Planning estimate only, not a formal site assessment. Uses state-average sunlight, dated electricity-rate defaults and an 80% system derate. A professional must assess your specific roof, shading, equipment and utility rules.
How to Calculate Your Solar System Size & Production Calculator Step‑by‑Step
- →US State — All 50 states plus DC. State-average sunlight and EIA 2024 residential electricity rates; national default: 5 peak sun hours/day and 16.48¢/kWh.
- →How do you know your electricity use? — Use a 12-month average. Bill mode estimates usage from the selected state's average rate; fixed charges can make it less accurate.
- →Average Monthly Electricity Usage — Add your last 12 months of kWh and divide by 12. This field is used only in monthly-usage mode.
- →Average Monthly Electricity Bill — Used only in monthly-bill mode. Divide dollars by the state's $/kWh rate to estimate monthly usage; exclude fixed charges if possible.
- →Sun-Exposure Quality — Illustrative roof multiplier applied to state sun hours, separate from the 80% equipment/system derate. Not a measured shading assessment.
- →Desired Energy Offset — Share of annual electricity consumption you want solar to cover. 100% annual energy offset does not mean a $0 utility bill or off-grid independence.
Hidden Factors Most People Forget
Plan Your Solar System Before Requesting Quotes
You do not need an installer proposal to start exploring solar. This calculator estimates the scale of a system that could cover a chosen share of your household electricity use. It translates your monthly consumption, state sunlight, and broad roof-exposure conditions into required kilowatts of DC capacity, a whole-panel count, expected annual generation, and potential electricity-bill savings. It is designed for early research, when you are deciding what to ask installers rather than comparing final contracts.
The results are planning estimates, not an equipment recommendation or a formal site assessment. State averages cannot see your roof, local weather, tree cover, utility tariff, or electrical service. A useful result gives you a starting question: could an array of roughly this size fit my property and meet my energy goal? A professional assessment must answer that question before you buy equipment, sign financing documents, or assume savings in a household budget.
Choose Electricity Usage or a Monthly Bill
Electricity consumption in kilowatt-hours is the better starting point. Collect twelve months of utility statements, add the kWh, and divide by twelve. This smooths seasonal changes such as summer air conditioning and winter electric heating. The calculator multiplies that monthly average by twelve to recover annual consumption. A single unusually hot or mild month can produce a misleading sizing estimate, even if you enter it accurately.
If you only know your bill, select monthly-bill mode. The tool divides your average monthly dollars by the selected state's residential electricity price in dollars per kWh. For example, a $150 bill at the national default of $0.1648 per kWh implies approximately 910 kWh per month. This is a rough conversion: fixed service charges, taxes, tiered pricing, discounts, and time-of-use rates mean your actual bill may not equal consumption multiplied by one average rate. Exclude unavoidable fixed charges when possible.
Why Your State Changes the Required System Size
A peak sun hour represents the equivalent of one hour of sunlight at a reference intensity of 1,000 watts per square meter. It is not an hour of daylight. Several hours of weaker morning, evening, or cloudy light can add up to fewer peak sun hours. Multiplying daily peak sun hours by 365 gives an annual sunlight baseline used in this simplified production model.
The state lookup includes all fifty states and the District of Columbia. Arizona and New Mexico use 6.5 peak sun hours per day, Nevada 6.4, and California 5.6. Washington uses 3.8, while New York and Vermont use 4.0. These annual-average planning values explain why identical household consumption can require different array sizes. Conditions vary within each state: coastal California, inland deserts, eastern Washington, and the Puget Sound region do not share a single roof-level solar resource.
Methodology: Sun Hours, Roof Exposure, and System Derate
The roof-exposure selector modifies the state's sunlight baseline. Excellent exposure retains 100%, good retains 90%, fair retains 75%, and poor retains 55%. These are transparent scenario assumptions, not measured shading percentages. Excellent describes favorable direction with little shade; good allows a modest penalty; fair represents more noticeable limitations; poor represents substantial shading or an unfavorable orientation. Change the setting to compare scenarios, not to certify a roof.
