What Does the EV Charging Mix Calculator Do?
The charging-mix calculator combines several electricity prices into one weighted purchased-energy rate. It then uses mileage, vehicle efficiency and charging efficiency to estimate annual electricity spending. This is more representative than assuming every mile is powered by the cheapest home rate or the most expensive rapid charger. It is particularly useful when occasional long journeys account for a significant part of the charging bill.
How to Use This Calculator
- Divide the electricity you expect to buy into home or off-peak, standard or public AC, and rapid charging. Use categories that correspond to the prices you actually pay rather than assuming every charger type has one national price.
- Enter the energy share and pence-per-kWh rate for each category. Shares are based on kWh, not visits. They may be normalised as weights, but an all-zero set cannot define a mix.
- Enter annual miles and miles per battery kWh. Set charging efficiency to translate battery use into purchased energy, or 100% if the driving-efficiency figure already uses purchased kWh.
- Calculate and inspect the blended rate and annual spending. Repeat with a larger rapid share to test a year containing more long-distance trips. Add fixed memberships or parking fees outside the energy total.
The Maths & Assumptions Behind the Tool
Blended pence per kWh = the sum of each rate multiplied by its share, divided by the sum of shares. Annual purchased energy = annual miles ÷ miles per battery kWh ÷ charging-efficiency fraction. Annual cost = purchased energy × blended pence ÷ 100. For example, equal energy at 10p and 70p averages 40p; a 90:10 energy mix averages 16p, regardless of the number of individual sessions.
Worked example
Buying 70% of electricity at 8p/kWh, 20% at 35p/kWh and 10% at 75p/kWh gives 0.70 × 8 + 0.20 × 35 + 0.10 × 75 = 20.1p/kWh. Shares are based on purchased energy, not the number of charging sessions.
At 10,000 miles, 3.5 miles/kWh at the battery and 90% charging efficiency, purchased energy is 3,174.60 kWh and annual electricity cost is £638.10, or 6.38p/mile. A higher share of expensive rapid charging increases the weighted rate. Membership fees are separate.
Why This Matters for UK Drivers
Shares should represent the proportion of electricity bought at each price, not the proportion of charging sessions. A short top-up and a large rapid charge are not equal quantities. Use charging-app records or meter readings if available. If the shares do not add to 100%, the tool treats them as relative weights and tells you it has normalised them; each individual share still needs to be non-negative and within the displayed limits.
The annual energy calculation allows for charging losses separately. Enter 100% charging efficiency if your miles-per-kWh figure already uses purchased electricity, otherwise the same losses would be counted twice. Membership fees and parking are not part of a weighted unit rate unless you deliberately calculate an effective rate including them. For an electricity-tariff decision, also compare the effect on non-EV household usage and standing charges using the whole-home tariff calculator.
Interpreting the result in practice
A practical way to build the shares is to total billed kWh by category over a representative period. Divide each category's kWh by the combined total. If a holiday month is unusual, either model it separately or give it the appropriate annual weight. Counting one home plug-in as equal to one rapid session can materially overstate or understate the expensive share.
This tool uses a common charging-efficiency assumption across the mix. If the losses differ substantially between charging methods, calculate the categories separately with their own efficiencies and add their costs. Likewise, a subscription fee does not become a unit rate until it is allocated across a quantity of energy. Keep that allocation visible because the effective price changes when usage falls.
Frequently asked questions
Do the percentages have to total exactly 100%?
No. The calculator normalises them automatically, although entering a true 100% split is clearer.
Can I use this figure elsewhere?
Yes. Use the blended p/kWh result in the Petrol vs EV or EV Break-Even calculators.
Why a blended charging price matters
What the number tells you
Many EV comparisons assume every kWh is bought at the same home tariff. Real use can be very different: cheap overnight charging may cover most weekly miles, while occasional motorway rapid charging is much more expensive. A weighted charging mix gives a better energy-cost estimate than either extreme alone.
Use realistic inputs
Base the percentages on your likely kWh rather than the number of charging sessions. A short rapid top-up and a long overnight home charge should not be treated as equal sessions. If you are unsure, start with your normal weekly pattern and create a second scenario for months with longer trips.
What is outside the calculation
The blended electricity price is an input to other decisions rather than the whole answer. Charging losses, winter efficiency and vehicle consumption still matter. Use the resulting pence-per-kWh figure with a realistic miles-per-kWh assumption when comparing annual running costs.