In short. Decide how many hours the site must run without a delivery. Hold that much diesel plus a margin, and no more than the set will burn within about six months. Rotate stock first-in, first-out, send samples to a laboratory on a fixed calendar, and design the tank so water and sediment can be seen and removed. Most fuel trouble on standby sets is not bad fuel; it is too much fuel kept too long in a tank nobody can inspect.
Who this page is for: facility managers, maintenance engineers and consultants who set fuel stock levels, write fuel procedures or specify tanks for standby and prime-power sets, from a 62.5 kVA set with a base tank to a multi-set plant with bulk storage and day tanks.
Key facts
- Stock level is a planning decision with two limits: the autonomy the site needs (hours of running without a delivery) and the turnover that keeps fuel fresh (stock used within about six months).
- Typical fuel use for planning: about 0.26–0.33 litres per kWh between 50% and 100% load, lowest near full load and rising as load falls. Confirm against the maker’s fuel curve.
- Fuel suction should sit about 50–75 mm above the tank floor; the drain must be at the true lowest point; bulk tank floors should fall about 1:50 towards the drain.
- A day tank typically holds 2 to 8 hours of full-load running and is refilled from bulk storage. Its small volume turns over fast; the bulk tank is where ageing happens.
- A laboratory panel at six-monthly intervals, plus a sample after any doubtful delivery, costs little compared with one failed start during an outage.
Why stock planning matters more than fuel type
Diesel ages in storage through water ingress, microbial growth and oxidation, and the practical storage life in Indian conditions is usually six to twelve months. Our fuel cleaning and polishing service page explains these mechanisms, the on-site checks and the treatment options in full. This page deals with the decisions that come before treatment: how much fuel to hold, how to rotate it, when to test it, and how to build and keep a tank so problems are found early.
Autonomy versus turnover: setting the stock level
Every site has a minimum fuel requirement: the running time it must survive without a delivery. A bank branch may need 8 hours. A hospital may need 48 to 72 hours by its own policy. A remote telecom site may need several days because deliveries are slow. That is the autonomy floor.
Every set also has a turnover ceiling: the stock it will burn within about six months at its actual running pattern. Hold more than this and some fuel will be old before it is used. The planning steps are:
- Find full-load consumption. Set kW (kVA × 0.8) × consumption rate at full load (typically about 0.26 L/kWh). This sizes autonomy, because an outage may bring full load.
- Calculate the autonomy floor. Full-load L/h × required hours.
- Find actual monthly use. Average running hours per month × average load × consumption rate at that load, which is higher per kWh at part load (about 0.30 L/kWh at 50% and more below that). Use log-book data, not guesses.
- Calculate the turnover ceiling. Monthly use × 6.
- Compare. If the ceiling is above the floor, hold a stock between them, nearer the floor plus a delivery margin. If the floor is above the ceiling, the site must hold old fuel by necessity, so test and polish it on a schedule.
Worked example (illustrative): three sites, three answers
Site A, 125 kVA set at an office, 8 h autonomy required, runs 10 h a month at 50% load.
- Full load: 125 × 0.8 = 100 kW × 0.26 L/kWh = 26 L/h.
- Autonomy floor: 26 × 8 = 208 L.
- Monthly use: 10 h × 50 kW × 0.30 L/kWh = 150 L/month.
- Turnover ceiling: 150 × 6 = 900 L.
- Decision: the ceiling is well above the floor. Hold about 300–400 L: the floor plus a margin for a late delivery. At 150 L a month that turns over in roughly 2 to 3 months.
Site B, 500 kVA set at a hospital, 48 h autonomy by policy, runs 6 h a month at 40% load.
- Full load: 500 × 0.8 = 400 kW × 0.26 = 104 L/h.
- Autonomy floor: 104 × 48 = 4,992 L, about 5,000 L.
- Monthly use: 6 h × 160 kW × 0.32 = 307 L, about 310 L/month.
- Turnover ceiling: 310 × 6 = 1,860 L.
- Decision: the floor is more than two and a half times the ceiling. Turnover alone would take 5,000 ÷ 310 ≈ 16 months. The hospital must hold the fuel, so the plan is: split storage into two tanks used alternately (below), a six-monthly laboratory panel, polishing when results drift, and a monthly run on real load to draw fuel.
Site C, 62.5 kVA set at a shop, 4 h autonomy, runs 25 h a month at 60% load.
- Full load: 50 kW × 0.26 = 13 L/h; autonomy floor 13 × 4 = 52 L.
- Monthly use: 25 h × 30 kW × 0.29 = about 220 L/month; ceiling about 1,300 L.
- Decision: the base tank can be kept full. It turns over within a month. Attention moves to delivery quality and filtration.
