What actually moves your bill (and why solar + battery flips it)

One question, in dollars

A comparison tells you which plan is cheapest. It does not tell you WHY, and without that you cannot sanity-check the answer or shop with any confidence.

There is a single question that settles it: if this rate were one cent better, what would that save me over a year? The answer is just the quantity you buy at that rate. A cent off a usage rate you buy 5,000 kWh at is worth $50 a year. A cent off the daily supply charge is worth $3.65 a year, because there are 365 days in a year and no way to buy fewer of them.

Rank the rates by that number and you have the list of what is worth shopping for, in order. Your comparison shows exactly this, computed on your own usage, above your ranked plans.

For most homes, the usage rate wins

A household using 5,000 kWh a year pays about $50 a year for each cent on the usage rate, against $3.65 for each cent of daily supply. The usage rate is roughly fourteen times the lever. That is why almost all energy advice tells you to look at the c/kWh figure first, and for most people that advice is right.

It also explains a common trap: a plan advertising a very low usage rate alongside a very high daily supply charge. On a low-usage home the supply charge can swamp the saving, which is why a comparison has to cost both together against your real consumption rather than compare headline rates.

Solar and a battery turn that upside down

A battery removes usage. It cannot remove the daily supply charge — that is billed for being connected, whether you draw a single kilowatt-hour or none at all.

So as self-sufficiency rises, the usage rate lever shrinks toward nothing while the supply lever stays exactly where it is. On a real customer with solar and a 30 kWh battery, grid imports fell from 8,600 kWh a year to around 1,700, and the daily supply charge went from about 12% of their charges to roughly 60%. The rate worth shopping for changed completely.

The part people get wrong

The instinct at that point is to hunt for a low peak rate, because peak is the expensive one. But a battery discharges into the evening peak first — that is the whole point of it. Peak imports are usually the FIRST thing to reach zero.

Which means a cent off the peak rate can be worth literally nothing to you, while you are still paying for it in a higher supply charge or a worse feed-in rate somewhere else on the same plan. Your comparison marks a rate as worth nothing when your usage in that band has gone, precisely so you do not shop for it.

What is usually left is a residue in the shoulder and off-peak bands — the winter evenings and early mornings when the battery has run down — plus the supply charge. Those are the levers that still matter.

Do not forget what you export

A battery reduces export as well as import, because solar that used to go to the grid now charges the battery instead. But it does not eliminate it: a battery fills by early afternoon on a long summer day and everything generated after that still goes to the grid, and in winter it may not fill at all.

That residue can be large. A household exporting 8,800 kWh a year that still exports 15% of it is sending 1,300 kWh to the grid — worth $39 a year at 3c and nothing at all on a plan paying 0c. On a total bill of a few hundred dollars, that is not a rounding error.

This is why we work out how much of your export your battery can genuinely absorb from your own bills, season by season, rather than assuming it swallows everything. A model that assumes perfect self-consumption makes a zero-feed-in plan look harmless, and it is not.

How to read your own list

1. Open your comparison and look at "What makes the difference for you", above the ranked plans.
2. Each row shows one part of your bill on a cheap plan and an expensive one, costed on your usage. The top row differs most between plans, so judge a plan on that first. If it is the daily supply charge, you are in supply-dominated territory.
3. If it says you use almost no power at a rate, ignore that rate — do not pay for a good one elsewhere on the plan.
4. Check the feed-in rate against your export figure before accepting a plan that pays little or nothing for it.
5. Remember the list moves. Add a battery, an EV or a pool and the ordering changes — upload the next bill and it recalculates.