Methodology & Data Sources
This page documents the data sources, calculation methodology, and assumptions underlying the DieselToSolar.com calculator. The tool is a preliminary planning and screening-level decision-support tool. It is not a project-finance or engineering-grade model. All key assumptions are transparent, user-adjustable, and referenced below.
Contents
- Overview and positioning
- Solar irradiance data (NASA POWER)
- Solar generation model and limitations
- Diesel consumption model
- Cost calculations and default assumptions
- Solar-diesel hybrid displacement
- Battery storage modelling
- CO₂ methodology
- Financial metrics
- Known limitations
- Worked example
- Data sources and references
1. Overview and Positioning
DieselToSolar.com compares the total cost of ownership of a diesel generator against an equivalent solar photovoltaic system over a 25-year period. It accounts for capital costs (solar panels, inverter, optional battery storage), operating costs (fuel, maintenance), and optional grid export revenue.
The tool supports three primary use cases:
- Full diesel replacement — solar (with or without storage) replaces diesel generation entirely
- Partial offset — solar reduces daytime diesel consumption while the generator covers nights and overcast periods
- Grid supplement — solar and storage reduce reliance on both diesel and an unreliable grid
2. Solar Irradiance Data (NASA POWER)
Solar irradiance data is sourced from the NASA POWER (Prediction of Worldwide Energy Resources) dataset, operated by NASA's Langley Research Center. This dataset is freely available with no commercial restrictions and is widely used in solar energy research and project development globally.[NASA POWER Project, Langley Research Center, power.larc.nasa.gov]
Parameter used
ALLSKY_SFC_SW_DWN — All Sky Surface Shortwave Downward Irradiance (kWh/m²/day). Returns the long-term climatological annual average of total solar radiation reaching the surface under all sky conditions (clear and cloudy). Data represents a multi-decade average, typically 1984–present.
The raw irradiance value (kWh/m²/day) is converted to an annual system yield (kWh/kWp/year):
Annual raw irradiance = daily average × 365
Effective yield = annual raw irradiance × performance ratio
Example: 4.7 kWh/m²/day × 365 × 0.75 = 1,287 kWh/kWp/year
Performance ratio (default 0.75)[IEA PVPS Task 2, 2014; Photovoltaic System Performance, IEC 61724] accounts for real-world system losses including inverter efficiency, wiring losses, soiling, temperature derating, and system availability:
| System type | Typical PR |
|---|---|
| Modern ground-mount, temperate climate | 0.78–0.82 |
| Modern ground-mount, hot climate | 0.73–0.78 |
| Agricultural / dusty environment | 0.68–0.75 |
| Rooftop with partial shading | 0.65–0.75 |
| Older or poorly maintained system | 0.60–0.70 |
| Tool default | 0.75 |
3. Solar Generation Model and Limitations
The solar generation model assumes near-optimal system orientation (panels tilted toward the equator at approximately latitude angle, south-facing in the northern hemisphere). No adjustments are made for:
- Tilt angle deviations from optimal
- Azimuth or orientation (east/west-facing systems will produce less)
- Snow cover losses (relevant in northern climates — can reduce annual yield by 2–8%)
- Inverter clipping (occurs when array output exceeds inverter capacity)
- Terrain or horizon shading beyond what the performance ratio captures
- Curtailment due to grid constraints
- Site-specific design constraints
4. Diesel Consumption Model
Diesel fuel consumption is estimated using a load-factor-dependent heat rate curve based on the thermodynamic efficiency of 4-stroke diesel engines.[Caterpillar Performance Handbook, Edition 47; Cummins Generator Technologies fuel consumption data] A diesel generator does not consume fuel in linear proportion to its output — efficiency peaks at 75–80% of rated capacity and deteriorates significantly at partial loads.
At ≥75% load: heat rate = 0.270 L/kWh
At 50–75% load: heat rate increases linearly to ~0.304 L/kWh
At 25–50% load: heat rate increases to ~0.370 L/kWh
Annual consumption is derived as:
kWh/year = generator kW × load factor × hours/day × days/year
Litres/year = kWh/year × heat rate (L/kWh)
Users may override the estimated consumption with a known figure from fuel purchase records or meter readings. This is recommended where available as it eliminates load factor estimation uncertainty. Actual fuel consumption records are always more reliable than derived estimates.