Expressing stock in days, not only litres
Site staff understand “we have two days of fuel” better than “we have 5,000 litres”. Show both on the log sheet and the tank gauge chart: days of autonomy = stock in litres ÷ (full-load L/h × 24). For Site B above, 5,000 ÷ (104 × 24) = 2.0 days. Set two trigger levels: a reorder level (autonomy floor + one delivery lead time of running) and a minimum level (the autonomy floor itself, never to be breached).
First-in, first-out rotation plan
Fuel ages from the date it is delivered, so the oldest fuel should be the first used. A single tank that is topped up constantly never empties: some of the fuel at the bottom can be years old. Rotation methods, from simplest to strongest:
| Method | How it works | Suits |
|---|---|---|
| Draw down before refilling | Let the tank fall to the reorder level before each delivery, rather than topping up small amounts often | Single-tank sites with steady use |
| Twin-tank alternation | Two tanks; one in service, one held full as reserve. Swap roles at each delivery or every three months | Hospitals, data rooms, sites with high autonomy floors |
| Bulk-to-day-tank transfer | Bulk tank feeds a small day tank through a filter; bulk is drawn down and refilled in cycles | Plant rooms and multi-set installations |
| Planned consumption runs | Monthly runs on real building load, timed to use the oldest stock | Lightly used standby sets |
| Transfer to a high-use set | Move older fuel to a set or process that burns it quickly, through a filter | Sites with several sets of different duty |
Whatever the method, label each tank with the date of its last delivery and record the age of the oldest fuel on the monthly inspection sheet.
The laboratory test panel and when to send samples
On-site checks catch gross water and dirt. A laboratory panel catches the slow drift that leads to filter blocking and injector wear. Ask the laboratory for the following tests.
| Test | What it tells you | Action level (typical) |
|---|---|---|
| Water content (mg/kg) | Dissolved and suspended water | Above 200 mg/kg: polish and find the ingress route |
| Particulate contamination or cleanliness code | Dirt load reaching filters | Worse than the engine maker’s limit, or a rising trend |
| Appearance (clear and bright) | Haze, free water, sediment | Any haze or sediment |
| Microbial count | Bacteria and fungi in fuel and water phases | Moderate or heavy growth |
| Oxidation stability | Tendency to form gums in further storage | Below specification: plan to use or replace |
| Density at 15 °C | Checks the delivery is diesel and not adulterated | Outside the delivery note figure or specification range |
| Flash point | Detects contamination with lighter fuels | Below specification: do not use |
When to send a sample
- Every six months on any tank whose fuel is older than three months.
- Before the monsoon and before summer on critical sites, so treatment is done before peak outage seasons.
- After any delivery that looks, smells or measures wrong, before it is mixed with other stock.
- After a filter blocks unexpectedly or the set loses power under load.
- Before buying or commissioning a used set that arrives with fuel in its tank.
Take samples from the lowest drain point and from mid-depth, in clean glass or approved sample bottles, labelled with the tank, date, point and person. A sample taken from the top of a tank tells you almost nothing.
Tank design and housekeeping
Features every tank should have
- Suction above the floor: 50–75 mm, so settled water and sediment stay in the tank.
- Drain at the lowest point: on a floor that falls towards it. A side-wall drain leaves a permanent pool.
- Sample points: one at the bottom (via the drain) and, on bulk tanks, a dip-sample access at mid-depth.
- Vent: with rain cap and insect mesh, sized for the fill rate; a desiccant breather on critical or lightly used tanks.
- Sealed, lockable fill point: with a fine strainer, kept closed except during delivery.
- Level indication: a gauge or sensor calibrated in litres, with a dip chart as a check.
- Return-line position: warm return fuel discharged away from the suction, so the hottest fuel is not drawn straight back.
- Compatible materials: no copper or galvanised fittings in contact with fuel.
- Containment: a bund or double-walled tank sized for at least the tank’s contents, kept clean and free of rainwater.
Day tank or bulk tank?
| Aspect | Base or day tank | Bulk tank |
|---|---|---|
| Typical capacity | 2–8 h at full load; base tanks often 8–24 h | Days of autonomy, often several thousand litres |
| Turnover | Fast | Slow on standby sites |
| Main risk | Heat from the engine and return fuel | Long storage, water at the floor, sediment |
| Key housekeeping | Overflow return to bulk, level controls, filter on transfer | Drain, sample, test, rotate |
| Inspection focus | Transfer pump and float switches | Floor condition, water, microbial growth |
Sizing bulk and day tanks against runtime is covered in our fuel tank sizing guidance; runtime itself in the fuel tank and runtime calculator.
Delivery discipline
- Check the delivery note: quantity, density and supplier.
- Look at the first litres from the tanker hose in a clear jar before accepting the delivery.
- Fill through a filter when decanting from barrels; never through an open funnel.
- Record the delivery date on the tank label and the log sheet.
