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Start with a like-for-like comparison
A steam project can have several legitimate output numbers at once: turbine gross generation, net plant generation after parasitic loads, energy exported at the revenue meter, and a theoretical maximum based on rated capacity. A comparison is meaningful only when the actual and forecast numbers share the same boundary, period, plant configuration, and operating assumptions.
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For the headline comparison, use revenue-grade net energy exported to the grid when that meter is available. If export data are unavailable, use net plant generation and state that choice. Do not compare gross turbine MWh with a net forecast, or an export forecast with gross generator readings.
Report these three metrics for every month and for the cumulative year-to-date period:
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- Absolute variance: actual net MWh minus base-case net MWh.
- Percentage variance: (actual net MWh minus base-case net MWh) divided by base-case net MWh, multiplied by 100.
- Actual-to-forecast ratio: actual net MWh divided by base-case net MWh.
A negative variance means the project delivered less than the base case; a positive variance means it delivered more. Keep the sign visible rather than reporting only an absolute difference.
Freeze the base-case forecast before touching the meter data
Forecasts change as engineering assumptions, well counts, equipment selections, and operating dates change. Select the version that was approved for the comparison and make it immutable for the analysis. Record the following items in the calculation workbook or data register:
- Forecast document name, revision, and revision date.
- Geography and the project’s planned operating start date.
- Whether the forecast is expressed as gross generation, net generation, or grid exports.
- Assumptions for auxiliary consumption, planned outages, commissioning, degradation, and resource decline.
- Whether the period is a calendar year, operating year, month, or lifetime average.
- The resource and uncertainty case, such as P50, P90, or another stated case.
- Any assumed dispatch, curtailment, or interconnection limit.
Do not silently replace the approved base case with a later reforecast. If a revised forecast is useful, show it as a separate scenario with its own revision date and assumptions.
Define the actual-output boundary
Steam projects often report several meters and counters. Create separate fields for each one, then select one boundary for the headline comparison.
| Field | What it represents | How to use it |
|---|---|---|
| Gross turbine MWh | Electrical energy produced at the generator before plant parasitic loads. | Use for conversion-efficiency analysis, not as a substitute for net output. |
| Auxiliary MWh | Electricity consumed by pumps, fans, cooling systems, controls, and other plant loads. | Report separately so a net shortfall is not wrongly attributed to the resource. |
| Net plant MWh | Gross generation less auxiliary consumption, normally at the plant boundary. | Use when export-meter data are unavailable and the forecast uses the same boundary. |
| Curtailed MWh | Energy the plant could have produced but did not export because of grid or dispatch restrictions. | Keep separate from technical underperformance. |
| Exported MWh | Energy recorded by the revenue-grade meter delivered to the grid. | Preferred headline actual when the forecast is an export forecast. |
Document meter IDs, calibration status, time zone, interval length, treatment of missing intervals, and whether readings are grossed up or adjusted. A revenue meter and a generator meter can legitimately disagree because they sit on different sides of transformers, station service, or other losses.
Align the period and plant configuration
Use identical calendar or operating windows. A forecast for a full operating year cannot be compared directly with a partial commissioning year. If commercial operation began partway through a month, either compare the same partial window in the forecast or show the commissioning period as a separate line.
Check for configuration changes before aggregating data:
- Changed well count, wellhead equipment, or gathering-system configuration.
- Turbine derating, replacement, or a balance-of-plant modification.
- Commissioning restrictions, staged ramp-up, or temporary test operation.
- Planned maintenance that moved between months.
- Resource-management changes, reinjection changes, or operating-pressure changes.
NREL’s System Advisor Model (SAM) geothermal model calculates monthly changes in resource and electrical output and produces annual and lifetime results. That monthly structure is useful when aligning a forecast to measured operating windows, but the project’s own approved forecast remains the controlling reference for the comparison.
