How is the federal ITC handled?

The Investment Tax Credit is modeled as a tax credit equal to the ITC percent times the gross system cost, realized in year one. At the 2024 rate of 30 percent on a 24,000 dollar system that is a 7,200 dollar credit added to the first year's benefit. The credit reduces the effective cost but is not a cash rebate at purchase.

What is the difference between NPV and IRR?

NPV is the present value of every future net cash flow discounted at your chosen rate, minus the up-front investment; a positive NPV means the project beats that discount rate. IRR is the discount rate at which NPV equals zero, found here by bisection, and represents the project's effective annual return regardless of your assumed discount rate.

Why does production fall and savings rise over time?

Panels degrade slightly each year, commonly around half a percent, so kilowatt-hours produced decline. At the same time utility rates tend to escalate a few percent per year, so each kilowatt-hour saved is worth more. The model applies both effects year by year, which is why later-year savings can still grow despite lower output.

How does financing change the result?

With financing, only the down payment is the year-zero cash outflow and the loan is repaid as a level annual payment over the loan term, which subtracts from each year's net during the loan. This usually lowers IRR versus a cash purchase but reduces the up-front capital, so payback timing shifts.

Is this tax and incentive advice?

No. It is a planning model. ITC eligibility, depreciation, SREC market prices, net-metering rules, and tax treatment vary by jurisdiction and by year. Confirm every incentive and the tax handling with a qualified solar installer and tax professional before relying on the numbers.

Solar Investment ROI & Cash Flow Calculator

A solar proposal lives or dies on its cash flow, not its sticker price. The incentives, escalating utility rates, panel degradation, and financing all interact over 25 years in ways a simple payback estimate misses. This calculator builds the full year-by-year cash-flow vector and reduces it to the three numbers that matter: net present value, internal rate of return, and payback period.

How it works

Year-one production is the system size times the local specific yield. Each year production degrades and the value of each kilowatt-hour escalates with utility rates:

production(t) = kW x yield x (1 - degradation)^(t-1)
rate(t)       = utilityRate x (1 + escalation)^(t-1)
energy_value  = production(t) x rate(t)

SREC income, O&M cost, and any loan payment are added to each year. The ITC and state incentive land in year one:

ITC = itc% x grossCost     (year-1 tax credit)
net(t) = energy_value + srec - om - loanPayment
NPV = sum( net(t) / (1 + discount)^t )
IRR = rate where NPV = 0   (solved by bisection)

Tips and example

An 8 kW system at 1,300 kWh/kW/yr produces about 10,400 kWh in year one. At a 0.16 dollar utility rate that is roughly 1,660 dollars of first-year savings. On a 24,000 dollar gross cost with a 30 percent ITC, a 3 percent rate escalation, and a 5 percent discount rate, the model typically shows a payback in the low-to-mid teens of years and a positive 25-year net.

Run the cash purchase and the financed case side by side: cash usually gives the higher IRR while a loan lowers the up-front capital. Whatever the headline number, confirm the incentive eligibility and tax treatment locally — those assumptions move the result more than any production estimate.