5. Cost Calculations and Default Assumptions
Diesel operating costs
- Fuel cost: litres/year × diesel price/litre. Diesel price defaults are sourced from Global Petrol Prices (globalpetrolprices.com) and should be verified against current local prices.
- Maintenance — fixed component: default $2,500/year. Reflects typical agricultural generator costs in Canada including seasonal recommissioning and scheduled servicing.[Based on industry surveys of Alberta agricultural diesel generator operators; adjust for local labour rates]
- Maintenance — variable component: default $25/kW/year. Covers oil and filter changes, fuel system maintenance, load testing, and minor unplanned repairs. Typical agricultural range: $3,000–$8,000/year total for a working irrigation generator.[EGSA (Electrical Generating Systems Association) maintenance cost guidance; Caterpillar O&M cost estimating guides]
- Excluded from diesel costs: major engine overhaul ($15,000–$25,000+ at 10,000–15,000 operating hours), fuel storage infrastructure, and environmental compliance costs. These omissions make the diesel cost estimate conservative (i.e. the solar advantage may be understated).
Solar capital costs
- Panel and installation: system kW × 1,000 × install cost per watt. Country defaults sourced from IRENA Renewable Power Generation Costs reports.[IRENA, Renewable Power Generation Costs, 2023]
- Battery storage (optional): sized as generator kW × load factor × backup hours. Default cost $400/kWh installed.[BloombergNEF Battery Price Survey 2023; IRENA Electricity Storage and Renewables, 2017 updated] Prices have declined significantly and vary by chemistry, supplier, and scale. Users should obtain current quotes.
- Incentives: applied as a flat reduction to total capital cost. Users must verify current eligibility — incentive programmes change frequently and vary by jurisdiction.
- Inverter replacement (optional): modelled as a lump-sum cost at a user-specified year (default year 12, default cost $0.15/W).[Wood Mackenzie inverter cost forecasts; NREL Solar PV O&M cost benchmarks] String inverters typically require replacement at 10–15 years. Central inverters and microinverters may have different lifespans.
Solar operating costs
Solar O&M: default $15/kW/year.[NREL 2023 Solar PV O&M Cost Benchmark Report; utility-scale ground-mount median $17/kW/year, small commercial $22/kW/year] Covers panel cleaning, inverter servicing, and minor electrical maintenance. The $15/kW/year default is at the low end of benchmarks and may understate costs for remote agricultural sites with higher cleaning frequency requirements. Users operating in dusty environments should consider $20–$30/kW/year.
Panel degradation: 0.6%/year.[Jordan & Kurtz, NREL, "Photovoltaic Degradation Rates — An Analytical Review", Progress in Photovoltaics, 2013; median degradation rate for monocrystalline silicon panels] Applied annually to solar generation output, reducing both energy offset and export revenue over the 25-year period.
25-year financial model
- Panel degradation: 0.6%/year compounded, applied to solar generation from year 1.
- Inverter replacement: lump-sum added to solar path at user-specified year.
- No fuel price escalation: diesel price held constant in real terms. Conservative — real diesel prices have trended upward in most markets historically.
- No inflation: all values in today's currency (real terms analysis).
- Simple payback only in v1: NPV and IRR with user-defined discount rate will be added in v2 (see Section 9).
6. Solar-Diesel Hybrid Displacement
The calculator assumes solar generation offsets diesel generation on a kWh-for-kWh basis:
1 kWh solar generated = 1 kWh diesel generation avoided
This limitation is most significant for:
- 24/7 facilities where the diesel must remain online as spinning reserve
- Systems without battery storage that must match solar variability with diesel throttling
- Generators that are oversized relative to the base load
The calculator includes an optional Hybrid Displacement Adjustment Factor (default 100%) that allows users to reduce the assumed fuel displacement efficiency. For example, setting this to 80% means 1 kWh of solar generation is assumed to displace only 0.8 kWh of diesel generation — reflecting real-world hybrid system behaviour.
For daytime-only loads (irrigation pumps, daytime processing) where the diesel generator is fully replaced by solar during operating hours, the 100% default is appropriate.
7. Battery Storage Modelling
Battery storage modelling in v1 is simplified and intended for initial capital cost estimation only. The following assumptions apply:
Sizing
Battery capacity (kWh) = generator kW × load factor × backup hours
This sizes the battery to meet the load at the generator's operating load factor for the specified backup duration. It does not account for round-trip efficiency losses in the sizing calculation — actual required battery capacity should be increased by approximately 10–15% to account for these losses.
