What Time-of-Use Battery Arbitrage Means
Time-of-use electricity pricing assigns different energy prices to different times of day or different tariff windows. A battery can sometimes reduce energy charges by importing extra electricity while the price is lower, storing it, and then supplying local loads when the grid price is higher. The economic value comes from the difference between the high-price energy you avoid buying and the lower-price energy you buy to recharge the battery.
The important detail is that a battery does not return every unit of charging energy as usable discharge energy. Charging, battery conversion and inverter operation introduce losses. If the battery delivers 9 kWh to the load and the complete charge-to-discharge path is 85% efficient, the grid must supply about 10.588 kWh during the charging period. That extra 1.588 kWh is why simply multiplying battery kWh by the difference between the peak and off-peak prices overstates savings.
This is an operating-energy model. It does not claim that a battery purchase is profitable, and it does not calculate total project payback. A positive monthly number means the assumed tariff arbitrage produces a positive energy-price difference before omitted costs. Battery purchase price, installation, finance, degradation, maintenance, taxes, fixed charges, demand charges, export compensation and other tariff details need separate treatment where they apply.
The Formula Used By This Calculator
The calculation starts with delivered battery energy per cycle. That is the usable energy that actually reaches the load during the high-rate period. Grid charging energy is calculated by dividing delivered energy by round-trip efficiency expressed as a decimal.
Grid energy required per cycle = delivered battery energy ÷ round-trip efficiency
The avoided high-rate cost is the delivered energy multiplied by the high-rate tariff. The charging cost is the grid energy required multiplied by the low-rate tariff. The difference is the estimated saving per cycle.
Saving per cycle = (delivered kWh × high tariff) − (grid charging kWh × low tariff)
Finally, the result is multiplied by the number of useful cycles expected during the month.
Monthly TOU saving = saving per cycle × monthly cycles
All tariff values should use the same currency basis and the same unit, normally currency per kWh. The output uses “currency units” because the arithmetic works the same way whether your tariff is expressed in dollars, pounds, euros, rupees or another currency. What matters is that both rates are entered consistently.
Worked Example Using The Default Inputs
The default calculator scenario assumes 9 kWh delivered from the battery during each expensive tariff period, a high-rate tariff of 0.36 per kWh, a low-rate tariff of 0.11 per kWh, 85% round-trip efficiency and 27 useful cycles in a month.
| Step | Calculation | Result |
|---|---|---|
| Grid energy needed per cycle | 9 ÷ 0.85 | 10.588 kWh |
| Peak-period cost avoided | 9 × 0.36 | 3.240 currency units |
| Off-peak charging cost | 10.588 × 0.11 | 1.165 currency units |
| Estimated saving per cycle | 3.240 − 1.165 | 2.075 currency units |
| Estimated monthly saving | 2.075 × 27 | 56.033 currency units |
The same scenario requires about 285.882 kWh of grid charging energy across 27 cycles. That is higher than the 243 kWh delivered by the battery because the model includes the 85% round-trip efficiency assumption. The difference between those two energy totals represents conversion losses within this simplified model.
The example is useful because it shows why the tariff spread alone is not enough. A high tariff of 0.36 and low tariff of 0.11 create a raw spread of 0.25 per kWh, but the battery does not capture the full 0.25 for every delivered kWh. The effective charging cost per delivered kWh is approximately 0.11 ÷ 0.85, or 0.1294. The energy-arbitrage margin is therefore approximately 0.36 − 0.1294 = 0.2306 per delivered kWh before other costs.
How To Choose The Delivered Energy Per Cycle
The first input is not necessarily the battery’s advertised capacity. It should represent the energy the battery can realistically deliver into loads during the high-price window. Several constraints can make this value smaller than nameplate energy.
- Usable state-of-charge window: a system may reserve part of the battery for backup, battery protection or operating preferences.
- Peak-period load: the home or site must have enough demand during the expensive window to consume the planned discharge. A 10 kWh battery cannot displace 10 kWh of grid imports if only 4 kWh of eligible load occurs while the peak rate is active.
- Inverter power limit: a battery may have enough energy but not enough discharge power to serve all peak-period loads at once.
- Charging opportunity: the low-rate period must be long enough, and the charger powerful enough, to restore the energy needed for the next shift.
- Solar interaction: if solar already supplies part of the expensive period, fewer grid kWh may remain for the battery to displace.
A practical way to choose this input is to examine interval consumption for the expensive tariff window and compare it with the battery’s expected usable discharge. Use the lower of the energy you can actually discharge and the grid energy you can actually avoid. This prevents an oversized battery from creating artificial savings in the model.
