How do you calculate service-parts days of supply?

Divide usable inventory by average daily demand to get raw days of supply, then divide by the criticality multiplier. With 960 usable parts, 24 parts/day demand, and a 1.25x multiplier, raw supply is 40 days and risk-adjusted protected supply is 32 days.

What is the service criticality risk multiplier?

A factor above 1.0 that shortens reported coverage for parts you cannot afford to stock out. A 1.25x multiplier turns 40 raw days into 32 protected days, building a margin against demand spikes and lead-time slip for critical charger spares.

What is a good days-of-supply target for EV charger spares?

It should comfortably exceed your replenishment lead time plus a safety margin. For long-lead power electronics with 60-90 day lead times, 32 protected days may be too thin; for fast-moving connectors with short lead times it can be plenty.

Why use usable inventory instead of total on hand?

Damaged, quarantined, in-transit, or wrong-revision parts cannot fill a real claim. Counting only usable, ship-ready spares prevents the buffer from looking healthier than it is when a stockout actually hits.

Days of supply vs safety stock — what's the difference?

Days of supply is a time horizon your current inventory covers; safety stock is the buffer quantity held against variability. This tool reports the horizon and uses the criticality multiplier to bake in a safety margin.

How do I raise protected days of supply?

Add usable inventory, reduce demand through reliability fixes, or lower the criticality multiplier by de-risking the part (second source, shorter lead time). Doubling usable inventory to 1,920 would push protected supply to 64 days.

EV Charging Infrastructure Manufacturing calculator

Service Parts Buffer Calculator

Service-parts days of supply tells an EV charging service organization how long its spares pool will last against field demand before stocking out. Field-service and after-sales planners use it to keep DC fast chargers and Level 2 units uptime-compliant under SLA, especially for critical, long-lead items like power modules, contactors, and payment terminals. Because charger downtime triggers SLA penalties and lost session revenue at public sites, this metric is risk-adjusted — a criticality multiplier discounts raw days of supply for parts that absolutely cannot stock out. It turns a raw inventory count into a forward-looking coverage horizon planners can act on.

What this calculator does

Estimate protected days of supply for EV charger service parts from usable inventory, daily demand, and service-risk multiplier.
a service parts planner needs to know whether spares cover field demand and supplier lead time risk
It computes risk-adjusted protected days of supply by dividing usable service-parts inventory by average daily demand, then dividing by a criticality multiplier that shortens the horizon for high-risk parts.

Formula used

Protected service-parts days = usable service-parts inventory ÷ average daily service demand ÷ service criticality risk multiplier

Inputs explained

Usable service-parts inventory:
Average daily service demand:
Service criticality risk multiplier:

How to use the result

Use it when sizing a spares pool against an uptime SLA, reviewing reorder timing, or stress-testing coverage for a critical, long-lead charger component.
It uses average daily demand and ignores demand variability and replenishment lead time, so a single regional failure cluster can exhaust the buffer faster than the average implies.

Current U.S. benchmarks

Global copper trades at $13,484 per tonne (IMF via FRED, May 2026), up 41.5% in a year, and U.S. industrial electricity averages 8.66 cents per kWh. Both feed electrified-hardware unit economics.

Common questions

How do you calculate service-parts days of supply? Divide usable inventory by average daily demand to get raw days of supply, then divide by the criticality multiplier. With 960 usable parts, 24 parts/day demand, and a 1.25x multiplier, raw supply is 40 days and risk-adjusted protected supply is 32 days.
What is the service criticality risk multiplier? A factor above 1.0 that shortens reported coverage for parts you cannot afford to stock out. A 1.25x multiplier turns 40 raw days into 32 protected days, building a margin against demand spikes and lead-time slip for critical charger spares.
What is a good days-of-supply target for EV charger spares? It should comfortably exceed your replenishment lead time plus a safety margin. For long-lead power electronics with 60-90 day lead times, 32 protected days may be too thin; for fast-moving connectors with short lead times it can be plenty.
Why use usable inventory instead of total on hand? Damaged, quarantined, in-transit, or wrong-revision parts cannot fill a real claim. Counting only usable, ship-ready spares prevents the buffer from looking healthier than it is when a stockout actually hits.
Days of supply vs safety stock — what's the difference? Days of supply is a time horizon your current inventory covers; safety stock is the buffer quantity held against variability. This tool reports the horizon and uses the criticality multiplier to bake in a safety margin.

Last reviewed 2026-05-12.