Separately, the calculator applies a fixed 80% system derate. DC panel ratings describe standard test conditions, while real production is reduced by inverter conversion, wiring losses, temperature, soiling, and other equipment or operating effects. Retaining 80% is an industry-style planning assumption, not a claim that panels have 80% conversion efficiency. Roof exposure and system losses are separate multipliers: good exposure combined with the system derate retains 0.90 × 0.80, or 72%, of the base ideal yield.
Effective annual sun hours = state daily peak sun hours × 365 × roof-exposure multiplier. Annual production per kW DC then equals effective annual sun hours × 0.80. Your target energy is annual household kWh × desired offset percentage. Finally, required system size in kW DC equals target energy divided by annual production per kW. These units matter: kW measures rated capacity, while kWh measures energy generated or consumed over time.
From Required Kilowatts to Whole Panels
This tool assumes standard 400W panels, each contributing 0.40 kW of DC nameplate capacity. It divides the required system size by 0.40 and rounds up to a whole panel. A theoretical requirement of 5.69 kW therefore becomes fifteen panels and a 6.00 kW array. Both the unrounded required capacity and the rounded practical array capacity are shown, so you can understand why the estimated production may slightly exceed your target.
Annual production uses the rounded array, not the theoretical requirement: whole-panel capacity × effective annual sun hours × 0.80. With 900 kWh monthly use, excellent exposure, and a 100% target, Arizona's annual yield is 6.5 × 365 × 0.80 = 1,898 kWh per kW. A 6.00 kW array produces an estimated 11,388 kWh per year. In Washington, the same calculation yields 1,109.6 kWh per kW, requiring twenty-five panels, or 10.00 kW, to produce about 11,096 kWh.
Energy Offset Is Not the Same as Bill Offset
Your desired offset is a share of annual electricity consumption. A 50% target sizes an array for half your annual kWh; a 100% target aims to match the full year's consumption. That does not mean the panels meet every hour of demand. Solar generates during daylight, while your household may need grid electricity after sunset. Annual production can match annual usage even when you still import energy at night and export it during the day.
Potential annual savings equal the smaller of estimated annual production and annual consumption, multiplied by the state's residential electricity rate. This cap prevents extra production from receiving unlimited savings credit. In bill mode, the cap also limits savings to your entered annual bill under the model's conversion. This is a retail-equivalent estimate, not a tariff simulation: low export-credit rates, unavoidable service charges, and a mismatch between generation and consumption can make realized savings lower.
What an Installer Still Needs to Check
A panel count does not establish that those panels fit safely on your roof. Installers evaluate roof dimensions, pitch, direction, age, structure, fire setbacks, shading throughout the year, electrical equipment, and local permitting requirements. They also model seasonal production, inverter sizing, and utility interconnection rules. Batteries, financing, installation prices, incentives, and long-term degradation are not calculated here. This annual grid-connected estimate should not be used to size an off-grid system for its darkest month.
The Next Stage: Cost and Payback
This calculator intentionally does not estimate installation price or break-even time. Once you have an installer quote, use the Solar Panel Payback Calculator to see when it pays for itself. Enter the quoted cost, incentives you are actually eligible for, and the expected electricity-bill offset supported by the proposal. Do not automatically copy the energy-offset percentage into a bill-offset field; ask how fixed charges and export-credit rules affect the dollar savings.
Sources and Assumption Dates
Residential electricity-rate defaults come from the US Energy Information Administration's Electric Power Annual, Table 2.10, using the 2024 residential column in cents per kWh. Sunlight defaults follow the published Unbound Solar state peak-sun-hours planning table. They are static assumptions, not live rates or address-level weather forecasts. For more precise location and roof modeling, compare your proposal with PVWatts and a professional site assessment.