Climate data · 15 cities
How much heat your windows lose, city by city
The same window loses 2.8 times as much heat in Dunedin as in Auckland. We added up every cold hour from 2021 to 2025 to show where glazing matters and where it barely registers.
What would your windows save?
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What we found
Our calculations, data as of 23 October 2026
- 25 m² of single-glazed aluminium windows lets out about 3,685 kWh of heat a year in Dunedin in a home heated mornings and evenings, and 1,311 kWh in Auckland. Heated around the clock, the Dunedin figure rises to 11,005 kWh.
- Replacing that heat with a heat pump costs about $522 a year in Dunedin and $180 in Auckland. That is the most any window upgrade could save, and no real window gets all of it back.
- Heating habits move the numbers more than latitude. A house heated all day accumulates about 3.0 times the degree-hours of one heated only from 7–9 am and 5–11 pm, in every city we ran.
- The Building Code’s climate zones do not follow the map neatly. Rotorua and Taupō sit in zone 4 with Upper Hutt and the Wairarapa, while Wellington City and Nelson are zone 3, and Tauranga is zone 1 like Auckland.
| Dunedin | 3,685 kWh |
| Queenstown | 3,581 kWh |
| Invercargill | 3,520 kWh |
| Christchurch | 2,894 kWh |
| Palmerston North | 2,646 kWh |
| Taupō | 2,639 kWh |
| Rotorua | 2,530 kWh |
| Nelson | 2,309 kWh |
Our model: single-glazed aluminium windows (R0.15), heated to 18 °C from 7–9 am and 5–11 pm. ERA5 hourly temperatures 2021–2025.
Degree-hours: the cold, measured
Heat flows out through a window in proportion to the temperature difference across it. To see how much of that difference each city produces, we took hourly air temperatures for 2021–2025 from the ERA5 climate reanalysis and counted, hour by hour, how far each one fell below 18 °C. Most New Zealand homes are heated in the morning and evening, not all day, so we report two totals: one for the hours 7–9 am and 5–11 pm, and one for every hour.
| City | H1 zone | Winter mean, °C | Degree-hours, part-day | Degree-hours, all day |
|---|---|---|---|---|
| Auckland | 1 | 12.5 | 7,863 | 23,429 |
| Whangārei | 1 | 12.0 | 9,554 | 28,207 |
| Napier | 2 | 10.7 | 11,141 | 33,721 |
| Tauranga | 1 | 10.7 | 11,195 | 33,930 |
| New Plymouth | 2 | 11.0 | 11,363 | 34,427 |
| Hamilton | 2 | 10.5 | 11,465 | 35,436 |
| Wellington | 3 | 11.0 | 12,366 | 36,725 |
| Nelson | 3 | 9.7 | 13,854 | 41,357 |
| Rotorua | 4 | 8.9 | 15,178 | 45,548 |
| Taupō | 4 | 8.6 | 15,836 | 48,377 |
| Palmerston North | 3 | 9.0 | 15,875 | 47,538 |
| Christchurch | 5 | 8.1 | 17,363 | 51,930 |
| Invercargill | 6 | 6.9 | 21,121 | 64,923 |
| Queenstown | 6 | 4.9 | 21,486 | 68,366 |
| Dunedin | 5 | 6.9 | 22,107 | 66,031 |
Queenstown has the coldest winters by a wide margin, a June–August mean of 4.9 °C against 6.9 °C in Dunedin. Yet Dunedin edges ahead on part-day degree-hours, because its evenings stay cool even in summer: 15 °C on average from 5 to 11 pm in December–February, against 18 °C in Queenstown.
From degree-hours to kilowatt-hours and dollars
Multiply the degree-hours by the window area and divide by the window’s R-value, and you get the heat that leaves through it. For 25 m² of single-glazed aluminium windows, roughly the window area of a mid-sized three-bedroom house, in a home heated mornings and evenings:
| City | Heat lost, kWh/yr | Heat-pump power to replace it | Power price, c/kWh | Cost a year |
|---|---|---|---|---|
| Dunedin | 3,685 | 1,228 kWh | 42.5 | $522 |
| Queenstown | 3,581 | 1,194 kWh | 42.4 | $506 |
| Invercargill | 3,520 | 1,173 kWh | 39.0 | $458 |
| Christchurch | 2,894 | 965 kWh | 38.6 | $372 |
| Palmerston North | 2,646 | 882 kWh | 43.1 | $380 |
| Taupō | 2,639 | 880 kWh | 42.2 | $371 |
| Rotorua | 2,530 | 843 kWh | 42.9 | $362 |
| Nelson | 2,309 | 770 kWh | 38.4 | $296 |
| Wellington | 2,061 | 687 kWh | 37.2 | $256 |
| Hamilton | 1,911 | 637 kWh | 39.8 | $254 |
| New Plymouth | 1,894 | 631 kWh | 43.5 | $275 |
| Tauranga | 1,866 | 622 kWh | 45.0 | $280 |
| Napier | 1,857 | 619 kWh | 41.3 | $256 |
| Whangārei | 1,592 | 531 kWh | 44.5 | $236 |
| Auckland | 1,311 | 437 kWh | 41.3 | $180 |
The last column is the bill for putting that heat back with a heat pump, which delivers about three units of heat per unit of electricity, at each city’s average price per kilowatt-hour in MBIE’s August 2026 survey. Wellington’s cheap power (37.2 c/kWh) offsets some of its cold; Tauranga’s dear power (45 c/kWh) does the reverse.
Zones in the Building Code
For new buildings, the Building Code’s energy clause H1 divides the country into six climate zones by district and sets minimum window R-values for each. The zone matters for a new house or an addition; for upgrading the windows in an existing house, the degree-hours above are the better guide, because they show how much heat there is to save. See H1 climate zones and windows for the full list.
What these numbers leave out
Heat lost is not the same as heat paid for. Winter sun through north-facing glass returns some of it, curtains drawn at dusk cut night losses, and many homes heat only the living area, so only those windows count. Use the calculator with the window area of the rooms you actually heat.
This week: Palmerston North
Single glazing in an aluminium frame runs below the dew point about 2,430 hours a year in Palmerston North (H1 zone 3); clear double glazing cuts that to 350. A low-E retrofit of 25 m² saves $190 a year heated mornings and evenings with a heat pump, or $569 heated all day. Fastest payback here: DIY magnetic secondary glazing, 17 years.
Cheap fix. EECA estimates that closing curtains before dark saves $80–90 a year. Lined, full-length curtains with a pelmet work best. Read more