How many solar panels do I need?

Start with electricity use in kWh and a roof-specific production estimate, not a national panel-count rule or the dollar total on one bill. The result is a preliminary size to discuss with an installer, not an approved roof layout.

Collect usage before dividing

Use a full year of metered consumption when available, noting missing or unusual months. Separate future EV charging or heating changes from current usage. Bills include fixed charges, taxes and sometimes time-varying rates; dividing the entire bill by one assumed rate can misstate energy use.

Use an explicit annual yield assumption

For a first-pass calculation, target annual solar energy divided by annual yield gives DC system size. Yield is expressed in kWh per kW DC per year. Obtain a location-specific estimate using the roof orientation, tilt, shading and losses. PVWatts exposes model inputs and cautions that site-specific performance and weather introduce uncertainty.

Hypothetical example: targeting 9,600 kWh/year at an assumed yield of 1,200 kWh per kW DC per year gives 8 kW DC. At 400 W per module, 8,000 W divided by 400 W gives 20 modules. The 1,200 yield and 400 W module are teaching assumptions, not a U.S. average, a roof survey or a measured production forecast.

Check roof fit and economic fit separately

Module dimensions, usable roof area, access paths, structural condition, inverter design and local requirements can constrain the layout. A professional design may differ from the arithmetic count. Do not translate watts into roof area without the actual module dimensions and required spacing.

Aiming to generate the same annual kWh that you consume does not guarantee a zero bill. Timing, onsite use, export credits and fixed charges determine value. Oversizing can add cost without comparable bill savings where exports receive lower compensation or credit use is restricted. Check utility sizing and interconnection limits rather than assuming a universal target.

Use calculators as planning screens

The panel-count tool helps explore a size assumption. The solar calculator needs explicit utility assumptions and does not automatically apply local tariffs. Ask the installer to provide both the physical design and a separately explained billing model.

Before you decide

• Collect 12 months of kWh and list expected new loads separately.
• Request the annual/monthly production model and its weather, shading, tilt and loss assumptions.
• Confirm DC array watts, module wattage and physical dimensions, and AC inverter capacity.
• Obtain a usable-roof layout and qualified structural/electrical assessment.
• Check export compensation, utility sizing limits and a smaller-system alternative before buying.

Sources and review limits

• U.S. EIA Photovoltaics and electricity — updated May 24, 2024; used for PV/DC/AC concepts, not as a 2026 market-price or tariff dataset.
• DOE national-laboratory PVWatts calculator — input fields and performance cautions reviewed; model estimates are not site surveys or production guarantees.
• CPUC Net Energy Metering and Net Billing — applies to PG&E, SCE and SDG&E retail programs, not every California utility. Public text reviewed through a reader because direct access failed; a reader snapshot is not an origin-page hash.

Source review: 2026-09-24. This is our access/review date, not the publication date of every source. The example is arithmetic only. No roof fit, permitting decision, tariff eligibility or savings outcome is guaranteed. Checklists are editorial buying questions, not individual tax or engineering advice.