Engineering Math
19 terms in the engineering math category.
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AC/DC Ratio (Inverter Loading Ratio)
AC/DC ratio, also called the inverter loading ratio (ILR) or DC/AC ratio, is installed DC array capacity divided by inverter AC rating. Designers set it in tools such as PVsyst (the Pnom ratio) to trade extra morning and evening energy against midday clipping. Most utility and commercial plants land between about 1.1 and 1.4.
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Availability
Availability is the share of time, or of potential energy, during which a solar plant is able to generate, after counting outages from inverter faults, maintenance and grid events. IEC TS 63019:2019 sets out an information model for PV availability. O&M contracts and lender models use it to measure operator performance and set guarantees.
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Bankability
Bankability is a solar project's ability to secure non-recourse or senior debt from lenders. It depends on an independent engineer's review of the energy yield assessment, usually at P90, equipment quality and warranties, a creditworthy offtaker, and permits. Developers and EPCs prepare design and yield reports with lender scrutiny in mind.
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Capacity Factor
Capacity factor is the actual energy a power plant produces over a period divided by the energy it would produce running at full nameplate capacity for the whole period. For solar, annual capacity factor equals specific yield in kWh/kWp divided by 8,760 hours. Lenders and planners use it to compare plants and technologies.
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Curtailment
Curtailment is a forced reduction of a solar plant's output ordered by the grid or system operator, usually because of transmission congestion, oversupply at low demand, or stability limits. In India, the Indian Electricity Grid Code gives solar must-run status, so curtailment is restricted. Developers and lenders model curtailment as an energy loss.
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Degradation Rate
Degradation rate is the yearly percentage loss in a PV module's power output during field operation. An NREL review of field data by Jordan and Kurtz (2013) found a median of about 0.5% per year for crystalline silicon. Designers use the manufacturer's warranted rate in long-term energy and financial models.
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EYA (Energy Yield Assessment)
An Energy Yield Assessment (EYA) is an engineering study that forecasts a solar plant's long-term energy production, usually with PVsyst or similar software, and expresses uncertainty as P50 and P90 values. P90 is the annual energy expected to be exceeded with 90% probability. Lenders, investors and buyers use EYAs for project financing.
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Inverter Clipping
Inverter clipping is the energy lost when a PV array's DC power exceeds what the inverter can deliver at its AC rating, so the inverter limits output to its nameplate. It is a deliberate design trade-off tied to the DC/AC ratio, and designers estimate it in PVsyst using hourly simulation.
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Inverter Loading Ratio
Inverter Loading Ratio (ILR) is the ratio of a PV array's DC nameplate capacity (kWp) to the inverter's AC rated capacity (kW), also called the DC/AC ratio. NREL's Annual Technology Baseline assumes an ILR of 1.34 for US utility-scale PV, and designers set it to balance clipping against inverter cost.
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IRR (Internal Rate of Return)
IRR (Internal Rate of Return) is the discount rate at which a solar project's net present value of cash flows equals zero, expressing its return as an annual percentage. Investors compare project IRR, before debt, and equity IRR, after debt, against their hurdle rate to decide whether to invest.
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kWh/kWp (Specific Yield)
kWh/kWp is the unit of specific yield: the energy a solar system produces over a period, usually a year, divided by its installed DC capacity. IEC 61724-1 defines this as final yield, and designers and investors use it to compare plant productivity across sites, sizes and module technologies.
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LACE (Levelized Avoided Cost of Energy)
LACE (Levelized Avoided Cost of Energy) is the value of the generation and capacity costs a new power plant lets the grid avoid, expressed per unit of energy. The US Energy Information Administration uses it in its Annual Energy Outlook, comparing it with LCOE to judge whether a project is economically competitive.
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LCOE (Levelized Cost of Energy)
LCOE stands for Levelized Cost of Energy, the lifetime cost of a power plant divided by its lifetime energy output, both discounted to present value. NREL and Lazard publish annual LCOE benchmarks using this method. Solar developers, lenders and EPCs use LCOE to compare design options and test whether a PPA tariff covers project costs.
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NPV (Net Present Value)
NPV (Net Present Value) is the sum of a project's future cash flows discounted to today's value at a chosen rate, minus the initial investment. A positive NPV means the project earns more than the discount rate. Solar developers, lenders, and buyers use it with IRR and LCOE to compare system designs and financing options.
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P50
P50 is the median annual energy yield forecast for a solar project, the output expected to be exceeded in 50% of years. P90 is the lower value exceeded in 90% of years. Both come from a yield simulation plus an uncertainty analysis in tools such as PVsyst or NREL SAM, and lenders size debt on P90.
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P75
P75 is the annual energy yield a solar plant is expected to exceed with 75% probability, taken from the uncertainty distribution in an energy yield assessment. Assuming a normal distribution, P75 equals P50 minus about 0.674 times the combined standard uncertainty. It sits between the median P50 and the lender-focused P90 estimate.
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P90
P90 in solar is the annual energy yield a plant is expected to exceed in 90% of years, or 9 years out of 10. It is calculated from the P50 estimate as P50 minus 1.28 times the combined uncertainty, assuming a normal distribution. Lenders size project debt and check DSCR against P90 rather than the expected P50.
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P99
P99 is the annual energy yield a solar plant is expected to exceed with 99% probability, derived from the uncertainty distribution in an energy yield assessment. Under a normal distribution it equals P50 minus about 2.326 times the combined uncertainty. Lenders and analysts use it mainly for stress testing and worst-case debt service checks.
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Payback Period
Payback period is the number of years it takes for a solar system's cumulative savings or income to equal its upfront cost. It is calculated by dividing the net installed cost by annual net savings, or by tracking cumulative cash flow year by year. Homeowners use it as a quick check, alongside NPV and IRR.
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