Google Solar Panel Calculator

Calculator Form

Monthly electric bill

Monthly electricity use, kWh

Retail electricity rate

Export credit rate

Peak sun hours per day

Panel wattage

Performance ratio, %

Solar offset target, %

Usable roof area, sq ft

Area per panel, sq ft

Installed cost per watt

Incentive percentage

Fixed rebate

Annual maintenance cost

Annual degradation, %

Analysis period, years

Grid CO2 factor, kg per kWh

Reset

Formula Used

Monthly kWh: Monthly kWh = monthly bill ÷ electricity rate. If monthly kWh is entered, the calculator uses that value.

Target energy: Target monthly solar kWh = monthly kWh × offset percentage.

Array size: Required kW = target daily kWh ÷ peak sun hours ÷ performance ratio.

Panel count: Panels = required kW × 1000 ÷ panel wattage. The answer is rounded up.

Roof area: Area needed = panel count × area per panel.

Production: Annual production = system kW × sun hours × performance ratio × 365.

Cost: Net cost = gross installed cost − incentive value − fixed rebate.

Payback: Simple payback = net system cost ÷ first year net savings.

How to Use This Calculator

Enter your monthly bill or monthly electricity use.
Add your retail electricity rate and export credit rate.
Enter local peak sun hours for your roof location.
Add panel wattage, system performance ratio, and offset target.
Enter usable roof area and area per panel.
Add cost per watt, incentives, rebates, and maintenance.
Press calculate to view panel count, cost, savings, and payback.
Use CSV or PDF buttons to save the estimate.

Example Data Table

Example	Monthly kWh	Sun Hours	Panel Watt	Offset	Estimated Panels	Estimated Size
Small home	550	4.8	400	85%	10	4.00 kW
Medium home	900	5.2	420	90%	16	6.72 kW
Large home	1,350	5.5	450	95%	24	10.80 kW

Solar Panel Planning Guide

Why Energy Use Matters

Planning a rooftop solar system starts with energy use. Your bill shows how many kilowatt hours your home buys each month. The calculator turns that use into a target solar production value. It then checks sunlight, panel rating, losses, roof space, cost, and incentives. The result is a practical estimate, not a final engineering design.

Sunlight and System Losses

Good solar sizing depends on local sun hours. A roof in a bright area needs fewer panels than a shaded roof. The performance ratio also matters. It covers inverter loss, wiring loss, dirt, heat, mismatch, and normal downtime. A higher ratio means more useful energy from the same panel size.

Panel Count Method

Panel count is based on direct current system size. The tool first finds the energy you want to offset. Then it divides daily energy by sun hours and performance ratio. The answer is the required array size in kilowatts. It divides that size by panel wattage. It rounds up, because you cannot install part of a panel.

Costs and Savings

Cost planning is also important. The calculator multiplies array watts by cost per watt. Then it subtracts percentage incentives and fixed rebates. Savings are estimated from self used energy and exported energy. This helps compare net cost with first year savings.

Roof Space Limits

Roof space can limit the plan. Each panel needs physical area, plus working space around edges. This tool compares panel area with your usable roof area. If the array does not fit, reduce the offset target, pick higher watt panels, or use more roof planes.

Long Term View

The long term view includes panel degradation. Solar panels normally produce slightly less energy each year. The calculator applies a yearly loss rate over your chosen analysis period. This gives lifetime production and simple return estimates.

Before Installation

Use this calculator before calling installers. It helps you understand the numbers. You can test rates, sunlight, incentives, panel sizes, and roof limits. Still, final results should include shade modeling, structural checks, code rules, utility policy, and a site inspection. A licensed installer can confirm wiring, permits, mounting, and interconnection.

Compare More Cases

Because electricity prices change, run more than one case. Try conservative and optimistic inputs. Compare cash purchase, loan, and lease quotes outside this tool. Keep notes from each estimate for installer discussions and future project updates.

FAQs

1. What does this solar calculator estimate?

It estimates panel count, system size, roof area, production, cost, savings, payback, lifetime value, and CO2 reduction. It uses your electrical, roof, sunlight, and pricing inputs.

2. Is this connected to Google Project Sunroof?

No. This is an independent calculator. It uses manual inputs instead of map, shade, or satellite data. You can enter values from any trusted solar source.

3. What are peak sun hours?

Peak sun hours measure useful daily solar energy at a location. More sun hours usually reduce the required system size and panel count.

4. What is performance ratio?

Performance ratio estimates real system output after losses. It includes inverter loss, heat, dust, wiring loss, shading, mismatch, and downtime.

5. Why is panel count rounded up?

Solar panels are whole units. If the formula gives 15.2 panels, the calculator rounds to 16 panels so the target can be met.

6. What does export credit mean?

Export credit is the amount paid for unused solar energy sent to the grid. It may be lower than your retail electricity rate.

7. Can this replace an installer quote?

No. It is a planning tool. Final quotes need shade analysis, roof inspection, permit review, utility rules, equipment selection, and installer pricing.

8. Why include degradation?

Panels lose a small amount of output each year. Degradation helps estimate long term production, lifetime savings, and return more realistically.