Generator Fuel Calculator - Runtime, Cost & Tank Size

🔋 Generator Fuel Calculator

Fuel Type

Generator Rated Power

Load (% of rated capacity)

Tank Capacity

Tank Fill Level

Fuel Price per Litre (for cost)

No. of Days to Run

🔌 Load Planner

Add appliances to calculate total load and required generator size.

Appliance

Watts

Qty

📦 Fuel Stockpile Planner

Calculate how much fuel to store for emergencies and planned outages.

Daily Usage (Litres/day)

Outage Duration to Prepare For

Fuel Price per Litre

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Generator Fuel Calculations

Fuel Consumption Formula

Fuel Consumption (L/hr) = kVA × Load% × Fuel Rate Fuel rates (at 100% load): Petrol generator: 0.30–0.40 L/kVA/hr Diesel generator: 0.20–0.28 L/kVA/hr LPG generator: 0.25–0.35 L/kVA/hr (kg/hr) Example: 5 kVA petrol generator at 75% load Fuel/hr = 5 × 0.75 × 0.35 = 1.31 L/hr Runtime from 15L tank: Runtime = 15 / 1.31 = 11.5 hours

Load Factor Impact on Fuel Use

Fuel consumption is NOT linear with load: 50% load: ~65% of full-load fuel consumption 75% load: ~82% of full-load fuel consumption 100% load: 100% of full-load fuel consumption Running at 75% load is the sweet spot: - Better fuel efficiency than 100% load - Some headroom for startup surges - Longer generator life vs running at 100% Never run a generator below 30% load for extended periods (wet stacking / carbon buildup).

Generator Sizing Rules

Step 1: Add up wattage of all appliances Step 2: Multiply by 1.25 (25% safety buffer) Step 3: Add startup surge for motors (motors need 2-3× running wattage to start) Example: AC 1.5T: 1500W running, 4500W start Refrigerator: 200W running, 600W start Lights + fans: 300W running, 300W start TV + misc: 200W running, 200W start Running total: 2200W With 25% buffer: 2750W = 3 kVA generator minimum With AC surge: ~5 kVA recommended

Frequently Asked Questions

A petrol generator typically uses 0.33–0.38 litres per kVA per hour at full load. A 5 kVA generator at 75% load: 5 × 0.75 × 0.35 = approximately 1.31 L/hr. A diesel generator is 20–30% more efficient: 5 kVA diesel at 75% load ≈ 1.0 L/hr. Actual consumption varies directly with load - a generator running at 50% load uses significantly less fuel per hour than the same unit at 90% load. Use the calculator above and enter your actual load percentage for an accurate result.

Divide total fuel in the tank by the hourly consumption rate. A 15-litre tank on a 5 kVA petrol generator at 75% load (1.31 L/hr) gives 15 ÷ 1.31 ≈ 11.5 hours. A 20-litre tank on the same generator ≈ 15 hours. A 5 kVA diesel generator at 75% load (1.0 L/hr) on the same 15-litre tank: 15 ÷ 1.0 = 15 hours. Runtime decreases with heavier loads and increases with lighter loads - enter your actual load percentage for the most accurate estimate.

Step 1: Add the running wattage of all appliances you want to run simultaneously. Step 2: Identify the appliance with the highest startup surge (usually AC or refrigerator) and add that surge wattage separately. Step 3: Add running total + largest single startup surge. Step 4: Multiply by 1.25 as a safety buffer. Step 5: Choose the next standard size above that figure. Typical Indian home with 1.5-ton inverter AC + fridge + fans + lights ≈ 3,000W running. With AC startup surge: 3,000 + 1,500 = 4,500W. × 1.25 = 5.6 kVA → choose 6.5 kVA or 7.5 kVA generator.

