Battery Energy Storage Models
Executive Summary
Key Takeaways
- ✓ Battery degradation is driven primarily by charge/discharge cycling — not simply the passage of time — and should be modelled against a projected cycling schedule rather than a generic annual degradation percentage borrowed from generation asset conventions.
- ✓ Round-trip efficiency (the ratio of energy discharged to energy charged) directly reduces net revenue captured from any price arbitrage activity and should be modelled explicitly, not ignored as a rounding factor.
- ✓ Depth of discharge — how much of the battery's capacity is used in each cycle — trades off against degradation rate, and the model should reflect the actual operating strategy's depth-of-discharge profile rather than assuming full-capacity cycling by default.
- ✓ An augmentation or replacement schedule, adding or replacing battery capacity partway through the asset's contracted or operating life to offset capacity fade, should be modelled explicitly with its associated capital cost where the project's plan includes one.
- ✓ Battery storage revenue is typically stacked across multiple distinct streams — energy arbitrage, capacity payments, and ancillary services — each modelled separately, since a single blended revenue-per-MWh assumption conceals which stream is actually driving project economics.
Objective¶
This guide covers the technical and revenue mechanics specific to battery energy storage systems (BESS), within Energy Financial Modelling, building on Power Project Financial Model Structure.
Cycling-Driven Degradation¶
Unlike a generation asset, whose degradation is primarily time-based, a battery's degradation is driven principally by charge/discharge cycling — the number of cycles and the depth of each cycle. The model should build a projected cycling schedule reflecting the asset's actual planned operating strategy (frequency of arbitrage cycling, ancillary service response cycling) and derive degradation from that schedule, rather than applying a generic annual percentage borrowed from solar or wind degradation conventions, which are calendar-driven rather than cycling-driven.
Round-Trip Efficiency¶
Round-trip efficiency — the ratio of energy discharged to energy used to charge the battery — reflects real energy losses in the charge/discharge process. This should be modelled explicitly in any energy arbitrage revenue calculation, since it directly reduces net captured revenue: a battery that charges at a low price and discharges at a high price still loses a portion of the arbitrage margin to round-trip efficiency losses, and ignoring this factor overstates achievable arbitrage revenue.
Depth of Discharge¶
The proportion of total battery capacity used in a given cycle — depth of discharge — trades off directly against degradation: cycling to a greater depth typically accelerates capacity fade relative to shallower cycling. The model should reflect the actual planned operating strategy's typical depth-of-discharge profile, since assuming full-capacity cycling by default when the actual strategy uses shallower cycling would overstate degradation, and vice versa.
Augmentation Schedule¶
Many battery storage projects plan an augmentation or replacement schedule — adding or replacing battery capacity partway through the contracted or operating life to offset cumulative capacity fade from cycling. Where a project's plan includes augmentation, this should be modelled explicitly, with its capital cost and timing built into the cash flow, rather than omitted or assumed to be covered by ordinary operating cost.
Revenue Stacking¶
Battery storage revenue is typically built from multiple distinct streams, each modelled separately:
- Energy arbitrage — capturing the price differential between charging (low price) and discharging (high price) periods, net of round-trip efficiency losses.
- Capacity payments — compensation for available capacity, independent of dispatch, following the same distinct treatment described in Capacity Payment Models.
- Ancillary services — grid stability services such as frequency regulation, typically priced and contracted separately from energy or capacity revenue.
A single blended revenue-per-MWh assumption applied across all of a battery's activity conceals which of these streams is actually driving project economics and makes it impossible to sensitivity-test any one stream (for example, a decline in ancillary service pricing) independently of the others.
Common Construction Pitfalls¶
Time-based degradation applied to a cycling asset. Using a generic annual degradation percentage, rather than a cycling-driven schedule reflecting the actual operating strategy, misrepresents the primary driver of battery capacity fade.
Round-trip efficiency ignored. Modelling arbitrage revenue as the full price differential without netting round-trip efficiency losses overstates achievable revenue.
Blended revenue stream. Combining arbitrage, capacity, and ancillary service revenue into a single figure prevents independent sensitivity testing of each stream.
Augmentation cost omitted. Failing to model a planned augmentation or replacement schedule's capital cost understates total lifecycle cost.
Recommended Practices¶
- Build degradation from a projected cycling schedule reflecting the asset's actual operating strategy, not a generic time-based percentage.
- Model round-trip efficiency explicitly in any arbitrage revenue calculation.
- Reflect the actual depth-of-discharge profile the operating strategy uses when deriving degradation.
- Model any planned augmentation or replacement schedule's capital cost and timing explicitly.
- Build revenue as the sum of separately modelled arbitrage, capacity, and ancillary service streams.
Continue Reading¶
Related Pillars¶
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Frequently Asked Questions
What drives battery degradation, and how does it differ from generation asset degradation?
Battery degradation is driven primarily by charge/discharge cycling — the number and depth of cycles the battery undergoes — rather than simply the passage of time as with solar panel or wind turbine degradation, and should be modelled against a projected cycling schedule reflecting the actual operating strategy, not a generic annual percentage.
What is round-trip efficiency, and why does it matter?
The ratio of energy discharged from the battery to energy used to charge it, reflecting energy losses in the charge/discharge process. It directly reduces the net revenue captured from any price arbitrage strategy (buying low, selling high) and should be modelled explicitly rather than ignored, since even a modest efficiency loss compounds across a high cycling frequency.
What is depth of discharge, and how does it affect the model?
The proportion of the battery's total capacity actually used in a given cycle. Cycling to a greater depth of discharge typically accelerates degradation relative to shallower cycling, creating a trade-off between revenue captured per cycle and battery life that the model should reflect based on the project's actual operating strategy.
What is an augmentation schedule?
A plan to add or replace battery capacity partway through the asset's contracted or operating life to offset capacity fade from cycling-driven degradation, restoring usable capacity toward its original level — where a project's plan includes augmentation, its capital cost and timing should be modelled explicitly, not omitted.
What does revenue stacking mean for a battery storage model?
Building battery revenue as the sum of multiple distinct streams — energy arbitrage (price differential capture), capacity payments (compensation for available capacity), and ancillary services (grid stability services such as frequency regulation) — each modelled separately with its own price and volume basis, rather than a single blended revenue-per-MWh assumption.
References
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