The 4.7-Cent Toll: Measuring What My Battery Charges Me

• 4.7c4.7cwear per kWh the battery hands back
• 90%round-trip efficiency, one measurement
• 180extra equivalent full cycles a year when trading

My battery charges me 4.7 cents for every kilowatt-hour it hands back. It never sends an invoice, which is why it is so easy to leave out of the sums.

The bill records every kilowatt-hour the grid sells this house and every one the house sells back, to the cent. But the battery’s price for its own services appears on no bill, and a cost that appears nowhere gets left out of every sum. That is how a battery strategy can look profitable on paper while quietly losing money in practice.

Last post ended with a confession: the 4.7-cent toll and the ten-in-nine-out round trip had been claimed but not earned. This post is the earning. One number came from a day’s experiment at this house’s own meters; the other came from pricing the battery the way you’d price anything else that wears out. Neither needed a laboratory, and both changed answers I’d thought were settled.

The kitchen-table experiment

Round-trip efficiency is the polite name for an impolite fact: a battery never returns everything you give it. Some of every charge is lost along the way, mostly as heat, and the share that survives the journey in and out is the round-trip efficiency. Any brochure will quote a figure. I wanted mine, counted at my own meter.

The design would not trouble a science fair. Pick a mild, clear day. Run the battery down to empty, then fill it completely, counting every kilowatt-hour that crosses the meter on the way in. Then run it back down to empty, counting every kilowatt-hour on the way out. Two totals, one division. The hardest part was patience: a 48 kWh battery takes the better part of a day, and the experimenter’s job for nearly all of it is to leave everything alone. The pot of Yorkshire tea was for the experimenter, not the method.

For roughly every ten kilowatt-hours that went in, about nine came back out. Call it 90 per cent, a one-in-ten toll paid in heat.

That is one observation: one charge and discharge, on one day, on one battery. A laboratory would repeat it across seasons and report a range. My figure landed about where such systems are expected to land, which is reassuring without being a certification. It’s good enough to budget with, and if a repeat test moves it, the spreadsheet moves with it.

The missing tenth is physics, not misbehaviour. Storing electricity chemically and retrieving it costs energy in any battery, of any make. The reason to measure it yourself isn’t suspicion. It’s that “about ten per cent” only becomes real to a household budget once it’s your ten per cent.

Photo to come

Pricing a machine that wears out

The round trip explains the smaller toll. The 4.7 cents comes from somewhere less visible: the battery is a consumable, and using it uses it up.

Nobody invoices you per kilometre for tyres, but every kilometre spends a little tread. A fleet manager puts a price on the kilometre: take what the consumable costs to replace, divide by the work it does before it’s spent. With made-up round numbers, a $1,000 set of tyres that lasts 50,000 kilometres costs two cents a kilometre, invoiced or not. The driver who ignores that isn’t avoiding the cost, just arranging to discover it all at once, later.

For a battery the work is energy handed back. Its working life is warranted in exactly those terms: a total quantity of energy it is expected to deliver before it’s done. Divide the price I paid by that lifetime of deliverable energy and every kilowatt-hour of discharge gets a price tag. For this battery it works out at about 4.7 cents. That’s the toll: the purchase price of the battery leaving the building one discharged kilowatt-hour at a time.

Batteries don’t expire at a cliff, so the 4.7 cents is an average across the working life, not a meter ticking in the garage. They also age with the calendar whatever you do, so this is a household reckoning rather than a laboratory-grade breakdown. But the warranted life is denominated in energy delivered, so energy delivered is what this house prices.

The unit that makes battery work countable is the equivalent full cycle: one complete use-up and refill, however it accumulates. Two half-discharges count the same as one full one; ten small sips the same again. For this 48 kWh battery that means 48 kilowatt-hours handed back, and at 4.7 cents each, a touch over two dollars of wear per cycle. Small enough to ignore on any given evening, which is precisely how it gets ignored.

The reckoning

With the toll priced, I went back and re-scored every battery strategy on the table, once the way the bill sees it and once with 4.7 cents docked from every discharged kilowatt-hour. Every strategy got worse, the busiest got worse fastest, and several crossed zero. Trades that had sat in the obvious-win column, selling hard into a high evening price and refilling cheaply later, flipped to losses once the energy sold had to pay for the battery it wore out on the way through. The strategies that looked best on the bill were the ones working the unbilled side hardest.

Chart to come

One comparison settles itself the moment wear is priced. A 3-cent feed-in tariff, the rate a plan pays for each kilowatt-hour exported, can never beat a 4.7-cent wear cost on battery-stored energy. Selling a stored kilowatt-hour at that price earns three cents and costs nearly five in wear, before round-trip losses. There is no volume at which that trade comes good; doing it more just loses more. Yet the same three cents for sunshine sent straight from the roof is still a win, because the direct route costs nothing to supply. Same price, same electrons, opposite verdicts.

How often the toll is paid

A toll only matters multiplied by traffic. Left to its quiet routine, soaking up the day’s surplus and running the house through the evening, this battery passes through its cycles at a sedate rate. The actively traded strategy, buying when prices are low and selling back when they’re high, day after day, adds roughly 180 equivalent full cycles a year, by this household’s model.

At a touch over two dollars of wear per cycle, those 180 cycles come to roughly four hundred dollars a year of toll, absent from any bill. That is what the trading has to out-earn before it has made a cent. A strategy doesn’t have to beat zero. It has to beat the toll.

Free to use is not free to own

If you own a battery, or you’re weighing up buying one, you will meet arithmetic that shows it earning money: charge cheap, discharge dear, keep the difference. Some of that is sound. But a great deal of it shares a blind spot: the revenue side of a battery trade appears on a bill, and the cost side mostly doesn’t. Wear never sends an invoice, and round-trip losses never appear as a line item. Before believing any battery-earnings arithmetic, including mine, ask where the toll is in it. If it isn’t there, the answer isn’t wrong, exactly. It’s half an answer.

Free to use is not free to own. The battery in my garage works beautifully, and it still charges this house about 4.7 cents for every kilowatt-hour it hands back, and keeps roughly one in ten as heat for the privilege of storage.

Suppose a strategy clears the toll comfortably on paper. Paper assumes the trades actually happen: that the forecasts were right, the automation acted, and nobody was on holiday when the price did something interesting. The gap between the value a plan offers in theory and the share a real household banks is the number the whole switching decision leans on hardest.

The spreadsheet doesn’t care what you hoped the answer was.