Diesel vs Petrol Generator Running Costs

Walk into a showroom and you’ll see a 5kW petrol generator for £1,200 sat next to a 5kW diesel unit at £2,400. Double the price for the same output.

Easy choice, right? Buy the petrol one and save £1,200.

Except that logic falls apart the moment you actually start using it.

I’ve watched people “save” £1,000 on a petrol generator only to spend £1,500 extra on fuel over three years. Then they come back asking why nobody warned them.

Let me show you the real numbers so you can make a properly informed decision.

Current UK Fuel Prices (2025)

Prices fluctuate, but here’s where we are right now:

Petrol (Unleaded): £1.45-£1.55/litre (average £1.50)
Diesel: £1.50-£1.60/litre (average £1.55)
Red Diesel (Agricultural): £0.95-£1.05/litre – only legal for specific off-road use

So diesel costs about 3% more per litre. That’s basically nothing.

The difference comes from how efficiently each engine converts fuel into electricity.

Fuel Consumption: Where Diesel Wins

Diesel engines are mechanically more efficient. They extract more energy from each litre of fuel. This isn’t marketing—it’s thermodynamics.

Real-World Consumption Comparison

Let me show you actual consumption figures from comparable generators:

5kW Generator at 75% Load (3.75kW Output)

10kW Generator at 75% Load (7.5kW Output)

Diesel uses roughly 30-35% less fuel for the same power output. That gap widens under sustained heavy load.

Annual Running Cost Calculations

Let’s work through some realistic scenarios. Your actual costs will vary, but these examples show the pattern.

Scenario 1: Occasional Home Backup

Usage: 50 hours per year (frequent but short power cuts)
Load: 3kW average

5kW Petrol Generator

• Consumption at 60% load: 1.8 L/hr
• Annual fuel use: 90 litres
• Annual cost: 90 × £1.50 = £135

5kW Diesel Generator

• Consumption at 60% load: 1.2 L/hr
• Annual fuel use: 60 litres
• Annual cost: 60 × £1.55 = £93

Annual saving with diesel: £42

Not massive. But the diesel unit cost £1,200 more. That’s a 28-year payback. Petrol wins here.

Scenario 2: Regular Commercial Use

Usage: 200 hours per year (weekly markets, events, backup)
Load: 6kW average

8kW Petrol Generator

• Consumption at 75% load: 3.2 L/hr
• Annual fuel use: 640 litres
• Annual cost: 640 × £1.50 = £960

8kW Diesel Generator

• Consumption at 75% load: 2.0 L/hr
• Annual fuel use: 400 litres
• Annual cost: 400 × £1.55 = £620

Annual saving with diesel: £340

Now we’re talking. The £1,500 price premium pays back in 4-5 years. After that, you’re £300-400 better off every year.

Scenario 3: Heavy Industrial Use

Usage: 1,000 hours per year (construction site, off-grid workshop)
Load: 15kW average

20kW Petrol Generator

• Consumption at 75% load: 7.5 L/hr
• Annual fuel use: 7,500 litres
• Annual cost: 7,500 × £1.50 = £11,250

20kW Diesel Generator

• Consumption at 75% load: 4.5 L/hr
• Annual fuel use: 4,500 litres
• Annual cost: 4,500 × £1.55 = £6,975

Annual saving with diesel: £4,275

The diesel generator costs £3,500 more upfront. It pays for itself in 10 months. Every year after that is pure savings.

For heavy commercial use, diesel isn’t just better—it’s the only sensible option. Check our diesel generator range for industrial-spec units.

The Break-Even Analysis

When does diesel start saving you money overall?

Formula: Break-even hours = (Diesel price – Petrol price) ÷ (Petrol fuel cost/hr – Diesel fuel cost/hr)

Let’s calculate for a 5kW generator:

• Price difference: £1,200
• Petrol cost at 60% load: £2.70/hr
• Diesel cost at 60% load: £1.86/hr
• Hourly saving: £0.84/hr

Break-even = £1,200 ÷ £0.84 = 1,428 hours

If you’ll use the generator for 1,500+ hours over its lifetime, diesel saves money overall. Under 1,500 hours? Petrol is cheaper.

Most portable generators run 200-500 hours before they’re replaced or retired. Petrol usually wins for portable use.

Standby generators might sit for years but accumulate hours through testing and outages. Over 10-15 years, they’ll hit 1,500 hours easily. Diesel wins for permanent installations.

Factors Beyond Pure Fuel Cost

Money isn’t everything. These factors affect your decision too.

Engine Longevity

Diesel engines typically last 2-3 times longer than petrol engines with similar usage.

