Rooftop Solar: Estimate How Much Electricity It Can Cover

This estimate compares your home’s annual electricity use with the electricity a rooftop solar system could generate. Enter your yearly use, proposed system size, and a local production estimate to see the projected annual coverage percentage and any estimated surplus or shortfall.

Formula

Estimated annual solar generation = system size × annual production per kilowatt.

Estimated use covered = 100 × annual solar generation ÷ annual electricity use.

Estimated balance = annual solar generation − annual electricity use.

Here, u is your household’s annual electricity consumption in kilowatt-hours per year (kWh/year); s is the planned system’s rated capacity in kilowatts (kW); and y is estimated annual production per installed kW, in kWh/kW/year. The result c is a percentage, g is estimated yearly generation in kWh, and b is the generation surplus or shortfall in kWh. A negative balance indicates less estimated generation than annual use. Coverage above 100% is possible mathematically; it does not necessarily mean your utility will credit the excess at the same value as electricity you buy.

Worked Example

Using the calculator’s defaults: annual use is 10,000 kWh, system size is 6 kW, and the production estimate is 1,200 kWh per kW per year.

1. Estimate generation: 6 kW × 1,200 kWh/kW/year = 7,200 kWh/year.
2. Calculate the share of annual use: 100 × 7,200 ÷ 10,000 = 72.0%.
3. Find the annual balance: 7,200 − 10,000 = −2,800 kWh/year.

Under these assumptions, the system generates an amount equal to about 72.0% of annual use, with a projected annual generation shortfall of 2,800 kWh. This is an energy estimate, not a promise that solar will supply that exact share of your home’s electricity at every moment.

What You’ll Need

• A recent electricity bill or utility account history showing annual consumption in kWh. If you only have monthly bills, collect all 12 months.
• A proposed solar system size in kW, preferably from a preliminary design. Do not confuse kW (capacity) with kWh (energy).
• A credible estimate of annual solar production for your location and roof, in kWh per kW of system capacity per year. A local installer’s preliminary estimate or a reputable solar resource estimator can help.
• Basic details about roof direction, shading, roof pitch, and any planned changes to household electricity use.

Step-by-Step

1. Find your annual electricity use. Look for “annual usage” or total kWh on your utility account. If unavailable, add the kWh from 12 consecutive monthly bills. Use actual consumption rather than the amount paid: bills can include fixed charges, delivery charges, taxes, or credits that do not measure energy use. If you have fewer than 12 months, note that the estimate may miss seasonal peaks.
2. Account for likely changes in demand. Consider whether you will add an electric vehicle, heat pump, electric water heater, or other major load, or improve insulation or replace equipment. Use a realistic future annual kWh estimate if the change is likely; otherwise, keep current use and calculate a separate scenario. Avoid assuming a small recent bill represents a full year.
3. Choose a system size to evaluate. Enter the proposed DC nameplate capacity in kW from the design. If a proposal gives watts, divide by 1,000: for example, 6,000 W is 6 kW. The final usable roof area, equipment layout, local rules, and electrical design may constrain the size, so treat an early figure as provisional.
4. Get a location- and roof-specific production estimate. Enter expected annual kWh per kW, not the system’s total generation. Production varies with location, roof orientation and tilt, shade, weather, equipment, and system losses. Prefer an estimate that uses your actual roof and accounts for shading and typical losses. If a source gives total annual kWh for the proposed system, divide that figure by system kW to obtain the input value.
5. Calculate and read the results. The headline percentage compares estimated annual generation with annual consumption. Check the generation and balance outputs too: they show the underlying kWh figures and make it easier to spot a unit or input error. A result of 80% means annual generation is estimated at 80% of annual use; it does not mean the home is solar-powered for 80% of each day.
6. Test more than one scenario. Repeat the calculation with a smaller and larger feasible system, or with lower and higher production assumptions. A range is more useful than a single precise-looking number because weather, shade, and future use are uncertain. Ask a qualified solar professional to validate the roof design and production estimate before making a decision.
7. Check timing and utility arrangements separately. Solar output changes by hour and season, while household use also changes. A grid-connected home can export excess generation and draw electricity at other times, but billing credits and export rules vary by utility and location. Battery storage changes when energy can be used; it does not increase the panels’ annual generation. Review the utility’s current terms rather than assuming annual matching means no bill or complete independence from the grid.

