How to Estimate EV Charging Cost From Your Power Bill
EV charging cost is straightforward to estimate once you know two numbers: how many kilometres you drive per week and the tariff rate at the time you charge. The common mistake is calculating from the car's battery size — most people do not charge from flat and most do not fill completely. Weekly kilometres driven is the accurate input.
- Formula: weekly km ÷ 100 × EV efficiency (kWh/100km) × tariff rate = weekly charging cost
- Most EVs use 15–20kWh/100km in typical Australian driving (urban slightly higher in stop-start traffic).
- Tariff rate matters: charging at 35c/kWh flat rate vs 15c/kWh off-peak rate doubles the weekly cost.
- The bill's kWh usage will increase noticeably after adding an EV — check the half-hourly graph on your retailer portal to confirm when the spike occurs.
The calculation
Step 1 — Estimate weekly kWh needed:
Weekly km ÷ 100 × kWh/100km = weekly kWh needed
| Weekly driving | At 15kWh/100km | At 18kWh/100km | At 22kWh/100km |
|---|---|---|---|
| 100km/week | 15kWh | 18kWh | 22kWh |
| 150km/week | 22.5kWh | 27kWh | 33kWh |
| 250km/week | 37.5kWh | 45kWh | 55kWh |
Your EV's kWh/100km figure is on the product sheet and in the car's dashboard energy display. Treat the WLTP efficiency as a best-case figure — real-world use with air conditioning, highway speeds and cold mornings is typically 10–20% higher.
Step 2 — Apply your tariff rate:
| Weekly kWh | At 15c/kWh (off-peak) | At 25c/kWh (shoulder) | At 35c/kWh (flat) |
|---|---|---|---|
| 15kWh | $2.25/week | $3.75/week | $5.25/week |
| 27kWh | $4.05/week | $6.75/week | $9.45/week |
| 45kWh | $6.75/week | $11.25/week | $15.75/week |
Annual cost estimate: multiply the weekly figure by 52.
Example: 150km/week, 18kWh/100km EV, charging at flat rate 30c/kWh → 27kWh × $0.30 = $8.10/week → $421/year.
How to find your tariff rate
Look at your electricity bill:
- Usage charge (flat rate plan): one rate for all kWh — typically 28–38c/kWh in 2025 depending on retailer and state
- Time-of-use plan: multiple rates — peak (5–9pm typically, 40–55c/kWh), off-peak (overnight, 10–20c/kWh), shoulder (mid-day/weekends, 20–30c/kWh)
If you have time-of-use pricing, the tariff rate applied to EV charging depends entirely on when the car charges. Overnight charging during the off-peak window at 12–15c/kWh is 2–3× cheaper than the same charging in the peak window at 40–50c/kWh.
What the bill will show after you start charging
If you charge a 15kWh/week EV at home, you are adding 60kWh to your monthly bill. At 30c/kWh, that is approximately $18/month additional.
To confirm this is showing correctly:
1. Log in to your retailer's portal
2. Check the half-hourly usage graph for the days after you first charged
3. The charging spike should appear as a distinct block — typically 2–6 hours of elevated usage during the charging period
If the spike appears during peak tariff hours and you expected off-peak charging, check the car's scheduled charging settings — the timer may have reset after a software update or battery swap.
Comparing EV charging cost to petrol cost
A rough comparison helps contextualise the new electricity cost:
- Petrol car at 8L/100km and $1.95/L: $15.60/100km
- EV at 18kWh/100km and 30c/kWh: $5.40/100km
- EV at 18kWh/100km and 15c/kWh (off-peak): $2.70/100km
For 10,000km/year: petrol costs approximately $1,560. EV at flat rate costs approximately $540. EV at off-peak rate costs approximately $270.
These figures exclude any overnight charging infrastructure cost (EVSE purchase, any socket installation). They also exclude public charging, which at DC fast charger rates (55–85c/kWh) is significantly more expensive than home charging.
Use your weekly kilometres and your actual tariff rate to calculate the real EV charging cost — not the car's battery size. For most Australian households at 30c/kWh flat rate, 150km/week of EV charging adds approximately $8–10/week to the electricity bill.
Analyse your bill to check your current tariff structure before estimating EV charging costs.