How Round-Trip Efficiency Changes The Break-Even Tariff
Efficiency changes the minimum high-rate tariff needed to cover the charging energy cost. Ignoring all other costs, the arbitrage is positive when the high-rate tariff is greater than the low-rate tariff divided by round-trip efficiency.
Break-even high rate = low rate ÷ round-trip efficiency
With a low rate of 0.11 and efficiency of 85%, the simplified break-even high rate is about 0.1294 per kWh. A high rate above that value produces a positive energy-only arbitrage margin in the calculator. A high rate below that value produces a negative result because the value of the displaced peak energy is not enough to pay for the off-peak charging energy after losses.
This break-even test is useful for screening tariff combinations, but it is not a full economic threshold. If you want to account for a battery cycling cost, you would need an additional cost per delivered kWh. In that expanded model, the high-rate tariff must cover both the efficiency-adjusted charging cost and the chosen cycling cost. The calculator deliberately leaves that assumption outside the core formula because battery wear depends on the specific system, operating strategy and cost basis.
Choosing A Realistic Monthly Cycle Count
Monthly cycles should represent days when a useful low-price charge and high-price discharge can actually occur. Entering the number of calendar days can overstate savings when the tariff does not apply every day or when operating conditions prevent a complete shift.
For example, a weekday-only peak tariff may provide fewer arbitrage opportunities than a tariff that applies seven days a week. Seasonal rate schedules can change both the price spread and the number of applicable days. Backup-reserve settings can prevent discharge on some days. Low household demand during the peak window can leave part of the battery unused. Solar production can reduce the peak-period grid import that would otherwise be displaced.
A “cycle” in this calculator is an economic event, not a claim about the battery manufacturer’s cycle-life definition. If the battery shifts only half of the entered delivered energy on a given day, it is better to reduce the energy input or model the partial shift explicitly than to count a full economic cycle that did not occur.
For annual planning, consider calculating representative months separately rather than multiplying one month by twelve. Tariffs, weather, solar generation, air-conditioning or heating demand, daylight hours and household schedules can all change the amount of energy available for shifting.
Tariff Details That Can Make A Simple Model Misleading
The calculator is designed for a straightforward two-rate energy arbitrage case. Real electricity tariffs can contain features that require a more detailed bill model. Before treating the result as bill savings, check how your utility applies each part of the tariff.
- Multiple TOU periods: some plans have super-off-peak, off-peak, shoulder and peak prices rather than a single cheap and expensive rate.
- Seasonal prices: summer and winter rates may differ substantially, so one annual average can hide the months that actually create value.
- Usage tiers: the marginal price of a shifted kWh may depend on total monthly consumption.
- Demand charges: reducing the highest measured kW can create value that is not represented by this energy-only calculator, while poorly timed charging could also increase a demand charge.
- Fixed charges: monthly connection or service charges normally do not change just because energy is shifted between tariff windows.
- Export credits: a solar-plus-battery system may face a choice between storing solar energy, exporting it, or charging from the grid. The opportunity cost of exported solar can matter.
- Taxes and adjustments: some bill components follow energy use while others do not. Use the marginal rate that changes with the shifted kWh rather than dividing the entire bill by total consumption.
If your tariff has several interacting components, use this calculator as one line item in a broader model. It remains useful for isolating the pure energy-price arbitrage created by one low-rate charge and one high-rate discharge.
Time-of-Use Savings With Solar
Solar changes the source and opportunity cost of charging. If surplus solar would otherwise be exported, storing it for later use can be compared with the export compensation you give up, not automatically with the off-peak retail tariff. If the battery is charged from the grid overnight and solar serves daytime loads, the calculation can still use the off-peak grid rate for charging, but the amount of peak-period load available to the battery may be reduced by solar production.
For a solar-plus-battery household, it can be helpful to model three cases separately: grid charging for TOU arbitrage, surplus-solar charging for later self-consumption, and a mixed strategy. Keeping these cases separate avoids assigning the same savings to the same kWh twice.
The Grid vs Solar Battery Charging Cost Calculator can help compare different charging sources, while the Home Battery Electricity Bill Savings Calculator is a better next step when you want to combine several household bill effects rather than isolate one arbitrage path.
Battery Degradation And Cost Per Cycle
Every time-of-use strategy should distinguish gross energy savings from net ownership economics. The calculator reports gross energy arbitrage after charging losses. It does not subtract battery degradation or spread the purchase price across expected delivered energy.
That omission is intentional because there is no single degradation cost that applies to every battery. Actual economic wear depends on purchase price, usable capacity, cycle depth, calendar ageing, temperature, warranty terms, replacement value, financing and how long the system remains in service. Treating one universal wear cost as fact would make the calculator look more precise than the available inputs justify.