Round-trip efficiency
Lithium-ion battery round-trip efficiency (charge to discharge) is typically 90–95%.[NREL, "Grid-Scale Battery Storage: Frequently Asked Questions", 2019; Tesla Powerwall and commercial LFP battery specifications] The v1 calculator applies a default round-trip efficiency of 92%, meaning that for every 1 kWh stored, approximately 0.92 kWh is available for use. This reduces the effective energy available from the battery and is factored into the energy balance calculation.
Battery degradation and replacement
Lithium-ion battery capacity typically degrades 20–30% over 10 years at standard cycling rates (1 cycle/day).[NREL, Battery Lifetime Analysis and Simulation Tool (BLAST); Schimpe et al., 2018] The v1 model does not simulate year-by-year battery degradation. Instead, it offers an optional battery replacement input (year and cost) that users can apply to account for end-of-life replacement. This is a simplification — detailed battery lifecycle modelling will be added in v2.
8. CO₂ Methodology
| Source | Factor | Reference |
|---|---|---|
| Diesel combustion (Scope 1) | 2.68 kg CO₂/litre | IPCC 2006 National GHG Inventories, Vol. 2, Table 2.2 |
| Solar PV lifecycle | 0.040 kg CO₂/kWh | IPCC AR5 WG3, Annex III, Table A.III.2 (2014), median utility-scale ground-mount |
Diesel: The 2.68 kg CO₂/litre factor covers direct combustion emissions (Scope 1) only. Upstream emissions from crude oil extraction, refining, and transport add approximately 0.5–0.7 kg CO₂/litre (Scope 3), making this a conservative estimate of diesel's true carbon intensity.
Solar PV lifecycle: The 0.040 kg CO₂/kWh factor covers full lifecycle emissions including manufacturing, balance of system, installation, operation, and end-of-life disposal. The IPCC AR5 median for utility-scale solar ranges from 0.018–0.048 kg CO₂/kWh depending on panel type and manufacturing energy source. Panels manufactured using coal-intensive electricity will be at the higher end of this range.
25-year CO₂: Annual solar generation is adjusted for 0.6%/year panel degradation. Diesel emissions are held constant year-on-year (conservative — no increase in consumption assumed).
9. Financial Metrics
The v1 calculator provides the following financial outputs:
- Simple payback period: net solar capital cost ÷ annual cost saving. Does not account for the time value of money. Appropriate for initial screening.
- Annual cost saving: diesel annual operating cost minus solar annual operating cost, plus any export revenue.
- 25-year cumulative cost saving: sum of annual savings over 25 years, less inverter replacement cost if applicable. Expressed in today's currency (real terms, no inflation).
10. Known Limitations
Solar generation model assumes optimal orientation
The model assumes near-optimal panel tilt and azimuth. East/west-facing systems, flat roofs, and suboptimal mounting will produce less than modelled. Snow cover losses are not modelled and can reduce annual yield by 2–8% in northern climates.
kWh-for-kWh diesel displacement may overstate savings
In hybrid systems where the diesel generator remains online at partial load, actual fuel displacement is lower than the kWh-for-kWh model assumes. Use the Hybrid Displacement Adjustment Factor to reduce this effect where applicable.
Battery cycling and degradation simplified
Battery round-trip efficiency (92% default) is applied but year-by-year capacity degradation is not modelled. Actual battery capacity declines 20–30% over 10 years at standard cycling rates. An optional battery replacement input is provided.
Export revenue is approximate
Export revenue estimates are indicative only and do not represent hourly dispatch modelling. Actual export revenue depends on load timing, utility tariff structure, battery operation, curtailment, and interconnection constraints. All export revenue outputs are labelled as approximate.
No NPV or IRR in v1
Financial outputs use undiscounted cash flows. For commercial investment decisions, NPV and IRR (with appropriate discount rate) are more appropriate than simple payback. These will be added in v2.
Grid connection and permitting costs excluded
Interconnection applications, metering upgrades, and permitting can cost $2,000–$25,000 depending on jurisdiction and system size.
Tax treatment not modelled
Capital cost allowance (Canada), depreciation, GST/HST recovery, and operating expense deductibility are not modelled. For commercial operations these can materially improve after-tax payback.