Common Mistakes

• Mixing up kW and kWh: kW describes system capacity; kWh measures energy used or generated over time. Enter annual consumption in kWh and system size in kW.
• Using a national average as a roof forecast: Two roofs in the same area can have different shade and orientation. A generic production figure may materially over- or underestimate output.
• Comparing a partial year with a full-year estimate: Solar output and household use are seasonal. Use 12 months of consumption or clearly label an extrapolation as rough.
• Treating annual coverage as hourly coverage: Panels may produce a surplus at midday and little or none at night. Annual totals do not show whether generation coincides with demand.
• Rounding too early or assuming exactness: Keep source figures as provided during the calculation, then round the final percentage. The estimate is only as reliable as its inputs.

Tips

• Use utility interval data, if available, to understand when you consume electricity; it complements, but does not replace, the annual estimate.
• Ask for the solar proposal’s estimated annual generation in kWh and the assumptions behind it, including shading, roof direction, and system losses.
• Compare estimates for different roof sections if panels would face different directions or have different shading.
• Keep a record of the bill period and assumptions so you can update the estimate when household use or the proposed design changes.

Troubleshooting

The calculator rejects an input: Enter numbers only, without commas or units. Use kWh/year for annual use, kW for system size, and kWh per kW per year for production. All three inputs must be zero or greater; a zero annual-use value cannot produce a meaningful coverage percentage.

The result is far above 100% or surprisingly low: Recheck whether you entered total system generation as the per-kW production figure. If a 6 kW system is expected to generate 7,200 kWh/year, enter 1,200 for production per kW, not 7,200. Also check whether system size was entered in watts instead of kilowatts.

Your bill has no annual total: Add the kWh usage across 12 consecutive bills. If the period is shorter, ask the utility for annual usage or use an estimate and mark it as provisional; avoid multiplying a summer or winter month by 12.

Different sources give different production figures: Compare their assumptions and input data. Check that they refer to the same location, system capacity, roof orientation, shading, and units. If the roof’s condition or layout is uncertain, request a site-specific assessment rather than selecting whichever estimate is most favorable.

Frequently Asked Questions

Does 100% annual coverage mean I will never buy electricity from the grid?
No. Solar generation varies through the day and year. You may export electricity at one time and import it at another; billing treatment depends on the utility’s rules.

What is a reasonable production-per-kW number?
There is no single reliable figure for every roof. Use a local estimate based on location, orientation, tilt, shade, and system losses rather than relying on a generic value.

Should I use my bill total or the amount I paid?
Use electricity consumption in kWh, not the dollar amount. Charges and credits do not directly indicate how much energy your home used.

Can solar cover more than 100% of my annual use?
Yes, estimated annual generation can exceed annual consumption. Whether excess energy is credited, and at what value, depends on the applicable utility arrangement.

Does adding a battery increase the coverage percentage?
Not by itself. A battery can shift some solar energy to a later time, but the panels’ estimated annual generation remains broadly the same, aside from storage and conversion losses.

How accurate is this estimate?
It is a screening calculation, not a production guarantee. Better roof-specific production data and a complete year of consumption improve it, but weather, equipment, shading, and future household use can still change actual results.

Related

• California AI Hiring Rules: Employer Guide
• Peel the Potato Event Schedule & Guide
• Moto G37 Power 5G vs OnePlus N6X 5G: Key Differences

in u | Annual electricity use | kWh/year | 10000 | 0
in s | Solar system size | kW | 6 | 0
in y | Solar production per kW | kWh/kW/year | 1200 | 0
out c | Estimated use covered | % | 100 * (s * y) / u | 1
out g | Estimated solar generation | kWh/year | s * y | 0
out b | Generation minus use | kWh/year | s * y - u | 0