The direct answer
Not every cheap interval deserves a charge. Not every high interval deserves a discharge. A battery moves value through time, but it also loses energy and consumes some of its finite economic life while doing it.
That means the useful question is not simply, is the later price higher? It is:
After the energy loss and a transparent wear allowance, is enough value left to justify this cycle?
For a strong opportunity, the answer can be yes. For a thin spread, the battery may be busy without creating meaningful value. This distinction matters most for households using time-of-use tariffs, live wholesale prices, export rewards, or aggressive automated schedules.
A good model makes that trade-off visible before automation repeats the same decision across dozens of future cycles.
Four terms that make the calculation clearer
Usable capacity
Nameplate capacity is not always the amount available to move. Use the battery's stated usable capacity, then apply the depth of discharge expected for the proposed action. A 12.2 kWh usable battery moved through 80% of its range shifts 9.76 kWh into the battery for this example.
Throughput and full-equivalent cycles
Throughput is the energy moved through the battery over time. Partial cycles accumulate: two separate 50% movements are broadly equivalent to one full-capacity movement, although warranty definitions vary. Always use the same throughput or cycle basis used in the battery documentation rather than mixing manufacturer definitions.
Round-trip efficiency
A battery returns less energy than it receives. The Australian Government's Solar Consumer Guide describes efficiency loss for a lithium-ion battery as typically around 10%, while actual results depend on the system and how it is connected. At 90% round-trip efficiency, 9.76 kWh charged becomes about 8.78 kWh delivered later.
Estimated wear
Wear in this article is an economic allocation, not a battery-health measurement. One transparent approach is to divide an assumed replacement value by expected lifetime usable throughput, then apply that cost per kWh to the proposed cycle.
The estimate is intentionally imperfect. Battery ageing also depends on time, temperature, chemistry, depth of discharge, state of charge, and usage. The public Battery Wear Estimator exposes those assumptions instead of pretending to know the battery's real state of health.
The value waterfall for one battery cycle
Net cycle value = (later value × delivered energy) − energy-in cost − estimated wear.
Decision one: grid arbitrage
Grid arbitrage means buying electricity to charge the battery, then using or exporting that energy when it is worth more. The Australian Energy Market Commission describes battery value in similar terms: charge when costs are low and discharge when they are high. It also notes that more volatile price exposure can create more arbitrage potential while increasing cycling and degradation risk.
The calculation must use the price paid to charge each kWh and the value of only the energy that returns after losses. If energy costs 9 c/kWh now, that does not make a later value of 12 c/kWh profitable. The difference is far too small once efficiency and wear are included.
Decision two: solar self-use versus export
Solar energy is not automatically free when it could have been exported. If the retailer would pay 7 c/kWh now, storing that solar creates a 7 c/kWh opportunity cost. The later avoided import or export value must clear that foregone payment, efficiency losses, and estimated wear.
This is why a high evening import tariff can make solar self-use attractive, while a generous daytime feed-in tariff can shift the answer. The best choice depends on the values at the household meter, not the wholesale market headline.
Two worked examples with the same battery
The cases below are hypothetical and deliberately conservative. They use the current estimator defaults: 12.2 kWh usable capacity, 80% average depth of discharge, 90% round-trip efficiency, a $9,000 replacement assumption, and 35,000 kWh of assumed lifetime throughput.
That produces 9.76 kWh of input throughput, about 8.78 kWh delivered, and an estimated wear allocation of 25.7 c per input kWh. These are assumptions for pressure-testing a decision, not claims about any particular battery.
The same 9 c charge can produce opposite results
28 c/kWh later value
- Later delivered value
- $2.46
- Charging cost
- −$0.88
- Estimated wear
- −$2.51
- Net per average cycle
- −$0.93
A 19 c headline spread does not clear the conservative wear and efficiency assumptions.
Check this case45 c/kWh later value
- Later delivered value
- $3.95
- Charging cost
- −$0.88
- Estimated wear
- −$2.51
- Net per average cycle
- +$0.56
The stronger later value clears the same assumptions, although the margin remains modest.
Open the prefilled caseUnder these assumptions, the later value needs to reach about 38.6 c/kWh before the grid-arbitrage case breaks even.
Why a positive answer can still be too weak
A result of +$0.05 is mathematically positive, but it is not a robust operating signal. Forecast error, tariff fees, a slightly different efficiency result, household load, or a missed export window can remove a narrow margin.
Use a buffer. A clearly positive result is more useful than a technically positive one. Borderline cases should usually lead to a more conservative rule, a higher trigger threshold, or no action at all.
A practical checklist before cycling
- Use the price at the meter. Include the import rate actually paid or the feed-in value actually given up, not an unrelated wholesale price.
- Use delivered energy. Apply a realistic round-trip efficiency instead of valuing every charged kWh as if it returns.
- Read the warranty basis. Check whether lifetime is expressed in years, cycles, throughput, retained capacity, or a combination. The Australian Government guide specifically recommends checking these measures and the warranty conditions.
- Protect non-financial value. Backup reserve, resilience, and confidence before a poor solar day may matter even when a narrow arbitrage calculation says discharge.
- Check the next opportunity. A discharge now can remove the energy needed for a more valuable evening period. A full overnight charge can remove room for tomorrow's solar.
- Demand a margin for uncertainty. Do not automate to an exact theoretical break-even threshold.
Where forecast and reserve fit
Cycle economics is not only a two-price calculation. Solar Victoria notes that smart battery software can use weather forecasts to decide how much grid pre-charging is needed before the next evening peak. A weak solar outlook can raise the value of charging tonight. A clear forecast can make battery headroom more valuable than another overnight cycle.
Reserve settings matter for the same reason. Stored energy can have resilience value that is not captured by a tariff spread. If backup is important to the household, treat the protected reserve as unavailable to routine arbitrage rather than assigning it a false zero value.
From one cycle to an annual strategy
A profitable cycle is not automatically a profitable annual strategy. Frequency matters. So do seasons, changing tariffs, missed opportunities, and how often the same spread actually appears.
Use the wear estimator to pressure-test one decision. Then use the Battery ROI Calculator to model recurring timing value and the uplift automation might capture over manual action. The older automation ROI guide shows how those decisions become a layered rule strategy.
Where SoCrates fits
The calculation should come before the automation. SoCrates can use buy price, feed-in price, battery state, time windows, and forecast context in explicit rules, but a rule still needs a sensible threshold. The goal is not maximum cycling. It is to act only when the decision has enough context and margin to be defensible.
Estimate break-even first. Model the annual opportunity second. Automate only when the margin is clear enough to survive real-world uncertainty.
Final word
A home battery cycle is not good because it happened during a cheap interval. It is good when the energy delivered later creates more value than the charging or export opportunity cost, efficiency loss, and estimated wear consumed along the way.
Sometimes that hurdle is easy to clear. Sometimes a spread that looks attractive disappears once the full calculation is visible. Both answers are useful.
The point is not to make battery owners afraid of cycling. It is to make each cycle earn its place.
Use the free estimator for grid arbitrage or solar self-use, adjust the assumptions to match your battery and tariff, and treat a borderline result as a reason to be cautious.