Diesel: 20–30% more fuel-efficient, longer engine life (20,000–30,000 hours vs 10,000–15,000 for petrol), better for continuous heavy use. Costs ₹30,000–1 lakh more upfront but saves on running costs. Best for: daily use, 4+ hour outages, high loads. Petrol: cheaper upfront, quieter at small kVA, easier to start in cold weather. Best for: occasional use, 1–3 hour outages, smaller homes without AC. At ₹103/litre petrol vs ₹95/litre diesel, and with diesel 25% more efficient, the fuel cost gap is significant for regular users.

Wet stacking is a diesel engine condition caused by running the generator at too low a load (below 30%) for extended periods. At light loads, combustion temperatures are too low to fully burn the fuel - unburnt fuel and carbon deposits accumulate in the exhaust system, turning it black and oily. Over time, this reduces performance and efficiency. Prevention: run diesel generators at 50–80% load. If you regularly need light loads, use a smaller generator. If wet stacking has occurred, run the generator under heavy load for 1–2 hours to burn off deposits.

Petrol: 3–6 months without additives, up to 12 months with a fuel stabiliser. Petrol oxidises and forms varnish deposits that can clog carburettors. Diesel: 6–12 months without treatment, up to 24 months with a biocide (kills microbes that grow in diesel) and stabiliser. In India's humid climate, diesel is particularly vulnerable to microbial contamination. Always label containers with fill date. Rotate your stockpile - use stored fuel every 3–6 months and replace it. Store in approved jerry cans away from heat and electrical sources.

Common causes: (1) Load above rated capacity - each additional kW significantly increases fuel consumption. (2) Poor maintenance - dirty air filter restricts airflow and forces richer fuel mixture; old spark plugs cause incomplete combustion. (3) Altitude - higher altitude means less dense air, richer mixture, and higher consumption. (4) High startup surges - appliances drawing heavy surge current during startup temporarily spike consumption. (5) Overdue service - generators should be serviced every 250–500 hours of operation (oil change, air filter, spark plugs). (6) Contaminated or old fuel.

Yes - inverter ACs are significantly more generator-friendly than non-inverter ACs. An inverter AC ramps up gradually and maintains a variable compressor speed, so its startup surge is relatively modest (1.2–1.5× running wattage). A 1.5-ton inverter AC drawing 1,200W has a startup surge of approximately 1,500–1,800W. A 5 kVA generator can comfortably run a 1.5-ton inverter AC plus a refrigerator, fans and lights. Non-inverter ACs have startup surges of 3–5× running wattage - a 1.5-ton non-inverter AC can surge to 4,500W on startup, requiring a much larger generator.

Generator Fuel Calculator - Runtime, Consumption Rates & Fuel Storage Guide

Power outages are unpredictable, but your fuel planning doesn't have to be. Whether you're sizing a generator for a home, calculating running costs for a business, or planning a fuel stockpile ahead of a monsoon season, knowing your generator's exact consumption rate and runtime is essential. This calculator handles all three scenarios - from a single outage estimate to a multi-day stockpile plan.

Quick example: A 5 kVA petrol generator running at 75% load consumes approximately 1.3 L/hr. With a 15-litre tank: runtime = 15 ÷ 1.3 = 11.5 hours. At ₹103/litre, cost per hour = ₹134. Daily cost (8 hours): ₹1,072. For a 3-day stockpile at 8 hrs/day: 1.3 × 8 × 3 = 31.2 litres needed.

Fuel Consumption Formula - How Generator Burn Rate Is Calculated

Generator fuel consumption is measured in litres per hour (L/hr) and depends on three factors: the rated capacity (kVA), the actual load applied as a percentage of capacity, and the fuel type. The standard formula:

Fuel consumption (L/hr) = Generator kVA × Load% × Consumption factor

Typical consumption factors: Petrol = 0.33–0.38 L/kVA/hr. Diesel = 0.25–0.30 L/kVA/hr. LPG ≈ similar to petrol by volume.

At 75% load, a 5 kVA petrol generator: 5 × 0.75 × 0.35 = 1.31 L/hr. The same 5 kVA diesel generator: 5 × 0.75 × 0.27 ≈ 1.01 L/hr - roughly 23% more efficient than petrol.