Expected Lifespan:

• Petrol: 1,000-3,000 hours before major service/rebuild
• Diesel: 3,000-10,000 hours before major service/rebuild

For light use, doesn’t matter. For heavy use, diesel’s durability is another cost saving.

Our Perkins, Cummins, and FG Wilson diesel units are built for 20+ year lifespans with proper maintenance.

Fuel Storage and Stability

Petrol:

• Degrades in 3-6 months without stabiliser
• More volatile (evaporation and fire risk)
• Cannot be stored in bulk without licensing

Diesel:

• Stable for 6-12 months (longer with additives)
• Less fire risk
• Easier to store in quantity

For standby generators that sit unused for months, diesel is less hassle. You’re not constantly draining old fuel and refilling.

Cold Starting

Petrol: Starts easily in cold weather. Pull and go.

Diesel: Can be stubborn below 0°C without glow plugs or block heaters. Modern diesels have these, but it’s still a consideration.

For winter outdoor use, petrol has an edge. Our Honda and Yamaha petrol units start first pull in any weather.

Maintenance Costs

Petrol:

• More frequent oil changes (every 50 hours)
• Spark plugs need replacement (100-200 hours)
• Carburettor cleaning if left with old fuel

Diesel:

• Less frequent oil changes (every 100 hours)
• No spark plugs (saves £30-50 per service)
• Fuel system more robust

Annual maintenance costs are similar, but diesel intervals are longer. Fewer service visits = lower overall hassle.

Usage Patterns: Which Fuel Suits You?

Let me break this down by application.

Choose Petrol For:

Occasional Home Backup (under 100 hours/year) You won’t use it enough to justify diesel’s price premium. A quality petrol unit from Honda or Yamaha will serve you well.

Portable/Leisure Use Camping, caravans, outdoor events. Petrol generators are lighter and easier to transport. Check our leisure generator options.

Small Capacity Needs (under 5kW) The price gap between petrol and diesel widens at smaller sizes. A 3kW diesel costs proportionally more than a 3kW petrol compared to their 10kW equivalents.

Infrequent Use in Cold Climates Petrol starts more reliably after sitting idle in freezing conditions.

Choose Diesel For:

Regular Commercial Use (200+ hours/year) Food trucks, market stalls, mobile workshops, regular backup. Fuel savings add up fast.

Permanent Standby Installations Home or business backup systems. Browse our standby generator range with options from SDMO, CAT, and Pramac.

Heavy Industrial Applications Construction sites, workshops, off-grid locations. Diesel’s efficiency and longevity justify the investment.

Large Capacity Requirements (10kW+) The efficiency advantage grows with size. At 20kW+, diesel is almost always cheaper over the unit’s lifetime.

The Third Option: LPG Conversion

Before you decide, consider LPG. It sits between petrol and diesel economically.

LPG Cost per Litre Equivalent: £0.60-£0.75
Consumption: Similar to petrol (slightly higher)
Running Cost: Significantly cheaper than both petrol and diesel

Many petrol generators can be converted to run on LPG. For details, see our LPG conversion guide.

Real-World Cost Summary Table

Here’s a quick reference showing total cost of ownership over 5 years for different usage levels:

5kW Generator, 5-Year TCO

Includes purchase price, fuel, and typical maintenance

The crossover point is around 150-200 hours per year for a 5kW generator. Use it more than that? Diesel wins. Less than that? Petrol wins.

Environmental Considerations

If you care about carbon emissions:

Diesel produces:

• 20% less CO₂ per kWh generated
• More NOx and particulates
• Better fuel economy = less total emissions for same work

Petrol produces:

• More CO₂ per kWh
• Cleaner combustion (less particulates)
• Worse fuel economy = more total emissions

Neither is environmentally brilliant. But diesel’s efficiency gives it an edge on overall carbon footprint.

Making Your Decision

Stop thinking “which fuel is better?” There isn’t a universal answer.

Ask instead:

1. How many hours per year will I actually run this?
2. What’s my budget including total cost of ownership?
3. Do I need portability or is it a fixed installation?
4. What size do I genuinely need?

For occasional home backup and portable applications under 5kW: Buy petrol. Lower initial cost, perfectly adequate for light use. Quality units from Honda or Yamaha will last decades.

For regular commercial use, standby installations, or anything over 10kW: Buy diesel. Higher upfront cost pays back through fuel savings. Professional units from Perkins, FG Wilson, or SDMO offer industrial reliability.

Do the maths for your specific situation. The calculator’s pretty simple: purchase price difference divided by annual fuel savings equals payback period. If that payback period is under 5 years, diesel makes financial sense.

Anything else is just preference.