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A 45 KB page transfer is small, but it is not a direct reading of the electricity or battery a phone used to load it. Per-byte carbon calculators turn transferred data into an estimate using average energy intensities for data centres, networks and user devices, then apply an electricity-grid factor. That makes the result useful for comparisons under stated assumptions—not a measurement of the marginal impact of one visit on one phone.
What does 45 KB tell you?
It describes the amount of data transferred under a particular measurement condition. It does not, by itself, tell you how much electricity a phone used, how much battery charge a visit consumed, or the page’s total carbon footprint.
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The result depends on what the 45 KB includes and how it was measured: for example, whether it represents a first visit or a cached return visit, and the browser, viewport and other test conditions. A small payload can still be assigned a nonzero footprint by a model, but that assigned amount should not be presented as a phone-level meter reading.
How does a per-byte carbon model turn data into emissions?
The Sustainable Web Design Model (SWDM) version 4 treats data transfer as an accessible proxy for allocating energy across the internet system. It estimates energy intensity by dividing aggregate energy use for a system segment by aggregate data transfer, then multiplies the transfer attributed to a page by those average intensities. The model includes operational and embodied emissions associated with data centres, networks and user devices; it is not simply a calculation of electricity used by the phone. See the SWDM v4 methodology for its boundaries and assumptions.
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Operational energy intensities in SWDM v4
| System segment | Operational energy intensity | Basis in the model |
|---|---|---|
| Data centres | 0.055 kWh/GB | Derived from aggregate energy and internet-transfer inputs; an average model input, not a measurement of a particular page. |
| Networks | 0.059 kWh/GB | Derived from aggregate energy and internet-transfer inputs; an average model input, not a measurement of a particular network visit. |
| User devices | 0.080 kWh/GB | Derived from aggregate energy and internet-transfer inputs; not a reading from a particular phone. |
SWDM v4 derives these figures from 290 TWh for data centres, 310 TWh for networks, 421 TWh for user devices and 5.29 ZB of total internet transfer. They are model inputs based on system-wide totals, not universal physical costs for each additional byte.
The grid factor changes the emissions result
Energy and emissions are different quantities. To estimate emissions, the model applies a carbon-intensity factor to estimated energy use. SWDM’s worked example uses 494 gCO2e/kWh as a global-average electricity factor and says a regional or local factor may be substituted where known. A result using that global example is not interchangeable with one using a different grid factor; the factor and its geographic relevance should accompany any comparison.
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Why bytes are not a phone-energy meter
Dividing aggregate energy by aggregate traffic produces an average intensity. It does not establish that every byte causes the same incremental energy use in every circumstance. SWDM itself describes data transfer as a proxy, retained partly because it is accessible and consistent with earlier model versions, while acknowledging ongoing debate about using it as an input.
Network energy can also behave differently from traffic volume. A 2023 study of Chinese mobile networks describes energy consumption that remains relatively high under light traffic and a mismatch between traffic load and energy use. That finding cautions against interpreting a per-GB average as the physical marginal cost of a page on every network. It concerns modeled mobile infrastructure in China, not the energy or battery use of an individual handset.
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A phone’s energy during a visit can involve activity beyond transferring the page’s bytes. The sources available here do not quantify the marginal electricity or battery impact of loading one 45 KB page on a specific phone. Therefore, “cost a phone” is best understood as pointing to what a byte-based estimate leaves unresolved, not as a measured device-level result.
Why first visits and return visits can have different estimates
Cache state changes how much data is transferred, so a page’s estimated footprint can change even when the page itself has not changed. Wagtail reports a mobile-emulated first-visit page weight of 507 kB and a return-visit weight of 41.9 kB. Its disclosure gives an estimate of 0.06 gCO2e per page view as of April 2026, using SWD v4; it specifies electricity factors of 458.29 gCO2e/kWh for network and user-device operational emissions and 256.54 gCO2e/kWh for operational data-centre emissions. These are Wagtail’s site-specific modeled figures, not a general page-size benchmark or a direct phone-energy measurement. See Wagtail’s digital-emissions disclosure.
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The example shows why an estimate needs a declared visit condition. A comparison between a cold first visit and a cached return visit may primarily reflect different transferred data, not a change in the site’s underlying design. The same caution applies when comparing results from different browsers, viewports or measurement setups.
Does a smaller website use less energy?
Reducing unnecessary transfers is a useful design goal, and a smaller transfer will generally lower the output of a model that allocates impact in proportion to data transferred, all else equal. But a byte-based estimate does not prove a particular reduction in real-world energy or emissions of the same size. System load, caching, device behavior, system boundaries and the electricity mix all affect what the model represents.
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Tracking is one source of avoidable transfer to consider. A 2023 study using browsing telemetry from 100,000 users and a crawl of 2.7 million websites reported up to 21% more data transmissions associated with third-party tracking, and modeled around 11 Mt of additional annual greenhouse-gas emissions. Those are the study authors’ findings under their methods, not a universal current measurement for every site or tracker. See the 2023 study on tracking-related transmissions and emissions.
How to report a page-carbon estimate responsibly
A useful report makes its method and conditions visible so readers can tell what is being compared. Include:
- Model and version: identify the calculator or methodology, such as SWDM v4, and distinguish a model estimate from directly metered energy.
- System boundary: state whether data centres, networks, user devices and embodied emissions are included.
- Transfer conditions: give the measured page weight and say whether it represents a first visit or a cached return visit.
- Test setup: identify the browser, viewport, device emulation and other conditions needed to interpret the result.
- Electricity factor: report the carbon-intensity value and whether it is global, regional or local.
- Claim strength: describe the result as an estimate based on the stated assumptions, not as the exact energy or battery use of a particular phone.
With those details, byte-based estimates can support transparent comparisons—for example, tracking whether a site’s modeled footprint changes under the same method and visit conditions. Without them, a precise-looking number can imply more certainty than the underlying average-based allocation provides.
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