No fuel price escalation
Diesel price held constant in real terms. Real diesel prices have trended upward historically — this is a conservative assumption that understates solar's long-term advantage.
Diesel generator overhaul not included
Major overhaul costs ($15,000–$25,000+ at 10,000–15,000 operating hours) are excluded from diesel maintenance. High-hours operations will find diesel lifetime costs understated.
Interannual solar variability not modelled
NASA POWER returns long-term average irradiance. Year-to-year generation can vary ±10–15%. P90 generation estimates (exceeded 90% of years) are not produced by this tool.
11. Worked Example
The following worked example illustrates the calculator's methodology using a representative irrigation pump scenario in Alberta, Canada. All intermediate values are shown to support auditing of assumptions.
Inputs
Step-by-step calculation
1. Diesel heat rate
Load factor 75% → heat rate = 0.270 L/kWh
2. Annual energy output
100 kW × 75% × 12 hrs/day × 214 days = 192,600 kWh/year
3. Annual diesel consumption
192,600 kWh × 0.270 L/kWh = 52,002 L/year
4. Annual fuel cost
52,002 L × $1.70/L = $88,403/year
5. Annual maintenance cost
$2,500 fixed + (100 kW × $25/kW) = $5,000/year
6. Total annual diesel operating cost
$88,403 + $5,000 = $93,403/year
7. NASA POWER irradiance (Lethbridge)
ALLSKY_SFC_SW_DWN annual average ≈ 4.72 kWh/m²/day → 4.72 × 365 = 1,723 kWh/kWp/year raw
8. Effective solar yield
1,723 × 0.75 (PR) = 1,292 kWh/kWp/year
9. Suggested solar system size
192,600 kWh ÷ 1,292 kWh/kWp = 149 kW (user selected 100 kW — partial offset scenario)
10. Solar capital cost
100 kW × 1,000 × $2.00/W = $200,000
11. Annual solar operating cost
100 kW × $15/kW = $1,500/year
12. Annual cost saving
$93,403 − $1,500 = $91,903/year
13. Simple payback
$200,000 ÷ $91,903 = 2.2 years
14. Inverter replacement at year 12
100 kW × 1,000 × $0.15/W = $15,000
15. 25-year cumulative saving
Diesel 25yr: $93,403 × 25 = $2,335,075 Solar 25yr: $200,000 + ($1,500 × 25) + $15,000 = $252,500 Net saving: $2,335,075 − $252,500 = $2,082,575 (before panel degradation adjustment)
16. Annual CO₂ avoided
Diesel: 52,002 L × 2.68 kg/L = 139,365 kg = 139.4 t CO₂/year Solar lifecycle: 100 kW × 1,292 kWh/kWp × 0.040 kg/kWh = 5,168 kg Net avoided: 139,365 − 5,168 = 134,197 kg = 134.2 t CO₂/year
12. Data Sources and References
| Source | Used for | Licence |
|---|---|---|
| NASA POWER | Solar irradiance by location (ALLSKY_SFC_SW_DWN) | Public domain, unrestricted |
| IPCC 2006 National GHG Inventories Vol. 2 | Diesel CO₂ emission factor (2.68 kg CO₂/litre) | Public |
| IPCC AR5 WG3 Annex III (2014) | Solar PV lifecycle CO₂ (0.040 kg CO₂/kWh) | Public |
| NREL PVWatts Calculator | Performance ratio reference; solar yield benchmarking | Public domain |
| NREL Solar PV O&M Cost Benchmark (2023) | Solar O&M cost default ($15/kW/year) | Public |
| Jordan & Kurtz, NREL (2013) | Panel degradation rate (0.6%/year median) | Academic |
| IRENA Renewable Power Generation Costs (2023) | Country-level solar install cost defaults | Public |
| BloombergNEF Battery Price Survey (2023) | Battery storage cost default ($400/kWh) | Commercial — figures cited only |
| Global Petrol Prices | Country-level diesel price defaults | Indicative — verify locally |
| Caterpillar Performance Handbook (Ed. 47) | Diesel generator heat rate curve by load factor | Commercial reference |
| IEA PVPS Task 2 / IEC 61724 | Performance ratio definition and typical values | Public |
| Alberta Utilities Commission (AUC) | Alberta electricity rates and micro-generation regulation | Public |