Critical spares are the parts that stop a generator when they fail and cannot be sourced quickly. Most sites either hold too much dead stock or nothing that matters. A simple criticality ranking and a min-max rule fix both problems.
Quick Answer
Rank every part by downtime impact, lead time and failure likelihood.
Keep at least one set of filters, belts and sensors for each generator model.
Set a minimum level that covers the supplier lead time plus a safety margin.
Review the list every 6 months and remove parts that never move.
Holding costs typically run 20 to 25 percent of stock value per year.
What is critical spares?
Critical spares are the components whose failure stops a generator and whose replacement cannot be obtained within the acceptable downtime window.
It means the small, selected stock that keeps the plant running while normal procurement catches up.
critical spares in practice
A practical three-tier stocking rule:
| Class | Examples | Stock rule |
|---|---|---|
| A - critical | Fuel shut-off solenoid, sensors, fuses | One spare on site at all times |
| A - critical | Filters, belts, hoses | One full service set per set |
| B - important | Starter motor, alternator, water pump | One shared spare for the fleet |
| B - important | Injectors, fuel lift pump | Order on failure, keep supplier contact |
| C - consumable | Oil, coolant, sealant, bulbs | Min-max based on monthly use |
| C - slow mover | Turbocharger, control board | No stock, service exchange only |
What does a critical spares programme cost in Nigeria?
A basic filter and belt kit for a 100 kVA set costs about 80,000 to 150,000 naira.
A starter motor or alternator spare runs about 150,000 to 450,000 naira.
Holding costs are roughly 20 to 25 percent of stock value each year.
A single day of unplanned downtime can cost a factory 500,000 naira or more.
A computerised inventory system costs 150,000 to 600,000 naira to set up.
Annual stock review and reordering take about 2 working days.
How is critical spares carried out?
- List every part that has failed or been replaced in the last 3 years.
- Score each part for downtime impact, lead time and failure rate.
- Assign each part to class A, B or C and set a stock rule.
- Calculate min-max levels from usage and supplier lead time.
- Store parts in a clean, labelled store and record every issue.
- Review the list every 6 months and adjust for actual consumption.
Sources
Manufacturer recommended spare parts lists, ISO 55000 asset management principles, CMMS inventory planning practice, Supplier lead time records
Why generic spare lists fail
A spare parts list copied from a manual assumes unlimited storage and a fast supply chain. Nigerian sites usually have neither.
The right list is built from the site's own failure history. A part that fails once in 5 years does not deserve shelf space.
Start with downtime cost. A cheap sensor that stops a 500 kVA set is more critical than an expensive part that can wait.
Ranking parts by criticality
Score each part from 1 to 5 for production impact, lead time and failure likelihood. Multiply the scores to get a simple priority.
Class A parts stop the set and take more than 72 hours to source. These must be on site.
Class C parts are consumables or slow movers. Manage them by usage, not by fear.
Setting min-max levels
The minimum level covers demand during the supplier lead time plus a safety margin. The maximum is the minimum plus the economic order quantity.
For a filter used four times a year with a 30 day lead time, a minimum of two and a maximum of six is sensible.
Record actual consumption monthly. A min-max level that never changes is usually wrong within a year.
Long lead times and local sourcing
Some control boards and turbochargers take 8 to 12 weeks to arrive. For these, keep a service exchange unit or a written supplier commitment.
Build relationships with two suppliers for the same part. A single source is a single point of failure.
Check that parts are genuine. Counterfeit filters and sensors cause failures that look like maintenance problems.
Storage and preservation
Keep the store dry, ventilated and free of dust. Damp air ruins electronics and corrodes bearings.
Rotate stock so older parts are used first. Rubber parts such as belts and hoses have a shelf life even when unused.
Label every bin with the part number, generator model and minimum level. A store that cannot be read is a store that gets duplicated.
Review and dead stock
Review the list every 6 months against actual issues. Parts that have not moved in 3 years are candidates for disposal or return.
Dead stock ties up cash and hides the parts that matter. Write it off rather than letting it fill the shelves.
Feed every breakdown back into the ranking. The list should get sharper each year, not longer.
Related guides from this series
- Generator Maintenance Contract in Nigeria
- Generator Hour Meter and Service Tracking in Nigeria
- Generator OEM vs Aftermarket Spares in Nigeria
About the author
Illuminate Engineering Limited maintains generator fleets and supports spare parts planning for industrial clients across Nigeria.
Frequently Asked Questions
Which generator spares should always be on site?
Filters, belts, hoses, fuses, sensors and the fuel shut-off solenoid. These are cheap, fail without warning and stop the set.
How much should a site spend on critical spares?
A common guide is 2 to 5 percent of the generator asset value per year. The exact figure depends on age, duty and downtime cost.
Should I stock a spare turbocharger?
Usually no. It is expensive, slow moving and available through service exchange. Keep a supplier agreement instead.
How often should the spares list be reviewed?
Every 6 months, and after every major breakdown. Usage data should drive the changes.
Conclusion
Critical spares planning is risk management, not shopping. Rank the parts, stock the few that stop the plant and let the rest come through the supply chain.
A short, accurate list beats a full warehouse. Review it twice a year and the generator stays online.