If a unit arrives while something is switched on, it is worth what you would have paid to import it — call it 30c. If it arrives while nothing is on, it goes down the wire for the buyback rate, which in New Zealand is more like 12c.
So the number that decides whether a system is worth having is not how many units it makes. It is how many of them land on top of something already running. That is a question about two shapes lying on top of each other, hour by hour, and it is the question a quote skips: “7,000 units at 30c is $2,100 a year” is the sentence every quote is built on, and it is wrong by the export share.
Which means the largest number on this page is usually free. A hot water cylinder is the biggest load in most New Zealand houses and one of the only big ones that does not care what time it runs. Moving it off the night rate and into the middle of the day is a timer setting.
This page computes when the power arrives, not how much. The when is astronomy and can be had exactly. The how much depends on cloud, dust, panel temperature and the neighbour's tree, so the clear-sky curve is used only as a shape and then scaled until the year totals whatever your quote or meter says. Nothing is stored: this page has no database and no session.
Your house
Of 7,000 kWh a year, 1,889 kWh arrives while the house is using power and is worth the 30c you would have paid for it. The other 5,111 kWh goes down the wire for 12c. That is $1,180 a year, not the $2,100 a quote gets by multiplying every unit by the price you pay.
An average day
The filled area is the only part of the array worth 30c. Everything above it is worth 12c, and everything under the load line that is not filled is still bought from the retailer.
A real average of all 365 days at each hour, not an average day — the two ends of the year are drawn separately below because no single day describes both.
The year
| A year | kWh | Worth |
|---|---|---|
| The house uses | 8,000 kWh | |
| The panels make | 7,000 kWh | |
| Used as it is made | 1,889 kWh | at 30c |
| Exported | 5,111 kWh | at 12c |
| Still imported | 6,111 kWh | at 30c |
| Saved | $1,180 | against $2,100 if it were all worth the import price |
The two ends of the year
The same house against the shortest day and the longest. Nothing about the load changes; the solar does, by a factor of three.
What hour the cylinder starts
The same cylinder, the same 6.0 kWh a day, started at each of the twenty-four hours. Nothing else changes.
$391 a year between the best hour and the worst, on a timer setting. The scale starts below the worst hour rather than at zero, so the difference between them is visible at all — every bar on it is above $1,180.
Which roof
The same array on a different roof, each with its own production — not the same total handed to all of them, which would answer a different question.
| Roof faces | Power made | Kept on site | Saves a year | Per unit made |
|---|---|---|---|---|
| north | 7,000 kWh | 27% | $1,180 | 16.9c |
| north-east | 6,684 kWh | 26% | $1,115 | 16.7c |
| east | 5,900 kWh | 28% | $1,002 | 17.0c |
| north-west | 6,684 kWh | 30% | $1,159 | 17.3c |
| west | 5,902 kWh | 33% | $1,063 | 18.0c |
Import 30c, buyback 12c. Nothing else on this page matters as much as that gap: it is why the shape of a day decides more than the size of an array does. Two of this exact house, identical in every way except what hour the cylinder heats, are $391 a year apart. The daily fixed charge is the same whether the panels are there or not, so it is in none of these figures.
It heats at 23:00 now, which is off-peak, and off-solar. The same 6.0 kWh a day heated from 10:00 lands on the solar instead of on the meter, and self-consumption goes from 27% to 58%. A hot water cylinder is usually the largest load in a New Zealand house and one of the only big ones that does not care what time it runs — which makes this the cheapest thing on the page, because it is a timer setting.
The same array facing west makes 84% of what it would facing north — 5,902 kWh against 7,000 kWh — but keeps 33% of it instead of 27%, because the sun is still on it during the evening peak. On totals north still wins here, $1,180 to $1,063. Per unit made, west is ahead: 18.0c against 16.9c. So a west roof is not the write-off it looks like, and if the north one is shaded or already full it is worth a lot more than the kWh figure says.
The first units land on the fridge and the heat pump and are worth 30c. The last ones arrive at noon on a Tuesday when nobody is home and are worth 12c. This system already exports 73% of what it makes, so a larger one adds mostly export — which is why the biggest quote is rarely the best value, and why moving load into the middle of the day beats adding panels almost every time.
What it does not know
It has no weather. The generation curve is clear-sky geometry, used only for its shape and then scaled to your annual figure. That is the right way round for the question — cloud moves the total, which you supplied, far more than it moves the time of day — but a run of grey weeks does move the shape too, and none of that is here.
The house uses the same power every day of the year. It does not: a New Zealand winter evening peak is bigger and later than a January one, and it lands exactly where the solar is weakest. So the winter half of this is optimistic and the summer half is pessimistic, and the year in between is closer than either.
The day shapes are a starting point, not data. They are what a household plausibly does, written down. If you have a smart meter, the half-hourly export from your retailer will beat them, and it is the single biggest improvement available to any of these numbers.
The battery figures are ceilings. Nothing here limits how fast it charges or discharges, nothing degrades it, and nothing accounts for the losses in a real inverter beyond a flat round trip. A ceiling is the useful shape for the question being asked of it, and it is still a ceiling.
Tariffs are one price each way. Time-of-use plans, free-hour plans, tiered buyback, buyback caps and export limits all exist and none of them is modelled. If you are on a plan with a free hour, moving the cylinder into it may beat moving it into the sun.
It has no view on whether to buy a system. It has no idea what one costs, and there is nowhere to type it in on purpose: the price of an installation is a quote, not a fact, and this page is about what the thing is worth once it is on the roof.