How accurate are our calculators?
Good estimates, not engineering-grade. Built on real NZ data from EECA, MBIE, NIWA and Stats NZ — accurate enough to make confident decisions, never a substitute for an installer quote.
Short answer: good estimates, not engineering-grade. Our calculators are built on real NZ data published by EECA, MBIE, NIWA and Stats NZ — accurate enough to make confident decisions, but never a substitute for a quote from a qualified installer.
Here's exactly what's behind each tool and the realistic accuracy band you should expect.
Where the numbers come from
| Calculator | Key data sources and assumptions |
|---|---|
| Home Energy Audit | EECA household energy use breakdowns; 4 NZ climate zones (Warm/Mild/Cool/Cold) with base heating demand of 35–90 kWh/m²/year; insulation factors based on H1 building code bands; hot water at 1,500 kWh/person/year baseline; daytime occupancy weighting for retirees and pre-schoolers. |
| Heat Pump Sizer | Region-specific watts-per-m² (Auckland 95, Wellington 125, Canterbury 160, Southland 185), with multipliers for ceiling height, glazing, sun exposure and insulation. Standard COP of 3.0–3.5. Multi-split diversity factor applied for systems with multiple heads. |
| Hot Water Sizer | NIWA mean annual cold-water inlet temperatures by city (Auckland 14°C → Invercargill 7°C). Region-specific heat-pump COPs from 3.6 (warm) to 2.7 (cold). Standing losses: 1,100 kWh/yr (old electric), 600 kWh/yr (modern wrapped), 500 kWh/yr (heat pump). |
| Solar Payback | City-specific peak sun hours (e.g. Nelson 4.3, Auckland 4.0, Wellington 3.7). 0.78 system loss factor, orientation adjustments. Self-consumption assumed 35–40% (no battery), 80–90% (with battery). Tiered $/kW install costs reflecting NZ market. |
| Carbon Footprint | NZ grid factor of 0.097 kg CO₂/kWh (MBIE 2022) — reflects our ~80% renewable grid. Natural gas 0.20, LPG 0.21, wood 0.02, petrol 2.31 kg/L. Average NZ household benchmark of 3,300 kg CO₂e/year (Stats NZ). |
| Retailer Comparison | Live rates loaded from our database covering 20+ NZ retailers. Buy-back rates currently 12.5–17 c/kWh. Updated regularly but always confirm with the retailer before signing up. |
Realistic accuracy bands
For a typical NZ home, expect our calculators to be within:
- ±10–15% for hot water and lighting (well-understood physics, narrow input ranges)
- ±15–20% for heating load and heat pump sizing (climate and insulation drive variance)
- ±20–30% for solar generation and payback (year-to-year weather, panel orientation, shading we can't see from here)
- ±25% for total household kWh (because behaviour — thermostat setpoint, shower length, who's home — dominates over building physics)
That's the honest range. Anyone who says their online calculator is "exact" is selling something.
How to get a more accurate result
- Use a recent power bill. If you can enter your actual annual kWh, our tools become much more accurate because we anchor everything to a real number.
- Be honest about your insulation. "Premium" should mean it really is — wall, ceiling and underfloor all done to current standards. If you're not sure, choose the lower band.
- Get a real quote. Use our calculators to scope the conversation, then ask a verified installer for a site visit. They can see things our tools cannot.
When you should trust other sources first
- Power retailer pricing — always confirm directly with the retailer before signing up. Plans change.
- Government grants (e.g. Warmer Kiwi Homes) — confirm eligibility on the EECA website. Rules update.
- Heat pump sizing for tricky homes — heritage, multi-storey, large open-plan or poorly-insulated homes need an in-person Manual J calculation by an installer.
- Solar for shaded or unusual roofs — get an installer to do a site survey with shading analysis.
We keep improving
We refresh our data when EECA, MBIE or NIWA publish new figures, and we welcome corrections. If you spot something that looks wrong, email hello@mypowerbuddy.co.nz and we'll review it.
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