Calculate the three headline metrics
Absolute MWh variance
Variance MWh = Actual net MWh − Base-case net MWh
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Use the same sign convention in every report. For example, a result of −2,000 MWh means the project delivered 2,000 MWh less than the base case.
Percentage variance
Variance % = (Actual net MWh − Base-case net MWh) ÷ Base-case net MWh × 100
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State the denominator explicitly. A percentage based on gross forecast energy is not comparable with one based on net exports. If the base-case value is zero, percentage variance is undefined; report the MWh difference and explain why the denominator is zero instead of forcing a percentage.
Actual-to-forecast ratio
Actual-to-forecast ratio = Actual net MWh ÷ Base-case net MWh
A ratio of 1.00 means the actual equals the base case, 0.90 means actual output is 90% of forecast, and 1.10 means actual output is 110% of forecast. Keep the ratio unrounded in the calculation and round only for display.
Report monthly and cumulative results
Monthly figures show seasonality, outages, and commissioning effects. The cumulative year-to-date figure shows whether a shortfall is being recovered or becoming persistent. Use a consistent precision, such as whole MWh and one decimal percentage point, and preserve the unrounded values in the source workbook.
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| Reporting field | Required content |
|---|---|
| Period | Calendar month, operating month, or cumulative year-to-date window. |
| Actual net MWh | Metered value at the selected boundary. |
| Base-case net MWh | Value from the frozen forecast revision. |
| Variance MWh | Actual minus base case, retaining the sign. |
| Variance % | Signed percentage using the base case as denominator. |
| Actual/base ratio | Actual divided by base case. |
| Main reconciled cause | Largest measured contributor, such as an outage, curtailment, or resource shortfall. |
Normalize performance without hiding outages
Capacity factor is useful for context, but it is not a replacement for the variance calculation. NREL defines capacity factor as annual average energy production divided by the energy that would be available if the plant operated at rated capacity every hour of the year. NREL identifies temperature, downtime, internal losses, technology, and dispatch as influences on actual capacity factor.
Calculate capacity factor using the same capacity definition used by the forecast:
Capacity factor = Net MWh ÷ (Rated MW × hours in the comparison period)
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Also calculate MWh per MW of available capacity when capacity changed during the period. If the plant did not operate for a full year, compare the identical operating window and show a clearly labeled full-year-equivalent metric only as a secondary view. Do not annualize away an outage or treat a commissioning gap as if the plant had been available.
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Track planned-outage hours, forced-outage hours, and available hours separately. A lower capacity factor caused by a documented turbine outage is operational information, not automatically evidence that the resource forecast was wrong.
Reconcile the output gap before calling it forecast error
The residual after measured operational causes have been removed is the part that can reasonably be attributed to forecast error. Work through the causes in a fixed order so the same MWh is not counted twice.
1. Resource and steam supply
Check reservoir temperature and pressure, well flow, steam quality, wellfield decline, scaling, reinjection behavior, and the number of producing wells. A steam shortfall can reduce turbine input even when the turbine itself is healthy. Compare measured resource conditions with the assumptions frozen in the base case, and identify whether the difference is a one-time condition or a trend across representative months.
2. Turbine and conversion performance
Compare turbine efficiency, inlet conditions, condenser performance, cooling temperature, and balance-of-plant losses with design assumptions. A plant can receive the forecast steam flow and still produce less net electricity if conversion efficiency or condenser vacuum has deteriorated.
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Separate pumps, fans, cooling equipment, and other parasitic loads from gross generation. Higher auxiliary use reduces net output even when gross turbine MWh matches the forecast. Record both the absolute auxiliary MWh and its share of gross generation.
4. Planned and forced outages
Use maintenance logs, trip records, startup delays, and commissioning reports to assign lost MWh to planned maintenance or forced outages. A one-month outage should not be spread across the rest of the year unless the forecast itself uses an averaged availability assumption and the treatment is documented.
