How Home by Numbers works

• Annual consumption is multiplied by the unit rate you enter.
• The daily standing charge is multiplied by 365.
• Electricity and gas are treated separately, then added for a whole-home total.
• “Every 1p…” insights show what a one penny change in a rate is worth over a year.
• If you don't know your own figures, clearly labelled GB default assumptions are used as a starting point.
• Rates you enter always take priority over any default.

• Your postcode is converted into approximate coordinates using Postcodes.io.
• PVGIS provides an estimate of solar generation for that location.
• Generation depends on location, system size, orientation, slope and system losses.
• How much you use directly and how much you export depends on your self-consumption assumption.
• Savings combine the imports you avoid and the income from exported electricity.
• Results are planning estimates, not an installer quotation or a guarantee of output.

• The comparison models a representative year hour by hour.
• Solar generation, home demand, battery capacity, reserve, power limits and round-trip efficiency are all considered.
• Charging and discharging losses are included.
• Stored solar carries an opportunity cost, because that electricity might otherwise have been exported for income.
• Cheap-rate grid charging is only worthwhile when the price difference covers the losses and any displaced export.
• Available solar and household usage patterns remain estimates.
• PVGIS hourly information may use an averaged representative period.
• If hourly information is unavailable, a simplified fallback is used — and the calculator says so on screen.

• Current-car fuel cost comes from your mileage, MPG and fuel price.
• EV cost comes from mileage, miles per kWh, charging losses and where you charge.
• Workplace, public, normal home, cheap home and surplus-solar charging can each have a different value.
• Solar charging is valued using the export income you give up.
• Surplus-solar charging is capped by the solar the estimate says is actually available.
• The annual solar approach cannot prove the car will always be plugged in at the moment a surplus occurs.
• For a plug-in hybrid, engine fuel and electric-driving costs are calculated separately.

• Systems are compared using the same amount of useful heat.
• Fuel consumption is converted to useful heat using your current system's efficiency.
• Heat-pump electricity demand uses a seasonal performance assumption (SCOP).
• Space heating, hot water and cooking are treated separately.
• Standing charges are retained or removed according to the gas-supply option you choose.
• Installation costs and grants are planning assumptions that you enter.
• A property survey is still essential before committing to a heating change.

• The page compares the tariffs you enter.
• It does not search the live market or cover every supplier.
• Standing charges, unit rates, time-of-use shares, exit fees and joining credits are handled separately.
• First-year and ongoing annual costs can differ, so both are shown.
• Connected-home effects on solar, battery, EV and heating are shown separately, to avoid double-counting them in the headline saving.
• Dynamic half-hourly tariffs are not currently modelled.

Where figures come from outside your own bill:

• Ofgem price cap and typical consumption values — default rates and typical household use (England, Scotland and Wales).
• European Commission PVGIS — estimated solar generation for your location and system.
• Postcodes.io documentation — approximate coordinates from a postcode.

Prices, price-cap levels and typical consumption assumptions change over time. Wherever you can, enter the figures from your own bill — they will always give you a more accurate answer than any default.