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Rooftop Solar Payback Calculator for Plants

A plant roof is often the cheapest place to generate clean power, because the load is on site and the daytime profile matches production. This calculator turns roof area into system size, generation, value and payback.

Your numbers

Roof area

Usable share of roof

60 %

After skylights, HVAC, setbacks, shading and access paths.

Array density

kWp/m2

Flat roofs with tilted racking are lower than flush mounted panels.

Specific yield

kWh/kWp/yr

Installed cost

$/kWp

Incentives or tax credit

30 %

Self consumed share

90 %

Avoided tariff

$/kWh

Export price

$/kWh

O&M cost

$/kWp/yr

Your results

Net annual benefit

$106,570

Simple payback

5.9 years

System size

816

kWp DC.

Year 1 generation

1,060,800

kWh.

Year 1 energy value

$118,810

Net investment

$628,320

CO2e avoided per year

424.32

Tonnes, using 0.4 kg CO2e per kWh. Replace with your grid factor.

Screening estimate without degradation, financing, inflation or tax depreciation. Get a site specific design and quote before deciding.

Save results:

Simple payback vs avoided tariff

Sweeps Avoided tariff from 0.04 to 0.3, holding your other inputs fixed.

Want energy data joined to production?

Optional expert review of linking utility meters to production and cost data.

Optional. The tool and every export work without this. No spam.

How to use the calculator

Enter total roof area and the share that is realistically usable after obstructions and fire access paths. Array density converts area to kWp; flat roofs with tilted racking typically fit about 0.12-0.18 kWp per m2 of usable area. Specific yield depends on location, so pull it from a solar resource tool for your site. Add installed cost, incentives, self consumed share and tariffs, then read net benefit and simple payback.

Check structural capacity and roof age first
Use your interval meter data to estimate self consumption
Use the avoided energy rate, not the total bill including fixed charges

The formulas

System size is area times usable share times density. Generation is size times specific yield. Energy value is self consumed kWh times the avoided tariff plus exported kWh times the export price. Net annual benefit subtracts O&M, which is per kWp. Net investment is size times installed cost times one minus the incentive share, and simple payback is net investment divided by net annual benefit. Avoided emissions are generation times the grid factor.

Reading the result

With the defaults, the roof holds about 816 kWp, generates roughly 1.06 GWh per year and pays back in about six years. The chart shows how sensitive payback is to the avoided tariff: at low tariffs it stretches beyond a decade, while high tariff regions see payback of three or four years. If self consumption is below 70%, test battery storage or a smaller array so export does not dominate.

Common mistakes

Common errors include using nameplate roof area without deducting obstructions, ignoring annual degradation of about 0.4-0.7%, and assuming every kWh offsets the top tariff tier. Financing, inverter replacement around year 12-15 and tax depreciation all change real returns, so treat this as a screen. Report market-based versus location-based emissions consistently when claiming avoided CO2.

Model inverter replacement separately
Use interval data for self consumption
Check interconnection and permitting lead times

Frequently Asked Questions

How many kWp fit on a factory roof?

As a rule of thumb, 0.12-0.18 kWp per m2 of usable flat roof with tilted racking, and up to about 0.2 kWp per m2 flush mounted. Usable area is often only 50-70% of the gross roof after obstructions.

What is specific yield?

It is annual AC energy in kWh per kWp installed. Typical values run from about 900 in cloudy northern sites to over 1,700 in sunny regions. Use a solar resource tool for your exact location and orientation.

Is simple payback good enough?

It is a quick screen. Finance teams usually also want NPV or IRR including degradation, inverter replacement, tax effects and financing. Use this tool to shortlist, then model the details.

How do I treat the CO2 avoided?

Multiply generation by the grid emission factor to show the location-based reduction in Scope 2. The default here is 0.4 kg per kWh, so replace it with your own grid factor before reporting.

Netray can bring meter, utility invoice and production data from your ERP into a single energy view, so projects like this are sized from real load profiles.

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