5. Grid curtailment and dispatch
Identify periods when the plant was technically available but could not export because of interconnection limits, transmission constraints, market dispatch, or an operator instruction. Keep curtailed MWh outside the technical-performance loss unless the forecast explicitly treats curtailment as net production lost.
6. Forecast conventions
Recheck gross-versus-net treatment, auxiliary-load assumptions, assumed operating hours, degradation, calendar-versus-operating-year basis, and the treatment of startup energy. Many apparent performance gaps disappear when the two models are placed on the same convention.
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Ormat’s 2024 Form 10-K states that a plant’s actual generation can differ from its generating capacity because of ambient temperature, geothermal-resource availability, and operational issues. Those categories are a practical starting point for the reconciliation, but they do not replace project-specific meter and event records.
Keep gross and net ratings distinct
Rated capacity and annual energy are not interchangeable. A California Energy Commission assessment for Morton Bay illustrates the distinction with approximately 157 MW gross capacity, 140 MW expected net capacity, and 1,226,400 MWh of maximum annual electrical production. The maximum annual figure equals 140 MW multiplied by 8,760 hours, which demonstrates that it is a net, full-availability ceiling under that stated convention—not evidence that the plant will deliver that amount every year.
Use the forecast’s exact boundary in every formula. If the actual record is gross turbine MWh while the base case is expected net MWh, first convert or restate one side using documented auxiliary and boundary assumptions. Never hide the conversion inside a percentage variance.
Understand what “base case” and P-cases mean
A base case is a modeled central scenario, not a guarantee. The UK government’s 2025 geothermal assessment presents base-case median inputs alongside P90, P50, and P10 output distributions. World Bank feasibility guidance lists P10, P50, P90, P95, and P99 as common cases and defines them by exceedance probability.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchState the convention in the table because the labels are easy to misread. Under the World Bank’s example, a P10 capacity of 50 MW means there is a 90% probability that capacity will be above 50 MW. In that convention, a lower-numbered P case represents a higher outcome threshold and a higher exceedance confidence is associated with the higher-numbered case.
If the original forecast supplies only one base-case number, report that limitation. Do not invent a P50, P90, or confidence interval from a single deterministic value.
| Scenario field | What to show |
|---|---|
| Base case | Forecast value, revision date, and stated central-scenario convention. |
| P50 | Forecast value and the document’s definition of the 50% exceedance case. |
| P90 | Forecast value and the document’s definition of the 90% exceedance case. |
| P10 | Forecast value and the document’s definition of the 10% exceedance case. |
| Actual | Metered net MWh at the stated boundary and period. |
Use external benchmarks carefully
There is no universal published geothermal percentage for actual output versus base-case forecast that can be applied to every steam project. A 2022 Australian Renewable Energy Agency study found that P50 forecasts averaged approximately 12% above actual dispatched energy across the analyzed portfolio for 2019–2020; individual projects ranged from nearly 24% overprediction to 9% underprediction. That study concerns a solar portfolio, so it is an example of variance-reporting practice, not a geothermal benchmark.
NREL’s Annual Technology Baseline uses representative assumptions and notes that its hydrothermal base-year costs draw on actual geothermal plant data. Those assumptions are useful for technology context, but they are not a substitute for the project’s own metered record, outage log, and approved forecast workbook.
Build a reproducible comparison workflow
- Lock the forecast. Save the exact report or workbook revision and record its date, geography, operating start date, net/gross convention, resource assumptions, availability, degradation, auxiliary load, and uncertainty case.
- Validate meters. Confirm the selected net or export meter, calibration status, time zone, interval completeness, and treatment of estimated or missing readings.
- Inventory parallel data. Load gross turbine MWh, auxiliary MWh, curtailment MWh, planned-outage hours, forced-outage hours, dispatch instructions, and availability alongside net MWh.
- Align the window. Match calendar or operating periods and flag commissioning, partial months, well-count changes, deratings, and plant modifications.
