Calculator Inputs
Use realistic tariffs and battery behavior. All currency values are in your local currency.
Example Data Table
| Scenario | Installed cost | Usable capacity | Cycles/year | Tariff / solar setup | Estimated simple payback* |
|---|---|---|---|---|---|
| Solar + export gap | $9,500 | 13.5 kWh | 260 | Import 0.22, export 0.06, surplus 2500 kWh/yr | ~6–9 years |
| Time-of-use arbitrage | $8,000 | 10.0 kWh | 300 | Peak 0.35, off-peak 0.12 | ~7–12 years |
| Peak shaving add-on | $12,000 | 15.0 kWh | 240 | Demand savings $15/month + mixed savings | Varies by demand profile |
Formula Used
Shifted_kWh = Usable_kWh × Cycles_per_year × (RTE% ÷ 100) × Degradation_factor
(1 − deg%)^(year−1).- Solar self-consumption:
Solar_savings = min(Shifted_kWh, Solar_surplus) × (Import_rate − Export_rate) - TOU arbitrage:
Arbitrage_savings = Remaining_kWh × (Peak_rate − Offpeak_rate) - Demand savings:
Monthly_savings × 12
- Net investment:
(Cost − Incentives) − Tax_credit, whereTax_credit = (Cost − Incentives) × Credit% - Simple payback (years):
Net_investment ÷ Year1_net_savings - NPV:
−Net_investment + Σ(Net_savings_year ÷ (1 + discount) ^ year) - Discounted payback: first year when discounted cumulative cashflow becomes non‑negative.
This tool applies escalation to rates and demand savings, and applies degradation to capacity each year.
How to Use This Calculator
- Enter the installed cost and any incentives or tax credit percentage.
- Set battery behavior: usable capacity, efficiency, cycles per year, and degradation.
- Choose your savings mode: solar only, arbitrage only, or both.
- Fill in rates: import/export for solar savings, peak/off-peak for arbitrage.
- Add optional demand savings if you reduce peak demand charges.
- Adjust economics: escalation, discount rate, and analysis years, then calculate.
For best accuracy, match cycles/year to your real charging pattern, and use your utility’s published tariff schedule.
Industry Notes
Payback drivers and energy value
Battery payback is the value of shifting kilowatt-hours into higher-priced moments. This tool blends solar self-consumption, time-of-use arbitrage, and demand savings, then subtracts operating costs. Yearly savings escalate with your tariff growth assumption, while usable capacity declines with degradation. Strong rate spreads and steady cycling shorten payback; low utilization pushes payback outward.
Usable energy, cycles, and degradation
Shifted energy starts with usable capacity and multiplies by cycles per year and round-trip efficiency. A 90% efficiency means 10% of charged energy is lost, so fewer kilowatt-hours are delivered to loads. Each year, capacity is reduced by a compounded degradation factor, so later savings are smaller even if rates rise. Set cycles to reflect your typical dispatch, not a perfect daily cycle.
Tariffs, exports, and arbitrage spread
Solar savings come from avoiding exports: each stored kilowatt-hour is valued at import rate minus export credit. If exports are compensated well, solar savings shrink. Arbitrage savings use any remaining shifted energy and the peak minus off-peak price gap. Demand savings are modeled as a monthly amount that escalates with rates. Enter annual solar surplus conservatively so it matches energy truly available after household use.
Reading payback, NPV, and IRR
Simple payback divides net investment by first-year net savings, giving a quick benchmark. Undiscounted payback uses cumulative cashflow and captures changing savings over time. Discounted payback applies your discount rate, so distant savings count less. NPV sums discounted net savings minus net investment; positive NPV indicates value above your hurdle rate. IRR, when solvable, is the annual return implied by the cashflow stream.
Calibrating inputs with real data
Better estimates come from aligning inputs to your utility plan and behavior. Use bills or interval data to confirm peak and off-peak prices and cycling frequency. Add realistic operating costs such as monitoring fees. For solar, compare midday exports to battery charge limits and seasonality. Use the graph to sanity-check trends: flat cumulative savings usually means surplus or spreads are overstated.
FAQs
How should I choose cycles per year?
Use the number of full-equivalent cycles you expect from your dispatch strategy. Many homes land near 200–300, while backup-first operation may be lower.
What does round-trip efficiency represent?
It captures charge and discharge losses. Lower efficiency reduces delivered energy per cycle, shrinking savings and extending payback.
How do incentives and tax credits apply here?
The calculator subtracts incentives first, then applies the credit percentage to the remaining cost to estimate net investment.
Why can discounted payback be much longer?
Discounting reduces the present value of future savings. Higher discount rates or back‑loaded savings delay the break-even year.
When is solar self-consumption more valuable than arbitrage?
When export credit is far below the import rate, storing surplus to avoid exports can outperform TOU arbitrage.
Does the model include battery replacement or warranties?
No. It assumes one system over the analysis period. If you expect replacement, raise cost, add extra O&M, or shorten the analysis years.