| Scenario | System cost | Incentives | APR | Term | Monthly savings | Estimated payback |
|---|---|---|---|---|---|---|
| Sample A | $12,000 | $2,500 | 7.5% | 84 mo | $140 | Depends on cash-flow assumptions |
| Sample B | $15,500 | $3,000 | 6.0% | 120 mo | $170 | Often after loan payoff |
| Sample C | $10,500 | $1,500 | 0.0% | 60 mo | $125 | Usually sooner than financed options |
- Amount financed = (Net cost after incentives - Down payment) + Origination fee.
- Monthly payment uses: PMT = P*r*(1+r)^n / ((1+r)^n - 1), where r = APR/12 and n = months.
- Monthly savings is either entered directly, or estimated as: daily_kWh * days * rate_spread * efficiency + demand_savings.
- Net cash flow per month = Savings - Loan payment - Upkeep/12 (plus any rebate in its receipt month).
- Payback month is the first month when cumulative net cash flow >= 0.
- NPV discounts each monthly net cash flow by a monthly rate derived from your annual discount rate.
- Enter the installed battery price and any tax.
- Add rebates and select when you receive them (upfront or delayed).
- Set your loan APR, term, and any extra monthly payment.
- Choose a savings method and provide realistic savings inputs.
- Adjust escalation, degradation, and discount rate for your scenario.
- Press Calculate to review payment, payback, NPV, and yearly cash flow.
Financing Inputs
The calculator starts with installed cost plus any sales tax, then subtracts incentives to estimate the net project cost. Amount financed is net cost minus down payment, plus an origination fee if your lender charges one. APR and term drive the monthly payment through amortization, while an extra payment shortens payoff and reduces total interest. These inputs matter most when savings are steady but rates vary.
Savings Drivers
You can enter monthly bill savings directly, or estimate them from energy shifting. In the kWh model, monthly savings equal daily discharge times days per month, multiplied by the peak-off-peak spread and round-trip efficiency, then add any demand-charge reduction. Example: 10 kWh/day, 30 days, a 0.18 spread, and 90% efficiency yields about 48.60 per month before demand savings.
Payback Interpretation
Two payback views are shown. Simple payback divides net cost after incentives by Year-1 savings, ignoring financing. Cash-flow payback is stricter: it tracks cumulative net cash flow after loan payments and upkeep, and marks the first month cumulative turns positive. If your loan payment exceeds savings, the payback point may occur only after payoff, even if simple payback looks short.
Scenario Stress
Long projects benefit from realistic assumptions. Savings escalation can reflect expected electricity price increases, while degradation reduces usable performance over time; together they shape the curve, not just the first year. The discount rate converts future cash flows into today's value and produces NPV, helping compare alternatives fairly. A positive NPV suggests the financed upgrade beats your chosen hurdle rate under the modeled conditions.
Using Outputs
Use the results to compare quotes and loan offers on the same footing. Start by matching the system cost, incentives timing, and expected savings, then test multiple APR and term combinations to see payment and payback tradeoffs. Review total interest, payoff months, and the year-by-year table to confirm the path to break-even. Finally, adjust extra payments to see how faster payoff changes cash flow. This helps align financing with your risk tolerance.
1) What is cash-flow payback in this calculator?
It is the first month when cumulative net cash flow becomes positive after including down payment, rebate timing, loan payments, and upkeep. It reflects real household cash movement, not just cost divided by savings.
2) How do delayed incentives affect results?
If you set a rebate delay, the model adds the rebate as an inflow in that month. Payback may improve later, but early cash flow can stay negative until the incentive arrives.
3) What monthly savings value should I enter?
Use an average based on bills or an installer estimate. If savings are seasonal, choose a conservative annual average. You can rerun scenarios with higher and lower savings to see sensitivity.
4) How does an extra payment change payback?
Extra payments reduce principal faster, lowering interest and shortening payoff months. That usually improves long-run profit and can bring the break-even month forward, especially when payment is close to savings.
5) What discount rate should I choose for NPV?
Pick a rate that matches your alternative use of money, such as your after-tax borrowing cost or a conservative investment return. Higher discount rates reduce NPV and emphasize near-term cash flow.
6) Is the kWh rate-spread method accurate?
It is a simplified estimate based on energy shifted and your price spread. Accuracy depends on how often the battery cycles, real efficiency, and tariff rules. Use it for planning, then refine with measured data.