If you have a defensible cycling-cost estimate, you can compare it with the calculator’s saving per delivered kWh. Using the default example, the energy-only margin is about 0.2306 currency units for each delivered kWh. A separate cycling cost lower than that leaves some operating margin; a higher cycling cost would eliminate the energy-only benefit even though the calculator still shows positive gross savings.
For a broader ownership comparison, use the Battery Storage Cost Per kWh Calculator and keep its assumptions visible beside the TOU result.
Why A Positive Monthly Saving Does Not Automatically Mean Good Payback
A battery can produce positive TOU arbitrage and still have a long or unattractive payback if the monthly value is small relative to the installed cost. Conversely, a battery purchased primarily for backup resilience or solar self-consumption may receive TOU savings as an additional benefit rather than the sole reason for ownership.
To evaluate payback, separate the benefits instead of combining them into one unsupported number. Possible benefit categories include TOU arbitrage, increased solar self-consumption, avoided demand charges where applicable, participation incentives, backup value and other program-specific payments. Then compare those benefits with installation, financing, maintenance and replacement costs using assumptions that match the actual project.
Do not add overlapping benefits twice. For example, one kWh discharged during a peak period cannot simultaneously be counted as both a grid-charged arbitrage kWh and a solar-stored self-consumption kWh unless the accounting clearly identifies two different energy flows.
Common Input Mistakes
The most common errors come from mixing values that describe different parts of the energy path. Check these points before relying on a result:
- Using battery nameplate kWh as delivered kWh: use the energy expected to reach loads during the high-rate window after reserve and operating limits.
- Ignoring efficiency: entering 100% makes charging energy equal delivered energy and can materially overstate savings.
- Using average electricity price: a TOU calculation needs the marginal prices that apply specifically to the charging and discharging windows.
- Counting every calendar day: use only days when the tariff schedule, load and battery state allow the planned shift.
- Assuming the battery always empties at peak: low peak-period demand may leave stored energy unused.
- Ignoring charging power: a cheap tariff window may be too short to recharge the required energy at the available charger or inverter power.
- Mixing currency units: both high and low tariffs must use the same currency per kWh basis.
- Treating gross savings as net profit: the calculator does not subtract battery ownership cost, degradation, financing or installation expenses.
A Better Way To Validate The Estimate
The strongest validation uses your own bill and interval data. First, identify the actual tariff windows and marginal rates. Next, total the grid energy consumed during the expensive period for several representative days. Then compare that load with the battery energy that can be delivered while maintaining the reserve and power limits you intend to use.
If the battery system provides historical charge and discharge data, compare the measured charging energy with the measured discharged energy over similar cycles. That gives a system-level efficiency estimate under your operating conditions. Use a sufficiently long period to avoid drawing conclusions from one unusual day.
After the battery is operating, compare predicted savings with bill changes carefully. Weather, occupancy, appliance use and solar production can change from month to month, so a simple before-and-after bill comparison can be misleading. Where possible, compare shifted kWh and tariff-period imports directly rather than attributing the entire bill difference to the battery.
When The Calculator Is Most Useful
This tool is most useful as a transparent screening calculator when you know the amount of energy you expect to shift, the relevant high and low energy rates, a reasonable round-trip efficiency, and the number of monthly opportunities. It can quickly show whether a tariff has a strong, marginal or negative energy-arbitrage spread under those assumptions.
It is less suitable as a complete billing engine for tariffs with several time bands, demand charges, export pricing, tiered rates or complex seasonal rules. It also cannot determine whether a particular battery, inverter, charger, wiring system or installation is safe or compliant. Those questions require the exact equipment documentation and any applicable professional or regulatory review.
Use the result as a decision aid: identify the assumptions that matter most, test realistic alternatives, and record the tariff and operating conditions used. A reproducible estimate is more valuable than a highly precise-looking number built on hidden assumptions.
Final Checks Before Using The Monthly Saving
Before carrying the result into a purchase, payback spreadsheet or operating schedule, confirm five things: the high and low prices are the actual marginal energy rates for the relevant periods; the delivered energy can be consumed during the high-rate window; the battery can recharge within the low-rate window; the efficiency assumption represents the full charge-to-discharge path; and the cycle count reflects real tariff opportunities rather than the number of days in the month.
Then list the items the calculator does not price. At minimum, decide whether degradation, capital cost, fixed fees, demand charges, solar export value or other tariff rules materially affect your situation. Keeping included and excluded items separate makes the estimate easier to audit and update when rates or operating conditions change.
If the result is strongly positive across conservative assumptions, the tariff may deserve a deeper project-level analysis. If the result is close to zero or changes sign with small adjustments to efficiency, delivered energy or cycle count, gather better measurements before treating TOU arbitrage as a meaningful source of savings.