EV Charging Cost Calculator: How to Estimate Your Own Numbers

Most EV owners can tell you roughly what they pay to charge, but far fewer can actually calculate it accurately enough to compare charging plans, budget for a longer trip, or work out whether switching power retailers is worth the hassle. The maths is simpler than it looks — here's how to build your own numbers rather than relying on generic per-100km figures that may not match your actual car or driving.

The basic formula

Charging cost comes down to one relationship: energy needed (kWh) × price per kWh = cost. The only real complexity is figuring out how much energy your specific driving actually uses, since that varies by car, driving style, and season far more than most people expect.

Step 1: find your car's real-world efficiency

Every EV's trip computer or companion app shows an efficiency figure, usually in kWh per 100km. Don't rely on the manufacturer's official spec — it's measured under test conditions rarely matched in normal NZ driving. Instead, note your car's actual displayed average after a few weeks of typical use. Most EVs on NZ roads land somewhere between 14kWh/100km for smaller, efficient models and 22kWh/100km for larger SUVs, with winter driving and heavy motorway use pushing figures 15–25% higher than a mild-weather average.

Step 2: find your actual home power rate

This is where a lot of owners undercount their real cost, because home power plans vary a lot:

Rate type	Typical NZD per kWh
Standard anytime rate	$0.29–$0.34
EV-specific off-peak overnight rate	$0.12–$0.19
Standard off-peak (non-EV-specific)	$0.20–$0.25

Several NZ retailers now offer EV-specific plans with a deeply discounted overnight window in exchange for a higher daytime rate — these are worth checking against your actual charging pattern, since the saving only applies if your car is genuinely charging during the discounted hours, not just plugged in.

Step 3: a worked home charging example

Say your car uses 17kWh/100km on average, and you drive 14,000km a year (about 270km a week).

• Weekly energy use: 270km ÷ 100 × 17kWh = 45.9kWh
• On a standard anytime rate of $0.31/kWh: 45.9 × $0.31 = $14.23 a week
• On an EV-specific overnight rate of $0.15/kWh: 45.9 × $0.15 = $6.89 a week

That gap — roughly $380 a year for this driving pattern — is exactly why checking whether your retailer offers a genuine EV overnight plan is worth ten minutes on their website.

Step 4: public charging costs

Public charging pricing in NZ is usually per-kWh, though some older or slower AC units still charge per-minute or a flat session fee. Rough ranges:

Charging type	Typical NZD per kWh
Public AC (destination charging)	$0.40–$0.55
DC fast charging (50kW+)	$0.65–$0.85
DC rapid/high-power (150kW+)	$0.75–$0.95

Using the same 17kWh/100km car, a 300km trip needing roughly 51kWh from a DC fast charger at $0.75/kWh comes to around $38 for that leg — noticeably more than the equivalent home charge, which is why most owners treat public DC charging as a top-up for travel rather than a routine habit.

Don't forget road user charges

Road user charges are billed separately from your power bill and need to be included in any genuine total cost of ownership calculation, even though they don't factor into a per-kWh charging cost comparison directly. Light EVs currently pay in the vicinity of $76 per 1,000km, purchased in advance as RUC licences rather than automatically deducted. For the earlier example — 14,000km a year — that works out to roughly $1,064 annually, or about $20.50 a week, which is worth adding on top of your calculated charging cost to get a genuine like-for-like comparison against what you'd pay in fuel excise built into a petrol car's pump price.

Putting it together: cost per 100km at a glance

Charging source	Approx. cost per 100km (17kWh/100km car)
Home, EV-specific off-peak	$2.55
Home, standard anytime rate	$5.27
Public AC destination charging	$7.65
Public DC fast charging	$12.75–$14.45

Building your own monthly estimate

1. Multiply your average weekly kilometres by your car's actual kWh/100km figure to get weekly energy use.
2. Split that between home and public charging based on your real pattern (most owners do 80–95% at home).
3. Apply your actual rate for each portion, not a generic published average.
4. Multiply the weekly total by roughly 4.3 for a monthly figure, and add road user charges separately if applicable, since RUC is billed independently of your power bill.

Our practical advice

Pull your actual kWh/100km figure from the car rather than trusting a spec sheet, confirm your real home power rate rather than assuming you're already on the cheapest available plan, and treat public DC charging cost as a trip-planning input rather than your everyday baseline. Once you've built the calculation once with your own numbers, adjusting it for a longer road trip or a potential plan switch takes seconds rather than guesswork.