Diesel vs Petrol vs LPG - Which Generator Fuel Type Is Right for You?

Diesel Generator

20–30% more fuel-efficient than petrol
Diesel engines last longer - typically 20,000–30,000 hours
Better for heavy loads and continuous operation
Quieter at higher kVA ratings
More expensive to buy upfront (₹30,000–1 lakh premium)
Best for: daily use during long outages (4+ hours), commercial use, high-load applications

Petrol Generator

Lower upfront cost - same kVA costs 30–50% less than diesel
Quieter at lower kVA ratings (1–3 kVA)
Easier to start in cold conditions
Better for light/occasional use
Petrol shelf life shorter (3–6 months)
Best for: infrequent use, 1–4 hour outages, small homes without AC

Sizing Your Generator - What Load Do You Actually Need?

The most common generator sizing mistake is accounting only for running wattage without factoring in startup surge wattage. Motors (in ACs, refrigerators, water pumps) require 2–3× their running wattage to start. A generator that's adequately sized for running loads can trip or stall when starting a compressor motor.

Typical appliance wattage (running / startup) for an Indian home:

1.5-ton inverter AC: 1,200W running / 1,500W startup (inverter ACs have low startup surge)
1.5-ton non-inverter AC: 1,500W running / 4,000–5,000W startup
Refrigerator (double door): 200W running / 500–800W startup
Ceiling fan: 70W running / 70W startup (no surge)
LED bulbs (4×10W): 40W running / 40W startup
LED TV (43 inch): 90W running / 90W startup
Water pump (0.5 HP): 375W running / 750–1,000W startup

Recommended sizing: add up all running watts, add the single largest startup surge (not all at once), multiply total by 1.25 for a safety buffer, then choose the nearest standard generator size above that figure.

The Optimal Load Percentage - Why 75% Is the Sweet Spot

Running a generator at the right load percentage significantly affects fuel efficiency, engine wear, and lifespan:

Below 30% load: "Light loading" causes inefficient combustion, oil fouling, and in diesel engines, wet stacking - unburnt fuel accumulates in the exhaust system. Avoid prolonged light loading, especially in diesel generators.
50–80% load: The optimal operating range. Good fuel efficiency, proper combustion temperature, manageable heat output, and adequate headroom for startup surges.
Above 90–100% load: Significantly increases heat, accelerates engine wear, and leaves no headroom for surge loads. Short-term spikes are fine; sustained 100% load shortens engine life.

The 75% load point is where most generator manufacturers rate their performance specs - it's also where fuel consumption per kWh output is lowest for most engine types.

Fuel Storage - How Long Petrol and Diesel Stay Usable

Stored fuel degrades over time, particularly petrol which oxidises faster than diesel. Planning a stockpile requires knowing your storage limits:

Petrol: 3–6 months without additives. Up to 12 months with a fuel stabiliser (available at auto parts shops). Store in approved metal jerry cans or HDPE containers. Keep cool, dark, and away from heat sources.
Diesel: 6–12 months without treatment. Up to 24 months with a biocide (prevents microbial growth in the tank) and stabiliser. Diesel degrades by oxidation and microbial contamination - particularly in humid climates like India.
Always rotate your fuel stockpile - use stored fuel and replace it regularly rather than letting it sit for 1–2 years. Label containers with the fill date.
Never store fuel inside a home, near electrical panels, or in direct sunlight. Fuel storage at home is regulated - check local rules for maximum permitted quantities.

How this calculator works, and where the numbers come from

The Generator Fuel Calc applies the standard formula for this calculation to the values you enter and updates the result as you type. The calculation itself happens in your browser, and the page explains the method so you can check any result by hand.

Please note: Results are estimates. Tariffs, appliance ratings and local rules vary; electrical work should follow local codes and be done by a qualified person.

Sources and further reading

Last reviewed: October 6, 2026 by the CalcQube Editorial Team. See our editorial policy for how we build and check calculators, or report an error.

🔋 Generator Fuel Calculator