# Rooftop Solar Payback Calculator for Plants

> Source: https://netray.co/tools/rooftop-solar-payback-calculator-for-plants
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Size a rooftop PV array from roof area and compute generation, savings, CO2 avoided and simple payback.

## Category

ehs-workforce

## Industry

Discrete Manufacturing

## Audience

- Sustainability lead
- Facilities manager
- CFO
- Plant manager

## Tool

**Kind:** calculator

**Currency:** $

**Inputs:**

#### Roof area

**Key:** area

**Type:** number

**Default:** 8000

**Min:** 0

**Step:** 100

**Unit:** m2

#### Usable share of roof

**Key:** usable

**Type:** slider

**Default:** 60

**Min:** 10

**Max:** 100

**Step:** 5

**Unit:** %

**Help:** After skylights, HVAC, setbacks, shading and access paths.

#### Array density

**Key:** density

**Type:** number

**Default:** 0.17

**Min:** 0.08

**Max:** 0.25

**Step:** 0.01

**Unit:** kWp/m2

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

#### Specific yield

**Key:** yield

**Type:** number

**Default:** 1300

**Min:** 500

**Max:** 2300

**Step:** 50

**Unit:** kWh/kWp/yr

#### Installed cost

**Key:** capex

**Type:** number

**Default:** 1100

**Min:** 0

**Step:** 25

**Unit:** $/kWp

#### Incentives or tax credit

**Key:** incentive

**Type:** slider

**Default:** 30

**Min:** 0

**Max:** 80

**Step:** 1

**Unit:** %

#### Self consumed share

**Key:** self

**Type:** slider

**Default:** 90

**Min:** 0

**Max:** 100

**Step:** 1

**Unit:** %

#### Avoided tariff

**Key:** tariff

**Type:** number

**Default:** 0.12

**Min:** 0

**Step:** 0.005

**Unit:** $/kWh

#### Export price

**Key:** export

**Type:** number

**Default:** 0.04

**Min:** 0

**Step:** 0.005

**Unit:** $/kWh

#### O&M cost

**Key:** om

**Type:** number

**Default:** 15

**Min:** 0

**Step:** 1

**Unit:** $/kWp/yr

**Outputs:**

#### System size

**Key:** kwp

**Expr:** area * usable / 100 * density

**Format:** decimal

**Description:** kWp DC.

#### Year 1 generation

**Key:** gen

**Expr:** kwp * yield

**Format:** integer

**Description:** kWh.

#### Year 1 energy value

**Key:** revenue

**Expr:** gen * self / 100 * tariff + gen * (1 - self / 100) * export

**Format:** currency

#### Net annual benefit

**Key:** net_year

**Expr:** revenue - kwp * om

**Format:** currency

**Primary:** true

#### Net investment

**Key:** cost_net

**Expr:** kwp * capex * (1 - incentive / 100)

**Format:** currency

#### Simple payback

**Key:** payback

**Expr:** if(gt(net_year, 0), cost_net / net_year, 0)

**Format:** years

**Primary:** true

#### CO2e avoided per year

**Key:** co2

**Expr:** gen * 0.4 / 1000

**Format:** decimal

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

**Chart:**

#### Simple payback vs avoided tariff

**Type:** line

**X:** tariff

**From:** 0.04

**To:** 0.3

**Steps:** 27

**Series:**

- payback

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

## Content

**Intro:** 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.

**Sections:**

#### How to use the calculator

**Body:** 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.

**Bullets:**

- 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

**Body:** 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

**Body:** 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

**Body:** 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.

**Bullets:**

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

**Cta:** 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.

**Faq:**

#### How many kWp fit on a factory roof?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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.

## Search terms

rooftop solar payback calculator, factory solar roof calculator, industrial solar ROI, solar PV payback period, commercial solar savings calculator, kWp from roof area, solar CO2 avoided, plant solar self consumption

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