What This Calculator Estimates
A home battery can reduce a household's electricity bill when stored energy replaces grid imports that would otherwise have been purchased at a higher price. The simplest example is time-shifting: charge during an off-peak period, then discharge during a peak period. The financial value comes from the difference between the avoided high-rate purchase and the cost of the extra low-rate energy needed to charge the battery.
The important word is extra. A battery does not return every kWh used to charge it. Inverter losses, cell losses, standby consumption and conversion losses mean the grid must normally supply more energy than the battery later delivers. This calculator handles that with a round-trip efficiency input.
The Savings Formula
For each cycle, the calculator first determines how much grid energy is required to deliver the selected amount of usable energy:
Grid energy required = delivered energy ÷ round-trip efficiency
It then compares the peak-rate cost avoided with the off-peak charging cost:
Net saving per cycle = (delivered energy × high tariff) − (grid energy required × low tariff)
Finally, it multiplies the per-cycle value by the expected number of cycles in an average month. Because the tariff fields are labelled generically, you can use dollars, pounds, euros or another currency as long as both rates use the same currency per kWh.
What each input means
- Delivered energy per cycle: AC energy the battery supplies to household loads during the expensive period. This is not automatically the same as nameplate battery capacity.
- High-rate tariff: the import price per kWh you avoid when the battery discharges.
- Low-rate tariff: the marginal import price per kWh paid while charging.
- Round-trip efficiency: the fraction of charging energy that returns as useful discharged AC energy across the full cycle.
- Monthly cycles: the number of equivalent tariff-shifting cycles expected in a typical month.
Worked Example Using The Default Values
With 8 kWh delivered per cycle, a high tariff of 0.33 per kWh, a low tariff of 0.14 per kWh, 88% round-trip efficiency and 25 cycles per month, the battery must draw about 9.09 kWh from the grid for each 8 kWh delivered.
| Step | Calculation | Result |
|---|---|---|
| Grid input per cycle | 8 ÷ 0.88 | 9.09 kWh |
| Peak cost avoided per cycle | 8 × 0.33 | 2.64 |
| Off-peak charge cost per cycle | 9.09 × 0.14 | 1.27 |
| Net saving per cycle | 2.64 − 1.27 | 1.37 |
| Net saving per month | 1.37 × 25 | 34.18 |
| Annualized at same pattern | 34.18 × 12 | 410.18 |
The annualized figure is not a forecast. It simply repeats the same monthly assumptions for twelve months. Seasonal tariffs, changing loads, solar production, holidays, battery temperature and control settings can all change the real dispatch pattern.
The Break-Even Tariff Spread
A battery only creates positive tariff-arbitrage value when the avoided high-rate cost is greater than the low-rate charging cost after efficiency losses. Rearranging the formula gives a useful break-even condition:
Break-even high tariff = low tariff ÷ round-trip efficiency
With a low tariff of 0.14 per kWh and 88% round-trip efficiency, the high tariff must exceed about 0.159 per kWh before the energy shift itself becomes positive. A higher peak price increases the margin; lower efficiency raises the break-even peak price.
This test is useful because a tariff can look attractive simply because the headline peak rate is high. What matters is the spread after conversion losses, not the peak rate in isolation.
Why Delivered Energy Is Better Than Nameplate Capacity
A 10 kWh battery does not necessarily shift 10 kWh of billable household energy every day. Some capacity may be held as backup reserve, the battery may have operating state-of-charge limits, the expensive tariff window may be too short to empty the usable portion, or household demand may be too low to absorb the full discharge.
For bill-savings analysis, use the amount of energy that is realistically dispatched during the high-rate window. If your monitoring portal reports daily battery-to-home energy, that measurement is usually more useful than assuming the full rated capacity is cycled every day.
If you do not yet own the battery, model a conservative usable-dispatch figure and compare it with a higher case. This prevents a larger battery from looking automatically more profitable just because its nameplate energy is larger.
Round-Trip Efficiency Can Erase A Narrow Price Spread
Efficiency matters most when the high and low tariffs are relatively close. At 100% efficiency, shifting one kWh requires one kWh of charging energy. At 80% efficiency, delivering one kWh requires 1.25 kWh of input energy. The low-rate price is therefore paid on a larger energy quantity.
Use an efficiency value that matches the system boundary you are modelling. For a household bill calculation, AC-to-AC round-trip efficiency is generally the most intuitive because both the charging cost and the avoided grid purchase occur on the AC side of the home. Mixing a cell-only efficiency figure with AC meter tariffs can make savings look better than they are.
How Solar Charging Changes The Calculation
Charging from surplus rooftop solar is not always equivalent to charging at a zero cost. If exported solar would otherwise earn a feed-in tariff or export credit, using that energy to charge the battery has an opportunity cost equal to the export revenue you give up. In that situation, the relevant "low-rate tariff" can be interpreted as the lost export value per kWh rather than a grid import price.
