Calculator Inputs
Use the form to estimate how much generator fuel and cost a solar generator can offset on a construction site.
Example Data Table
This example shows a typical jobsite scenario and the corresponding outputs.
| Scenario | Demand (kWh/day) | PV (kW) | Sun (h/day) | Derate | Fuel type | kWh/unit | Days/year | Offset | Fuel saved |
|---|---|---|---|---|---|---|---|---|---|
| Small site office + tools | 30 | 5 | 4.5 | 85% | Diesel | 3.2 | 250 | ~64% | ~1,500 units/year* |
| Night work lighting | 18 | 4 | 5.0 | 80% | Gasoline | 2.7 | 220 | ~71% | ~1,040 units/year* |
| Remote pump + comms | 12 | 3 | 5.5 | 88% | Propane | 2.2 | 300 | ~100% | ~1,636 units/year* |
Formula Used
Solar_year = Solar_day × Days
Displaced = min(Demand_year, Solar_year)
Fuel_with = (Demand_year − Displaced) ÷ kWh_per_unit
Fuel_saved = Fuel_without − Fuel_with
CO2_avoided = Fuel_saved × kgCO2_per_unit
Annual_cash_savings = Fuel_cost_saved + (Maint_gen − Maint_solar)
Payback_years = Net_capex ÷ Annual_cash_savings
Annualized_capex = Net_capex ÷ Life_years
How to Use This Calculator
- Enter demand: Estimate daily energy in kWh for tools and facilities.
- Select solar method: Use PV estimate or enter measured kWh/day.
- Set fuel details: Choose fuel type, unit, cost, and generator efficiency.
- Add financial inputs: Include purchase cost, incentives, and maintenance.
- Submit: Review results above the form and refine assumptions.
- Export: Use CSV or PDF downloads for procurement and reporting.
Energy Baseline for Construction Loads
Start with a realistic daily kWh total for tools, lighting, charging, cabins, and temporary pumps. Log run times and nameplate watts, then apply a utilization factor to avoid overestimating. Convert to kWh/day and multiply by operating days to get annual demand. This baseline drives the displaced energy, fuel savings, and the offset percentage reported by the calculator. For mixed shifts, average weekday and weekend profiles separately.
Solar Output and Derate Assumptions
The PV method estimates daily solar energy using array kW, peak sun hours, and a derate factor. Derate captures inverter losses, temperature, wiring, dust, and battery round-trip efficiency when applicable. Use conservative values during monsoon or winter schedules. If you already have metered production, switch to direct kWh/day to reflect actual field performance. Document the derate you choose so audits are straightforward later.
Fuel Displacement and Generator Efficiency
Fuel avoided depends on the generator's delivered kWh per fuel unit. Well-maintained units at steady load often perform better than lightly loaded sets cycling on and off. Enter site-specific efficiency from fuel logs when possible. The calculator translates displaced kWh into fuel saved and then multiplies by your unit cost to estimate annual savings. Avoid assuming full rated output unless load banks confirm it.
Emissions Accounting for Reporting
To support ESG or permit reporting, the calculator converts fuel saved into avoided CO2 using fuel-specific factors per liter or per gallon. Select Custom to align with your corporate inventory factors. Results are shown in kilograms per year, which can be converted to metric tons by dividing by 1,000. Pair the CO2 figure with offset percent to communicate both environmental and operational impact. When reporting, note that upstream fuel emissions are excluded.
Financial Indicators and Decision Support
Net capital cost equals purchase cost minus incentives, while annual cash savings combine fuel savings and the maintenance difference. Simple payback divides net capex by annual cash savings and returns N/A when savings are insufficient. The 5-year NPV discounts savings at your rate to compare options. Use CSV or PDF exports to document assumptions, approvals, and change-order discussions. For procurement reviews.
What does “offset” mean in this report?
Offset is the share of annual site demand supplied by solar energy. It equals displaced kWh divided by annual demand, capped by demand. Values near 100% indicate solar can cover most daytime loads for the selected operating days.
How do I estimate generator kWh per fuel unit?
Use fuel logs and a kWh meter over several typical shifts. Divide delivered kWh by liters or gallons consumed. Avoid short tests at idle or low load, because efficiency improves when the generator operates closer to its optimal loading range.
Why should I use a derate factor for solar?
Derate accounts for real-world losses such as inverter conversion, wiring, temperature, dust, and battery cycling. Using a conservative derate reduces the risk of overstating savings and helps align the estimate with measured site performance.
What if the calculator shows more than 100% offset?
That means estimated solar production exceeds the demand you entered. Increase the demand inputs, reduce PV size or sun hours, or switch to direct kWh/day using measured output. Oversizing can still be valid if you plan to add new loads later.
Does this include battery limitations or generator runtime controls?
It assumes solar energy can be used to displace an equal amount of demand over the operating period. For battery-backed systems, validate usable storage, inverter limits, and dispatch settings. Adjust demand or direct kWh/day to reflect curtailment or downtime.
How is the 5-year NPV calculated?
The calculator discounts the same annual cash savings for five years using your discount rate, then subtracts net capex. It is a screening metric, not a full lifecycle model. For longer projects, evaluate additional years and replacement costs separately.