Solar and battery storage glossary

The system

BESS: Battery Energy Storage System. A battery installed to move electricity from one hour to another — charging when energy is cheap or free, discharging when it is expensive.→ How a BESS earns its money
C&I: Commercial and Industrial. The consumer segment this product serves: factories, commercial buildings and other HT-connected sites. Distinct from residential and from utility-scale generation.
MWp: Megawatt-peak. Solar array capacity measured at standard test conditions. Solar is sized in MWp; batteries are sized in kWh of energy and kW of power, which are separate numbers.
C-rate: How fast a battery can charge or discharge, expressed as a multiple of its capacity. A 1,000 kWh battery at 0.5C moves up to 500 kW, taking two hours to fill or empty. Charge and discharge C-rates are often different and both matter.
Round-trip efficiency (RTE): The share of energy that survives being stored and returned. Frequently quoted per direction rather than round trip: 92% per direction is about 85% round trip, meaning roughly one unit in six is lost.
Depth of discharge (DoD): How far down a battery may be drawn before dispatch stops discharging it. Converts nameplate capacity into usable capacity — 90% DoD on 1,000 kWh leaves 900 kWh to work with — and protects cycle life at the cost of usable energy.
State of charge (SoC): How full the pack is at a given moment. Tracked hour by hour through a dispatch simulation, because what a battery can do in the evening peak depends entirely on what it did at midday.
State of health (SoH): Capacity remaining as a battery ages and cycles. A fifteen-year projection that assumes year-one capacity throughout overstates every year after the first.
Dispatch: The hour-by-hour decision of what the battery does: charge from surplus, discharge into an expensive hour, or stay idle. A dispatch model resolves that decision for all 8,760 hours of a year rather than applying an annual rule of thumb.
Behind the meter: Generation on the consumer’s own premises, consumed before the DISCOM’s meter measures it. Each unit consumed on site avoids the full landed tariff, which is what makes C&I solar economics work.
Open access: Generation located elsewhere and wheeled to the site across the grid, against wheeling charges, cross-subsidy surcharges and state open-access regulations.

The tariff

A commercial bill charges for two different things read off the same month. The area under the whole curve is energy, billed in kVAh; the single tallest half-hour sets the demand charge for every day of the month. Holding that one peak down changes the bill without changing the area much at all — which is what peak shaving is.

DISCOM: Distribution Company — the utility that bills the site. The most consequential single fact about a project: the tariff, its Time-of-Day zones and therefore the entire sizing answer depend on which DISCOM the site is connected to.
SERC: State Electricity Regulatory Commission. Issues the tariff orders that set rates for the distribution licensees in its state — MERC in Maharashtra, KERC in Karnataka, and so on.
Tariff order: The published, dated regulatory document setting rates for a financial year. Typically revised annually, and the document any defensible saving figure should be traceable to.
Time-of-Day (ToD): Electricity priced differently by hour of the day. Universal for Indian C&I consumers since April 2024. Indian tariff orders say "ToD"; "Time-of-Use" or "ToU" means the same thing and appears in older material.→ How ToD tariffs are built
ToD zone: A named block of hours with its own price — off-peak, normal, solar hours, evening peak. Boundaries are set by the DISCOM’s order, are defined to the half hour in several states, and can move when the order is revised.
ToD arbitrage: Charging a battery when energy is cheap or free and discharging it when the tariff is expensive, keeping the difference less round-trip losses. The mechanism that justifies a battery on a bill.
Peak shaving: Discharging a battery into a site’s highest-demand moments to reduce the maximum demand the demand charge is billed on. Unforgiving: a single unshaved spike can reset the month’s billed maximum.
Demand charge: A charge billed on the site’s maximum demand in the period, per kVA, regardless of how long that maximum lasted. Often a large share of a C&I bill and a large share of what storage saves.
kVAh vs kWh billing: kVAh bills apparent energy, so the site pays for its power factor; kWh bills active energy. They are not interchangeable, and savings computed in kWh on a kVAh-billed site are understated.
Consumer category: Which tariff schedule applies to a connection. This product covers six: HT Industrial and HT Commercial, each at 11 kV, 33 kV and above 33 kV (EHT).
Landed rate: What a unit actually costs after everything that rides on the published energy charge — Time-of-Day adder, fuel adjustment, electricity duty and any riders. The correct rate at which to value a displaced unit, and the one most DISCOMs do not publish enough components to assemble.
Electricity duty: A state-levied percentage on the bill. Scales with whatever the bill is, so it scales down with the saving.
Fuel adjustment (FAC / FPPPA): A per-unit adjustment passing fuel and power-purchase cost variation through to the consumer. Revised periodically, published monthly in some states, and volatile enough that it is quoted as a range in others.
Wheeling charge: A per-unit charge for using the distribution network to move energy. Relevant to open access; generally not to behind-the-meter generation.
Cross-subsidy surcharge: A per-unit charge on open-access consumption, compensating the DISCOM for the subsidised consumers it retains. Rarely published in a form that can be applied without reading the order.