- Do not deliver into a tank during heavy rain with the manhole open.
Tank inspection checklist
Print this and keep it with the set’s log book. One sheet per tank.
| Check | Frequency | Acceptable | Result / date / initials |
|---|---|---|---|
| Stock in litres and in days of autonomy | Weekly | Above minimum level | |
| Stock against reorder level | Weekly | Reorder raised if below | |
| Age of oldest fuel (months since delivery) | Monthly | 6 or less, or tested | |
| Bottom drain: water removed (litres) | Monthly | Recorded; nil or falling | |
| Fill point sealed, locked, strainer in place | Monthly | Yes | |
| Vent cap and mesh intact; desiccant colour | Monthly | Intact; not saturated | |
| Level gauge agrees with dip reading | Quarterly | Within 5% | |
| Primary filter bowl after test run | Each run | No water or debris | |
| Filter restriction indicator | Each run | Within maker’s limit | |
| Day-tank transfer pump and float switches | Monthly | Start and stop at set levels | |
| Overflow return line clear | Quarterly | Clear | |
| Tank exterior: leaks, rust, seams, supports | Quarterly | No defects | |
| Bund clean, dry, drain valve closed | Monthly | Yes | |
| Laboratory sample sent; result filed | Six-monthly | Within action levels | |
| Rotation step done (draw-down, swap, consumption run) | As planned | Done |
What to record
A fuel record is what turns housekeeping into evidence. Keep these entries for each tank, in the set’s log book or a simple register:
- Deliveries: date, litres, supplier, density from the delivery note, visual check result, person accepting.
- Stock: weekly level in litres and days of autonomy.
- Consumption: running hours and fuel used per month, giving actual L/h to refine the stock plan.
- Water drained: litres per drain. A rising figure points to a leaking cap, vent or seam.
- Laboratory results: date, sample point, each test value, action taken.
- Treatment: polishing, tank cleaning, additive or biocide use, with dates and quantities.
- Filter changes: date and running hours; early changes are a fuel-quality signal.
Our DG set log book format includes fuel pages you can use directly.
Exceptions and cautions
- Hospitals, fire-safety installations and other regulated sites may have fixed autonomy requirements that override turnover. Hold what the rule requires and manage quality by testing.
- Consumption figures on this page are typical planning values. Use the maker’s fuel curve and your own log data for final numbers.
- Bulk storage above certain quantities needs statutory approvals and specific tank construction and spacing. Check what applies before installing or enlarging a tank.
- Never enter a fuel tank. Cleaning inside a tank is confined-space work for trained teams under a permit.
Common mistakes
- Filling every tank to the top “for safety” on a set that runs a few hours a year.
- Topping up small amounts every week, so the bottom of the tank is never renewed.
- Setting autonomy by habit rather than by the site’s real outage pattern and delivery lead time.
- Specifying a tank with no bottom drain or sample point, so nobody can see what is in it.
- Sending one laboratory sample after a failure, with no earlier results to compare.
- Recording running hours but never fuel received and fuel used.
What to send us
For a stock plan or tank review, send: set ratings, tank types and capacities, the autonomy your site requires, average monthly running hours (from the log book), delivery lead time from your supplier, and photographs of each tank’s fill point, vent, drain and level gauge. Send these through the service and AMC enquiry or WhatsApp 98852 01608. A service engineer will return a stock level, rotation method and testing calendar for your site.
Questions engineers ask
How much diesel should a standby generator site keep in stock?
Enough for the autonomy the site needs (full-load litres per hour × required hours) plus a delivery margin, but ideally no more than the set will burn in about six months at its actual running pattern.
How do I convert fuel stock into hours or days of running?
Divide the stock in litres by the full-load consumption in litres per hour. Divide again by 24 for days. Use full load, because an outage may bring full load.
What is first-in, first-out rotation for generator fuel?
A plan that makes sure the oldest fuel is used first: drawing a tank down before refilling, alternating two tanks, or timing consumption runs to burn older stock. Constant small top-ups defeat it.
Which laboratory tests should be done on stored generator diesel?
Water content, particulate contamination or cleanliness code, appearance, microbial count, oxidation stability, density and flash point.
Do I need a day tank as well as a bulk tank?
For plant rooms and multi-set installations, yes. The day tank keeps a small, fast-turning supply near the engine, fed through a filter from bulk storage, and limits the fuel volume inside the generator room.
What should a generator fuel register record?
Deliveries with density and checks, weekly stock in litres and days, monthly consumption, water drained, laboratory results, treatments and filter changes.
Related pages
- Fuel cleaning and polishing service
- Fuel tank sizing
- Fuel tank and runtime calculator
- Generator fuel consumption
- Fuel theft, security and refuelling control
- DG set log book format
- Generator monsoon precautions
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 5 November 2026.