- Calculate monthly and year-to-date metrics. Produce actual MWh, base-case MWh, signed MWh variance, signed percentage variance, and the actual-to-forecast ratio.
- Add normalized views. Calculate capacity factor and MWh per MW of available capacity while keeping outage hours visible.
- Reconcile the gap. Assign measured MWh to resource, conversion, auxiliary load, planned maintenance, forced outages, curtailment, dispatch, and other documented causes.
- Compare uncertainty cases. Where P50 or P90 values exist, show actual against each case and print the forecast’s exceedance convention in the table footnote.
- Assess persistence. Separate one-off events from recurring bias across representative months or operating windows.
- Archive the evidence. Preserve meter exports, event logs, calculation formulas, assumptions, and sign-off so another analyst can reproduce the result.
Interpret common shortfall patterns
One bad month followed by recovery
A single forced outage, grid restriction, or unusual ambient-temperature period can explain a monthly shortfall. If subsequent representative months return to the forecast range, classify the event as a discrete operational cause rather than revising the resource model immediately.
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Persistent shortfall with normal availability
If availability is near plan but net MWh remains below forecast, examine resource temperature, pressure, flow, steam quality, turbine efficiency, condenser performance, and auxiliary load. Repeated divergence points to an assumption or degradation issue that deserves a formal forecast update.
Gross output near forecast but net output low
Focus on parasitic consumption, cooling performance, pumps, fans, and electrical losses. The resource and turbine may be performing as expected while station service consumes more energy than the base case allowed.
Plant available but exports low
Check curtailment notices, interconnection limits, dispatch instructions, and settlement meters. Do not label curtailed energy a steam-resource failure when the plant was capable of producing it.
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Verify that the meter boundary, operating window, and configuration match before treating the result as genuine outperformance. A gross-versus-net mismatch or a forecast that excludes a later capacity addition can create an apparent surplus.
Frequent comparison mistakes and their fixes
- Mixing gross and net energy: Restate both values at one documented boundary.
- Comparing a partial year with a full-year forecast: Use the same operating window and show commissioning separately.
- Annualizing outages away: Report actual outage hours and lost MWh instead of smoothing them out.
- Counting curtailment as equipment failure: Keep grid-limited MWh in a separate field.
- Using capacity factor as the only test: Pair it with signed MWh and percentage variance.
- Changing the forecast mid-analysis: Freeze one revision and identify later revisions as separate scenarios.
- Misreading P labels: Print the exceedance definition supplied by the forecast or feasibility document.
- Calling every residual “forecast error”: Remove measured resource, conversion, outage, auxiliary, dispatch, and convention effects first.
- Relying on a generic benchmark: Treat published statistics as context only; geothermal projects require their own metered and operational evidence.
Minimum evidence for a defensible conclusion
A project-specific conclusion requires four core inputs:
- The frozen base-case report or workbook, including its revision and assumptions.
- Meter-quality actual MWh at a defined net or export boundary.
- Outage, maintenance, curtailment, dispatch, and availability logs.
- The forecast’s stated uncertainty cases and exceedance convention, if supplied.
Without those records, you can describe a measured variance, but you cannot reliably assign its cause or claim that the base case is biased.
Decision framework for updating the forecast
Keep the base case unchanged when the variance is explained by a documented one-off event, a temporary commissioning restriction, or grid curtailment outside the plant’s control. Open a forecast-review action when a representative set of months shows recurring resource, availability, auxiliary-load, or conversion variance after those events are removed.
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What a clear result should say
A useful conclusion names the boundary, period, forecast revision, headline variance, and reconciled cause in one sentence. It then distinguishes measured operational losses from the remaining unexplained residual and states whether the pattern is isolated or persistent. That format gives owners, lenders, operators, and engineers a common basis for deciding whether to fix equipment, investigate the resource, revise dispatch assumptions, or update the forecast.
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