This page lists the formula behind every calculator on DIY Home Calculators, the assumptions built into each one and the limits you should keep in mind. The same code runs the calculators in your browser and produces the worked examples in the guides.
General rules
- Prices are inputs. Example values are placeholders and are labelled as such. Replace them with figures from your own bill.
- Prices should include VAT, since that is what households pay, unless a field says otherwise.
- Results are rounded only for display. Intermediate steps use full precision.
- Currency is a label. The calculators do not convert between currencies, so enter prices in one currency.
- Nothing you enter leaves your browser.
Heat pump vs gas boiler
The calculator compares the cost of delivering the same amount of heat with each system.
- Gas cost per year = heat demand ÷ boiler efficiency × gas price
- Heat pump cost per year = heat demand ÷ SCOP × electricity price, using the heat pump tariff if you enter one
- Break-even SCOP = electricity price × boiler efficiency ÷ gas price
- Net upfront cost = installed cost − grant − boiler replacement avoided
- Simple payback = net upfront cost ÷ yearly saving, where the saving includes any gas standing charge you stop paying
It does not model servicing, radiator upgrades, hot water cylinders, solar power or changes in prices over time.
EV charging cost
- Energy added = battery capacity × (end % − start %)
- Energy from the socket = energy added ÷ (1 − losses)
- Cost = energy from the socket × price per kWh
- Range added = energy added ÷ consumption per 100 km × 100
- Time = energy from the socket ÷ charger power
Time ignores the slowdown in charging at a high state of charge.
EV vs petrol or diesel
- Blended price = home share × home price + (1 − home share) × public price
- EV cost = distance ÷ 100 × consumption × blended price
- Fuel cost = distance ÷ 100 × litres per 100 km × fuel price
- Saving = fuel cost − EV cost + yearly service saving
- Payback = extra purchase price ÷ saving
Depreciation, insurance, tax and tyres are not included.
Solar panel payback
For each year from 1 to 25:
- Production = size × yield × (1 − degradation) raised to the power (year − 1)
- Benefit = production × self-use share × grid price × (1 + price rise) raised to the power (year − 1), plus production × (1 − self-use share) × export rate, minus upkeep
- The inverter replacement is deducted in year 12
- Payback is the point where cumulative benefit passes the system cost, interpolated within that year
Export rates are held flat. Batteries and financing costs are not modelled.
Appliance running cost
- kWh per year = watts × number × hours per day × days per year × share at full power ÷ 1,000
- Cost = kWh × price
The typical appliances in the drop-down are examples. Wattage and hours vary, so measure when you can.
Gas m³ to kWh
- kWh = volume in m³ × volume conversion factor × calorific value in kWh per m³
- Calorific value in kWh per m³ = MJ per m³ ÷ 3.6
- 100 cubic feet = 2.83168 m³
LED savings
- Energy saved = (old watts − LED watts) ÷ 1,000 × hours per day × 365 × number of bulbs
- Payback in months = cost of new bulbs ÷ yearly saving × 12
Bulb lifetime and the heating effect of old bulbs are not included.
Electricity bill
- Usage charge = kWh × unit rate
- Standing charge per month = daily standing charge × 365 ÷ 12
- Total = (usage + standing) × (1 + extra VAT)
- Effective price per kWh = total ÷ kWh
How defaults are chosen
Defaults are round numbers that give readable examples. They are not forecasts and they do not describe any specific country. Efficiency and yield ranges mentioned in hints, such as SCOP between 2.5 and 4 or 850 to 1,050 kWh per kWp for solar in central Europe, are broad ranges based on published industry sources and are labelled as ranges.
Found a mistake?
Tell me at mudassar@hevizonetech.com. The editorial policy explains how corrections are logged.