If surplus solar would otherwise be curtailed or exported for no compensation, a zero marginal charging cost may be a reasonable scenario. If your system sometimes charges from solar and sometimes from the grid, calculate separate scenarios or use a weighted charging cost based on actual energy flows.
Solar self-consumption can also change the number and depth of cycles across the year. A single monthly-cycle assumption cannot capture every seasonal pattern, so annual savings are best built from monthly data when accuracy matters.
Time-Of-Use Tariffs, Flat Rates And Dynamic Pricing
This calculator is most directly useful when there is a predictable difference between the price paid to charge and the price avoided when discharging. Traditional time-of-use plans often provide defined off-peak and peak windows, which makes the inputs straightforward.
On a flat-rate tariff, routine grid-to-battery-to-home cycling usually loses money on energy alone because the same price applies on both sides while efficiency is below 100%. A battery may still provide backup value, solar self-consumption or other benefits, but those are different value streams.
Dynamic or real-time tariffs can produce larger spreads on some days and smaller or negative spreads on others. For those plans, a monthly average based on actual dispatch data is more defensible than assuming the battery captures the maximum daily spread every day.
What The Calculator Deliberately Leaves Out
The result should not be described as profit or payback because several economic items are outside the formula. Depending on your system and utility plan, you may need to consider:
- battery purchase price, installation cost, finance interest and incentives;
- capacity degradation and the value of battery wear per cycle;
- warranty throughput or cycle limits;
- fixed daily utility charges that do not change with energy use;
- demand charges, tiered rates, taxes or minimum bills;
- export credits and lost solar-export revenue;
- virtual power plant payments or grid-service incentives;
- maintenance, replacement equipment and inverter losses not represented by the chosen efficiency value;
- future tariff changes and changes in household consumption.
These omissions do not make the calculator useless. They define what the number means: a transparent estimate of the variable-energy bill saving from shifting kWh between two prices.
Battery Degradation And Cycling Cost
Every cycle contributes some amount of battery wear, although the relationship between cycling and degradation depends on chemistry, depth of discharge, temperature, calendar age, charge rate and manufacturer design. A savings estimate that ignores wear can overstate the economic value of frequent cycling when the tariff spread is small.
One way to stress-test the economics is to assign an estimated wear cost per delivered kWh outside this calculator and subtract it from the energy-arbitrage saving. Another is to compare expected lifetime energy throughput with the installed battery cost. Both methods require assumptions that are specific to the product and warranty, which is why they are not hard-coded into this simple tariff model.
Use Your Electricity Bill To Build Better Inputs
Start with the tariff schedule, not with a generic national electricity price. Locate the per-kWh import rates for the periods in which your battery will charge and discharge. If the bill bundles taxes or adjustments into the variable energy rate, decide whether those charges really change with each imported kWh.
Next, estimate how much load occurs during the expensive period. A battery cannot avoid more peak-rate imports than the home would otherwise consume. If your peak-window household demand is only 4 kWh, entering an 8 kWh discharge simply because the battery can deliver 8 kWh would overstate savings.
Finally, compare the assumed cycle count with actual behavior. Twenty-five monthly cycles may be reasonable for a battery that shifts energy on most weekdays, while a backup-focused system with a high reserve setting may cycle much less.
Common Mistakes That Inflate Estimated Savings
- Using rated capacity as daily delivered energy. Reserve settings, load and tariff-window length can prevent a full cycle.
- Ignoring charging losses. The battery has to buy more low-rate energy than it returns at the high rate.
- Counting fixed charges as avoidable. A battery normally cannot eliminate a daily service charge just by shifting kWh.
- Treating solar as automatically free. Exported solar may have a real credit or feed-in value.
- Assuming 30 or 31 full cycles every month. Dispatch may be limited by weekends, weather, reserve settings or insufficient price spread.
- Calling bill savings "payback." Payback requires installed cost and other revenues/costs, not just reduced energy purchases.
- Using the best tariff spread all year. Seasonal and dynamic pricing can materially change the annual result.
How To Interpret A Negative Result
A negative number means the charging energy costs more than the peak-rate purchase that the battery is expected to avoid under the entered assumptions. That can happen when the tariff spread is too small or efficiency is too low. It does not mean the battery has no value; it means this particular grid-charging arbitrage strategy does not save money on the variable energy charge.
A battery can still be valuable for outage backup, solar self-consumption, demand management or participation in utility programs. Those benefits should be evaluated separately so they are not double-counted.
From Monthly Savings To A Purchase Decision
Use the calculator output as one line in a broader financial model. If you are considering a new home battery, compare conservative annual bill savings with the installed cost, incentives, expected service life, warranty terms and any additional revenue streams. Run low, central and high cases rather than relying on one precise-looking number.
For an existing battery, the calculator is useful for checking whether a new tariff or control schedule is economically sensible. Compare the estimate with meter data after a few billing cycles. If observed savings differ, investigate actual charge/discharge energy, tariff windows, reserve settings and efficiency before changing the model.