Methodology
Every Wattlyst result comes from one tested calculation engine that uses standard electrical formulas, published data and the default values listed below. Each calculator shows its formula and the values it assumed, and lets you change the main ones, so you can see exactly how a number was produced.
How a calculation works
- Your inputs are checked first. Blank, negative or impossible values show a clear message instead of a wrong number.
- The formula is applied exactly as written in our formula specification, for example amps = watts ÷ (volts × power factor) for single-phase AC.
- Defaults fill any field you leave blank. They are listed below; most can be changed in each calculator's advanced settings, while physical constants and code limits are fixed.
- The result is rounded only for display. Breaker and charge controller sizes are rounded up to the next standard size, and counts of panels and batteries are rounded up to whole units, because rounding down would undersize them. Other sizes, such as battery Ah, inverter watts and generator kVA, are shown as the minimum required: choose the next larger product.
Default values
| Assumption | Default |
|---|---|
| Single-phase voltage (Pakistan) | 230 V |
| Three-phase voltage (Pakistan) | 400 V |
| Single-phase voltage (US option) | 120 V |
| Power factor, mixed home loads | 0.8 |
| Power factor, resistive loads (heaters, irons, kettles) | 1 |
| Power factor, generators | 0.8 |
| Power factor, line-interactive UPS (VA to W) | 0.6 |
| UPS inverter efficiency | 85% |
| Hybrid solar inverter efficiency | 90% |
| Usable depth of discharge, lead-acid and tubular | 50% |
| Usable depth of discharge, lithium (LiFePO4) | 90% |
| Sizing safety margin | 1.25 × |
| Solar performance ratio | 78% |
| Days per billing month | 30 |
| Default solar panel size | 585 W |
| Watts per horsepower | 745.7 W |
| Voltage drop limit, lighting | 3% |
| Voltage drop limit, other circuits | 5% |
Data sources and update policy
- Engineering defaults: Standard electrical practice (IEC 60364, NEPRA performance standards for 230/400 V, 50 Hz supply), listed in the table above. Last checked 2026-10-13.
- Pakistan electricity tariff: NEPRA decision 11 Feb 2026, notified SRO 279(I)/2026 on 12 Feb 2026 (A-1 domestic, uniform for CY2026). Last checked 2026-10-13.
- Peak sun hours by city: Global Solar Atlas long-term averages (api.globalsolaratlas.info/data/lta), optimum tilt. Last checked 2026-10-13.
- Appliance wattages: Virginia Tech Extension 2901-9014 (DOE appliance wattage table) https://pubs.ext.vt.edu/2901/2901-9014/2901-9014.html; Murdoch's generator wattage chart https://www.murdochs.com/learning-center/power-equipment/how-to-choose-a-generator/; Gree GS-18PITH11G datasheet (Japan Electronics PK) https://japanelectronics.com.pk/products/gree-ac-split-inverter-1-5-ton-gs-18pith11g-aaa; PowerHouse Express PK: AC watts https://powerhouseexpress.com.pk/blogs/news/how-many-watts-does-an-air-conditioner-use; PowerHouse Express PK: 6kW solar load guide https://powerhouseexpress.com.pk/blogs/news/how-many-ac-can-run-on-6kw-solar-system; ARY News: inverter fan watts https://arynews.tv/do-inverter-fans-really-consume-only-30-watts-unveiling-the-truth/; Haier HSU-18 non-inverter listing (Star Tech) https://startech.com.bd/haier-1-5-ton-non-inverter-ac. Last checked 2026-10-13.
Values that change over time, such as tariffs and prices, carry a "last checked" date on the page. We re-check them at least monthly and whenever NEPRA, a DISCO or a manufacturer publishes a change. Electricity bill estimates follow the tariff checked on 2026-10-13.
Testing
Before any change goes live, the engine runs an automated test suite: hand-worked cases for every calculator, checks that results are never blank, negative or infinite, and checks that page titles and descriptions follow our rules. A change that fails any test is not merged into the live site.
Technical review
Technical review by a PEC-registered electrical engineer is in progress. Until it is complete, no page claims an engineer sign-off.
Limits
Results are estimates for planning. Real appliances, batteries and panels differ from their labels, and bills include charges that change month to month. Read the disclaimer before relying on a result for a purchase or installation.
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