Methodology, Constants and Data Sources
Every number this site produces comes from either an exact definitional factor or a documented average. This page lists all of them, states which category each belongs to, links the source, and explains where the assumption stops being universal.
Last updated · Next scheduled review January 2027 · Maintained by the SnapUnitConverter editorial team
Two kinds of numbers
Definitional factors are fixed by international agreement and carry no uncertainty. One kilowatt-hour is exactly 3.6 megajoules; one therm is exactly 100,000 BTU. These never change and are safe to use for billing, specification and engineering.
Modelling assumptions are averages used to give a result meaning — the price of electricity, the carbon intensity of a grid, the efficiency of a heat pump. These vary by country, tariff, season and installation quality. Wherever this site shows a real-world equivalence, the assumption behind it is stated next to the figure and listed below.
A third category deserves explicit mention: conversions that are not fixed unit pairs at all. Watts to amps depends on supply voltage, and for alternating current also on power factor and phase. Milliamp-hours to watt-hours depends on cell voltage. Those converters require you to enter the missing quantity rather than presenting a single answer built on a hidden assumption.
Exact conversion factors
| Quantity | Value used | Basis / source |
|---|---|---|
| 1 kWh in megajoules | 3.6 MJ (exact) | SI definition: 1 W = 1 J/s, 1 h = 3,600 s BIPM SI Brochure (9th ed., 2019) |
| 1 kWh in joules | 3,600,000 J (exact) | SI definition BIPM SI Brochure (9th ed., 2019) |
| 1 BTU (International Table) | 1,055.05585262 J (exact) | International Steam Table 1956, as tabulated by NIST NIST SP 811 (2008) |
| 1 kWh in BTU | 3,412.142 BTU | Derived from the IT BTU definition ISO 80000-5 (energy quantities) |
| 1 therm (US) | 100,000 BTU = 29.3001 kWh | US gas trading definition (105.5 MJ). The UK therm uses the same 100,000 BTU basis. NIST SP 811 (2008) |
| 1 thermochemical calorie | 4.184 J (exact) | IUPAC / NIST convention NIST SP 811 (2008) |
| 1 mechanical horsepower | 745.6999 W | 550 ft·lbf/s, NIST SP 811 NIST SP 811 (2008) |
| 1 metric horsepower (PS/CV) | 735.49875 W | 75 kgf·m/s NIST SP 811 (2008) |
| 1 ton of refrigeration | 12,000 BTU/h = 3.51685 kW | ASHRAE convention ASHRAE Handbook — Fundamentals |
| European nominal supply voltage | 230 V ±10% single-phase, 400 V three-phase | Harmonised standard voltage IEC 60038 — Standard voltages (230/400 V) |
Modelling assumptions used in decision context
These constants power the “What does this result mean?” panels, the flagship calculators and the guided journeys. They are deliberately conservative and rounded to avoid implying more precision than the underlying data supports.
| Assumption | Value used | Source / basis |
|---|---|---|
| Average household electricity use | 10.5 kWh/day (≈3,800 kWh/year) | EU household consumption average Eurostat — Energy consumption in households (2024) |
| Electricity price | €0.25/kWh including taxes | Eurostat household electricity price band, EU average Eurostat — Electricity price statistics (2025 H1) |
| Grid carbon intensity | 0.35 kg CO₂e/kWh | Approximate global average generation intensity for 2024 — local grids range from 0.02 to 0.8 IEA — Emissions from electricity (2024 data) |
| EV usable battery capacity | 77 kWh | Representative long-range passenger EV pack |
| EV consumption | 18 kWh/100 km | WLTP mixed-cycle figures for mid-size EVs EEA / WLTP electric vehicle consumption data |
| Heat pump seasonal COP | 3.2 | Typical air-source SCOP for a well-installed system in a temperate climate NREL — Heat pump performance research |
| Condensing gas boiler efficiency | 90% | Seasonal efficiency of a modern condensing boiler |
| Whole-house peak heat demand | 5 kW for ~120 m² | Typical insulated dwelling; a proper heat-loss survey overrides this |
| Kettle boil | 0.11 kWh (1.7 L) | 2.2 kW element for approximately three minutes |
| Reference battery sizes | Phone 12 Wh, laptop 60 Wh, 18 V tool pack 90 Wh | Common consumer product capacities |
| Air-transport battery thresholds | 100 Wh / 160 Wh | Carry-on limits and airline-approval band for lithium batteries IATA — Lithium battery guidance (100/160 Wh) |
| E-bike consumption | 12 Wh/km | Mid-level pedal assistance on mixed terrain |
| Continuous circuit loading limit | 80% of breaker rating | Standard derating practice for loads exceeding three hours |
Reference bodies we follow
- BIPM — SI Brochure, 9th edition (2019) — definitions of the joule, watt, ampere and volt, including the 2019 redefinition of the base units.
- NIST Special Publication 811 (2008) — imperial-to-SI conversion factors, including horsepower, BTU and the thermochemical calorie.
- IEC 60038 — Standard voltages — supply voltage harmonisation (230 V ±10% in Europe, 400 V three-phase).
- ASHRAE Handbook — Fundamentals — refrigeration tons, BTU/h conventions and HVAC load practice.
- IEA — Data & statistics (2024 data year) — electricity generation carbon intensity and global energy statistics.
- Eurostat — Electricity price statistics (2025 H1) and Energy consumption in households (2024) — the European price and consumption averages.
- IATA — Lithium battery guidance — watt-hour thresholds for air transport (100 Wh and 160 Wh).
Source links point to the publishing body rather than a mirrored copy, so they stay valid when a body republishes an updated edition.
Where these numbers stop being reliable
- Grid carbon intensity varies by more than a factor of thirty between countries and by the hour within a country. Use your own grid figure for any reporting purpose.
- Electricity and gas prices change with tariff, region and contract. Every cost figure on this site is illustrative.
- Heat pump performance depends on climate, emitter temperature and installation quality. A quoted SCOP is not a guarantee.
- Electrical results assume a resistive single-phase load. Motors, three-phase supplies and inverter loads need power factor and phase corrections.
- Battery capacity degrades with age and falls in cold weather; usable capacity is always below nameplate.
- Nothing here replaces a qualified electrician, heating engineer or accredited energy assessor for work that will be installed or certified.
Corrections and updates
If a factor here is wrong or out of date, we want to fix it. Constants are reviewed whenever a source body publishes an update, and every calculator states its assumptions inline so errors are visible rather than hidden.