Net Present Worth (NPW) of a Cash Stream
The value of a stream of payments is called the Net Present Worth (NPW).
The present value of a stream of payments  Net Present Worth (NPW) or Net Present Value (NPV) 
can be calculated with a discounting rate
P = F _{0 } / (1 + i) ^{ 0 } + F _{ 1 } / (1 + i) ^{ 1 } + F_{2}/ (1 + i)^{2}+ .... + F _{ n } / (1 + i) ^{ n } (1)
where
P = Net Present Worth (or Value)
F = cash flow in the future
i = discounting rate
(1 + i) ^{ n } is known as the " compound amount factor ".
Example  the Net Present Worth of an Investment Transaction with a variable Cash Flow
The Net Present Worth  NPW  of investing an amount of 1000 today and saving 250, 200, 300, 310 and 290 the next 5 years  and selling the investment for 310 in the last fifth year  at an interest or discount rate of 10% , can be calculated as
P = (1000)/ (1 + 0.1) ^{ 0 } + (250) / (1 + 0.1) ^{ 1 } + (200) / (1 + 0.1)^{2}+ (300) / (1 + 0.1) ^{ 3 } + (310) / (1 + 0.1) ^{ 4 } + (290 + 310) / (1 + 0.1) ^{ 5 }
= 202
Net Present Worth is positive and the investment is profitable.
Example  Investment In Renewable Energy  Discounted Cash Flow
 how to calculate discounted net present value in a typical renewable energy project
Net Present Worth Calculator  Variable Cash Flow Stream
The calculator below can be used to estimate Net Present Wort  NPW  in an investment project with up to 20 periods and variable cash flows.
 money out  negative values
 money in  positive values
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SUM  
Discount rate (%)  
Net Present Worth Calculator  Investment with Fixed Cash Flow and Growth Rate
The calculator below can be used to calculate the Net Present Worth for a project with a fixed investment value and fixed return cash flows with a growth rate.
Example  Investing in a Solar Power System
An investment of 30000 in a solar power system saves an amount of 1000 on the electricity bill the first year. The electricity price is in the future assumed to rise 10% (growth rate) each year. The alternative use of the money is to invest in a savings account with 3% interest rate (discount rate).
The lifespan of the solar power system is estimated to 20 years (no. of periods).
Using these values in the calculator above gives
P = 12819
IRR = 6.1 %
 and the investment is profitable compared to the savings account.
Using the same values  but for a shorter lifespan of 15 years  gives
P =  3581
IRR = 1.6 %
 and the investment is not profitable compared to the savings account.
Net Present Value  Spreadsheet
A flexible Net Present Value and Internal Rate Calculator in Excel spreadsheet format is available here !
Related Topics

Economics
Engineering economics  cash flow diagrams, present value, discount rates, internal rates of return  IRR, income taxes, inflation.
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