Factory Solar and Diesel Hybrid Payback Calculator for Unreliable Grids
Where grid supply is unreliable, a generator is often the real price setter for power. This calculator estimates what adding solar PV to a diesel-backed factory saves, what it costs, and how long it takes to pay back on a simple basis.
Your numbers
Without batteries, solar usually cannot replace more than a fraction of generator output because generators need a minimum load.
Typical large sets run near 0.25 to 0.35 L/kWh depending on load.
Depends on site irradiance, tilt and losses.
Your results
Simple payback ignores financing, degradation, tax allowances and battery costs. Use it for screening, then ask an installer for a full yield study.
Simple payback as diesel price changes
Sweeps Delivered diesel price from 0.6 to 1.8, holding your other inputs fixed.
Energy cost reporting from your ERP
Optional: we show how to combine fuel, meter and production data into one monthly energy cost view.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the energy your generators supply each year, the diesel price you actually pay delivered to site, and the fuel use per kWh. Set the share of that energy you want solar to replace, then enter yield and installed cost per kWp. The tool sizes the array, estimates diesel avoided and net saving after O&M, and returns simple payback in years.
- Use metered generator kWh, not nameplate capacity.
- Use delivered fuel price including transport and losses.
- Keep the solar share realistic for a no-battery design.
The method
Solar energy used equals generator energy times the replaced share. Capacity equals that energy divided by annual yield per kWp. Diesel avoided equals solar kWh times litres per kWh times price. Payback is capital cost divided by net annual saving. It is deliberately simple so a finance team can reproduce it in a spreadsheet and then compare it with an installer proposal.
Reading the result
Short paybacks are typical where diesel is the marginal source, but the result depends heavily on fuel price, which the chart sweeps for you. Check that the array fits roof area and structure, and that your generator controller can run in parallel with PV without reverse power or minimum load trips. If you also buy grid power, the saving per kWh is lower than diesel, so model that mix separately.
- Treat battery storage as a separate costed option.
- Add inverter replacement to long-term cost views.
- Check insurance and landlord consent for roof systems.
Common mistakes
Overestimating the solar share, using the grid tariff instead of diesel cost, ignoring annual degradation, and assuming perfect yield are the usual errors. Another is forgetting that production patterns matter: if most load runs at night, daytime PV saves less. Match the sizing to your actual load profile from a meter log.
Frequently Asked Questions
How much diesel does a generator use per kWh?
Many large diesel sets use roughly 0.25 to 0.35 litres per kWh at reasonable load, and more at light load. Use your own fuel and meter records for the best estimate.
Can solar run a factory without batteries?
Partly. PV can reduce generator fuel during the day, but generators need a minimum load and the system needs controls to avoid reverse power. Full replacement normally needs storage.
What is simple payback?
It is capital cost divided by net annual saving. It ignores financing, tax and degradation, so it is a screening measure rather than a full investment appraisal.
Is this the same as the load shedding generator cost calculator?
No. That tool prices the cost of running generators during outages. This one estimates the benefit of adding solar to offset generator fuel.
If you track generator hours, fuel and downtime in an ERP or spreadsheet, Netray can pull those figures into a monthly energy cost report with the query shown.
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