looking duration without its starting state of charge, reserve, load and loss assumptions is not decision-ready. The useful output is a range under stated conditions, not a universal “days of backup” claim.

Model solar recharge during an outage separately

Solar panels do not automatically keep operating when the grid is down. The installed system needs compatible equipment and an approved backup configuration that can isolate the home from the utility grid. When that configuration is present, available solar may serve loads and recharge the battery, subject to inverter limits, battery state of charge, weather and controls. The Department of Energy describes solar-plus-storage resilience as an islanded operating mode, but the actual capability depends on the installed design.

Do not assume the array will refill the battery every day. Compare a clear summer day with a cloudy winter day and allow for simultaneous household loads. Ask what happens when solar production exceeds the combination of household demand and available charging capacity. Confirm whether the proposed system can restart after the battery reaches its minimum state of charge, and what homeowner actions—if any—are needed to conserve energy or restore operation.

Connect daily dispatch to the California tariff

For most new residential solar systems in PG&E, SCE and SDG&E territory, California's Net Billing Tariff—called the Solar Billing Plan by the utilities—uses hourly export values that are usually below retail import prices but can be higher during some late-summer evenings. The CPUC says battery storage can help customers use or export stored energy during higher-value hours. That does not mean every battery should be charged and discharged the same way.

Identify the utility, generation provider, rate schedule and export-credit source used in the model. A community-choice provider may set generation pricing while the investor-owned utility handles delivery and interconnection. Municipal utilities use their own programs. Ask the proposal to show direct solar use, energy sent to storage, battery discharge, grid imports and exports by relevant time period. Then compare results with more reserve held for backup. Software settings and tariffs can change, so a forecast should identify its date and assumptions rather than guarantee a bill outcome.

Read the exact Q.HOME CORE G3 configuration

Qcells' August 2026 Q.HOME CORE G3 data sheet illustrates why model-level review matters. One Q.HOME CORE A7.6B13.1 cabinet lists 13.12 kWh total energy, 12.46 kWh usable energy at 95% depth of discharge and 7.6 kW rated backup output. With Q.HOME HUB G3, the documented two-cabinet configuration lists 26.24 kWh total energy and 15.2 kW rated backup output. The sheet also lists an approximately 250-millisecond grid-loss switchover and 89.7% system round-trip efficiency.

Those figures are not a recommendation for every home and do not describe a generic “Q.HOME” installation. Qcells says grid-support configuration without the Q.HOME HUB G3 does not support off-grid operation. The proposal must identify the exact battery model, cabinet count, hub or gateway, compatible solar equipment, backed-up circuits and configuration. Current manufacturer documents and the final engineered scope control if specifications change.

Check placement, interconnection and electrical scope

The cabinet is only one part of a storage project. The scope may include a hub or gateway, compatible inverter, metering equipment, load controls, a backup subpanel, service-panel work, communications and utility interconnection. Location affects clearances, environmental exposure, cable routes and the work needed to connect selected loads. Local fire and building requirements, manufacturer instructions and the serving utility's rules all apply.

Ask the proposal to show the intended mounting location and whether it is wall- or floor-mounted, indoors or outdoors. Confirm who obtains permits and utility approval, who performs the installation, and which appropriately licensed contractor appears in the agreement. All Star Solar is an independent authorized Axia by Qcells sales partner, not the battery manufacturer or installation contractor. Do not accept an office photo, product rendering or sales-partner label as evidence of the final installation details.

Compare warranty term, throughput and operating conditions

A long warranty term does not mean unrestricted cycling or unchanged capacity. Qcells' limited warranty effective April 1, 2026 lists a 15-year product warranty for covered Q.HOME CORE G3 products and a battery performance period ending at the earlier of 15 years or 68 MWh of energy throughput. The table states a 60% warranted state of health at 15 years under its measurement conditions. Registration, installation, operation, connectivity, claim timing and exclusions matter.

Read the complete warranty that applies to the exact model and ownership structure. For a lease or PPA, also read the provider agreement to learn who monitors, services and replaces equipment and what happens at transfer or contract end. Ask how the intended operating mode relates to throughput, whether internet connectivity is required, and who receives alerts. A sales summary is useful orientation, but it does not replace the manufacturer warranty and signed customer agreement.

Treat incentives as eligibility questions—not sizing shortcuts

California's Self-Generation Incentive Program maintains current handbooks, forms, eligibility materials and an Energy Storage Sizing Tool. Its February 2025 announcement required a sizing workbook for certain single-family storage projects above 15 kWh. Program budgets, steps, waitlists, customer categories and documentation can change. A potential incentive should not be counted as approved money until the responsible program administrator confirms eligibility and reservation status.

Do not enlarge a battery merely to chase a headline incentive or select a smaller unit that fails the home's documented objective. Ask whether the quoted price assumes SGIP funding, which budget category is involved, who prepares the application, and what happens if funding is unavailable. The energy design must still make sense under the actual tariff, loads, backup goals and written contract.

Use a proposal checklist before signing

A decision-ready proposal should list the battery manufacturer, exact model, cabinet count, total and usable kWh, continuous kW, surge or load-start capability, coupling method, hub or gateway, selected circuits, operating mode and reserve. It should disclose the data period, tariff, utility or CCA, load assumptions, estimated direct solar use, battery charging and discharge, imports, exports, losses and remaining utility charges. Backup estimates should state starting charge, household load and solar-recharge assumptions.

Also confirm equipment location, permits, interconnection, service-panel work, monitoring, communications, manufacturer warranty, provider service terms, installer identity and responsibility for future settings. Request a smaller and larger configuration when the choice is close, then compare the reason for each—not just monthly payment. Keep the final proposal, site plan, single-line diagram, permission-to-operate notice, manuals and warranties. The right size is the configuration whose stated capability matches the home's priorities under reviewable assumptions.

Apply this to your California home

Compare the matching solar battery installation in California page, the Axia by Qcells pathway and Qcells equipment. Local permit and utility questions are covered in our Irvine, Anaheim and Orange County pages.