The study

Project: One site. Holds its bills, location, tariff, contact details and every analysis run against it. One site is one project; alternative sizes are scenarios inside it, not separate projects.
Analysis profile: A saved set of parameters within a project — the optimisation goal plus the technical and commercial assumptions. Runs and custom scenarios belong to a profile, so changing an assumption does not overwrite earlier work.
Scenario: A specific solar and battery size, whether the optimiser named it or a user entered it. Both are simulated by the same engine, so a customer’s own number can be compared on equal terms.
8,760: The number of hours in a non-leap year, and the unit of the whole simulation: twelve monthly bill totals in, 8,760 hourly readings out.→ How the hourly year is rebuilt
The sweep: The exhaustive search across candidate configurations — 3,171 solar and battery combinations, each run through a complete hourly dispatch and scored against the chosen goal.
Knee point: The capacity at which the savings curve flattens and further capacity stops paying for itself. The build named Headroom sits here.
Today: The grid-only baseline: what the site pays with nothing built. The anchor every other option is read against.
Recommended: The configuration ranked best by the selected optimisation goal.
Lean: The cheapest battery that still retains at least 98% of the best available savings. The option for a constrained budget or a customer optimising return per rupee.
Headroom: The knee-point build — the most capacity the site can usefully absorb before returns flatten.
NASA POWER: The satellite irradiance and weather dataset used to model a site’s solar year from its latitude and longitude, when measured generation data is not available.
Reference 1 MWp year: The modelled hourly output of one megawatt-peak at a specific site, from which every candidate array size is scaled — so all sizes are compared against the same weather.

The numbers

Capex payback: The number of years for savings to repay the capital cost. The most quoted return figure and the most ambiguous, because it depends entirely on how the capital was priced.
Break even: A second payback framing that prices solar on a different basis from capex payback. The two are meant to differ; a proposal quoting a payback should say which basis it means.
IRR: Internal rate of return — the project framed as an investment rather than as a bill reduction. The right frame for a finance team, the wrong one for a plant manager.
Lifetime benefit: Cumulative saving over the modelled life of the system. Sensitive to the tariff escalation assumed, which should always be stated alongside it.
Effective tariff: Total cost divided by total units after the build. The most intuitive figure on a comparison sheet, because it is directly comparable to the rate printed on the bill.
Grid dependence: The share of the site’s consumption still drawn from the grid after the build.
Green share: The share of consumption met by solar, whether used directly or delivered through the battery.
Surplus (wasted solar): Generation with nowhere to go — the load is met, the battery is full, and export is unavailable or unpaid. The clearest reason a larger array is not automatically a better one.
Self-consumption: Solar used on site rather than exported. What solar-only sizing maximises, because a consumed unit is worth the landed tariff and an exported one usually is not.
Curtailment: Generation deliberately not produced because it cannot be used or exported. The inverters back off and the energy never exists.

Terms this product avoids

Not everything in circulation is worth repeating. These are avoided deliberately, and a proposal is stronger without them:

“Bankable” — a claim about financial diligence that a sizing study has not performed. “Guaranteed” or “exact” applied to a modelled output — the method models a year, it does not measure one. “Real-time” applied to tariff data — orders are transcribed by hand from published documents, and saying otherwise invites a question best not invited.

If a figure cannot survive the question “on what basis?”, it is not ready to go in front of a customer.