Software Carbon Intensity for web products: measure the rate, then lower it

Most carbon reporting answers “how much did we emit last year?” A product team needs to know whether each visit, search and checkout gets cleaner with every release. The Software Carbon Intensity specification is built for that. It expresses emissions as a rate per unit of useful work, so the number moves when the software changes.^{1 2}

The equation

SCI = ((E × I) + M) per R

O   = E × I                          operational emissions
M   = TE × (TiR / EL) × (RR / ToR)   embodied share:
      TE   total embodied emissions of the hardware
      TiR  time reserved for this software   EL   expected lifespan
      RR   resources reserved                ToR  total resources

Two rules make SCI useful for design decisions. Carbon intensity must be location-based, for the grid the software draws from. Carbon offsets and market-based measures such as renewable-energy certificates cannot reduce the score.¹ The only way to lower it is to change the software, or where and when it runs.

Draw the boundary first

The specification asks you to include everything that significantly supports the software and to disclose what you included and left out.¹ That covers compute, storage, networking, logging and monitoring, build and deploy pipelines, backup and failover, plus the end-user devices it runs on. For a website that usually means servers, the CDN, the network and the visitor’s browser.

Choosing R for a website

R gives the number its meaning. For a content site, per page view. For a product, per active user per day, or per completed task such as a search or a checkout. Choose the unit your team already uses to describe value and apply it to every component inside the boundary. Keep it stable so trends stay comparable.

A worked example

Suppose a campaign site serves one million page views a month. Inside its boundary, the servers, CDN share and database use 120 kWh that month on a grid averaging 700 gCO₂e per kWh. The month’s share of the hardware’s embodied emissions is 30 kgCO₂e.

E × I        = 120 kWh × 700 gCO₂e/kWh   =  84,000 gCO₂e
M                                         =  30,000 gCO₂e
(E × I) + M                               = 114,000 gCO₂e
R            = 1,000,000 page views
SCI          = 114,000 / 1,000,000        =  0.114 gCO₂e per page view

What lowers the score

• Send less. Right-sized images in modern formats, fewer fonts, less JavaScript. Work that improves Core Web Vitals also cuts energy use on the network and the visitor’s device.
• Cache more. A response served from a nearby cache does far less work than one rebuilt at the origin.
• Compute less. Remove unused tags and third-party scripts; render unchanging pages once instead of on every request.
• Shift work in time and place. Reports, image processing and model training can run where and when grid intensity is lower.³
• Right-size the hardware. Idle servers still carry embodied emissions. Fewer, busier machines lower M per unit.

Make it part of the release

Treat SCI like a performance budget. Measure a baseline for the templates or journeys that carry most of your traffic, set a target rate and check it at release. The Web Sustainability Guidelines, a draft group note from the W3C Sustainable Web Interest Group, collect the design and engineering practices that lower it.⁵ We include this in Audits & Assessments and in every Websites & Apps build.