Time-of-Use Arbitrage Calculator

• Usable_kWh = Battery_kWh × (Usable_Depth% ÷ 100)
• Charge_limit_kWh = Charge_kW × OffPeak_Hours
• Discharge_limit_kWh = Discharge_kW × Peak_Hours
• Energy_in = min(Usable_kWh, Charge_limit_kWh)
• Energy_out = min(Energy_in × (RTE% ÷ 100), Discharge_limit_kWh)
• Arbitrage_value = Energy_out × Peak_Rate
• Energy_cost = Energy_in × OffPeak_Rate
• Fees = Energy_cost × (Fees% ÷ 100)
• Degradation = Energy_out × Degradation_Cost_per_kWh
• Net_per_cycle = Arbitrage_value − Energy_cost − Fees − Degradation
• Net_per_month = Net_per_cycle × Cycles_per_day × Days_per_month + Demand_rate × Peak_shave_kW − O&M
• NPV = −Capex + Σ( Annual_Net × (1−Fade)^(y−1) ÷ (1+Discount)^y )

1. Enter your off-peak and peak rates exactly from your tariff.
2. Set battery capacity, usable depth, and round-trip efficiency.
3. Input charge/discharge power and peak/off-peak window hours.
4. Add degradation cost and monthly O&M if you track them.
5. Optional: include demand charge rate and shaved peak kW.
6. Click Calculate; results appear above the form for review.
7. Use CSV/PDF downloads to share scenarios with stakeholders.

Tariff spread and dispatch window

A typical residential TOU spread ranges from 0.10 to 0.30 per kWh. If off‑peak is 0.10 and peak is 0.30, the gross spread is 0.20. Seasonal spreads can vary widely. Savings grow only when time windows allow full dispatch. With 6 off‑peak hours at 5 kW charge power, the charge window can accept up to 30 kWh, but usable capacity may cap it. With 4 peak hours at 5 kW discharge power, the peak window can deliver up to 20 kWh.

Efficiency and usable energy limits

Usable energy equals capacity times usable depth. With a 20 kWh battery at 90% usable depth, usable energy is 18 kWh per cycle. At 88% round‑trip efficiency, 18 kWh charged returns about 15.84 kWh at peak. If the peak window caps discharge, delivered kWh drops and the model reduces the implied charge.

Degradation and operating expenses

Degradation can be modeled as a cost per kWh discharged. At 0.03 per kWh and 15.84 kWh out, degradation is 0.48 per cycle. Add fees as a percent of off‑peak energy cost; a 3% fee on a 1.80 charge adds 0.05. Fixed O&M matters too: a 10 monthly service fee reduces annual benefit by 120.

Demand charge stacking and peak shaving

Commercial tariffs may include demand charges like 12 per kW‑month. Shaving 3 kW during the billing peak adds 36 per month, independent of kWh. This stacks with arbitrage and can dominate when energy spreads are modest. Reliable peak clipping often improves results more than adding extra cycles.

Financial metrics and decision thresholds

Simple ROI equals annual net benefit divided by capex, while payback equals capex divided by annual net benefit. NPV discounts future cashflows and applies capacity fade, such as 2% per year. A break‑even rule is: Peak_Rate should exceed (OffPeak_Rate ÷ Efficiency) plus Degradation_Cost plus Fees_per_kWh. Positive NPV indicates the project beats the discount rate.

What does this calculator estimate?

It estimates net savings from charging during off‑peak hours and discharging during peak hours, including efficiency losses, fees, degradation cost, demand savings, and monthly operating costs.

How do I know if arbitrage is profitable?

Arbitrage is usually positive when the peak rate exceeds off‑peak rate divided by efficiency, plus degradation and fees. If net per cycle is negative, reduce costs or increase the price spread.

Why do charge and discharge power limits matter?

Power limits and short tariff windows can cap kWh moved per cycle. Even a large battery cannot earn more if it cannot charge enough off‑peak or discharge enough during the peak window.

How should I set degradation cost per kWh?

Use your internal estimate of battery wear cost per discharged kWh, based on cycle life, replacement cost, and usable capacity. Conservative inputs prevent overstating savings and shorten unrealistic payback claims.

What are demand charge savings in this model?

Demand savings are monthly bill reductions from lowering peak kW demand. The calculator multiplies shaved kW by the demand charge rate, then adds that value to the energy arbitrage benefit.

Can I use it for solar plus storage?

Yes. Treat off‑peak rate as the effective charging cost. If you charge from solar, you can enter a low cost or opportunity cost, then compare savings against peak offset value and degradation.