Model battery economics with degradation, escalation, and maintenance. Add replacement timing, grants, and salvage values. See IRR and charts, then export results anytime easily.
| Scenario | Upfront Net Cost | Annual Savings | Project Life | Degradation | Escalation | Replacement |
|---|---|---|---|---|---|---|
| Home backup + bill savings | $8,450 | $1,800 | 12 years | 2%/yr | 3%/yr | Year 8: $2,500 |
| Commercial demand shaving | $24,000 | $6,500 | 15 years | 1.5%/yr | 4%/yr | Year 10: $6,000 |
| Small cabin off-grid support | $6,900 | $1,050 | 10 years | 2.5%/yr | 2%/yr | None |
The calculator builds yearly cashflows and solves for the internal rate of return (IRR), which is the rate that makes net present value (NPV) equal to zero.
Note: If cashflows change sign multiple times, IRR may be non-unique; this tool will report “not solvable” when it cannot find a reliable root.
IRR is driven by the net savings you can capture each year. Value streams include demand charge shaving, time of use shifting, and avoided outage costs. Start with measured bills and load data, then use year one savings as a baseline. Apply escalation only when tariff history supports it. Degradation reduces delivered energy and peak power, so the decline rate should align with the warranty curve and operating strategy.
Models can mislead when soft costs are missing. Installation labor, permits, interconnection fees, controls integration, and metering upgrades add cost without raising savings. Incentives reduce the initial outlay, but confirm eligibility, timing, and tax treatment in your area. This calculator applies incentives in year zero, helping you compare vendor quotes and avoid unrealistic payback assumptions.
Some systems need inverter service or component replacement during the analysis window. A midlife replacement can cut IRR sharply, especially if it arrives before payback. If warranty coverage applies, model only the expected net cost after coverage. When replacement is uncertain, run a none case and a worst case cost to quantify downside risk and guide contract negotiations.
IRR is a breakeven rate, while NPV measures value at your discount rate. Use a discount rate that reflects financing cost and risk. A project can have a positive IRR but negative NPV if your required return is higher. The discounted cumulative line shows when the project becomes positive in today’s money, not just in nominal cashflow terms.
Review year by year cashflows to confirm the pattern matches operations. Escalation and degradation should change values gradually, while replacement and tariff changes should create step effects. If IRR is not solvable, cashflows may never cross zero or may cross multiple times. Adjust assumptions, shorten project life, or remove spikes to improve stability and interpretation.
IRR is the annualized rate that makes the project NPV equal zero. If IRR exceeds your required return, the cashflows are strong enough to justify the investment under your assumptions.
If cashflows never switch from negative to positive, or switch multiple times due to spikes, there may be no unique IRR. Review the table, then adjust project life, replacement timing, or savings assumptions.
Use recent bills and interval data to estimate demand charge reduction and time based shifting. For backup value, estimate avoided outage costs conservatively. Enter year one savings, then apply escalation only when justified.
Start with the manufacturer warranty or expected capacity retention curve. If your strategy relies on high peak power, consider additional performance decline over time. Keep the rate modest unless you have site specific evidence.
Enter rebates or grants as incentives so the calculator reduces year zero cost. If you expect delayed payments, you can approximate by lowering incentives and adding a positive cashflow in the year you receive them.
Payback is easy to interpret but ignores value after the payback year and the time value of money. NPV incorporates discounting and captures total value, so it is usually better for comparing projects.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.