Cutting your dependence on diesel generators
Primary generator power is the most expensive electricity a farm can buy. Here's how the hybrid approach dismantles that bill, with real numbers from a real quoted system.
The short answer: a correctly designed solar and battery system doesn't eliminate your generator, it demotes it. Solar carries the days, batteries carry the nights, and the generator drops from running thousands of hours a year to a few hundred as backup for the darkest weeks. Fuel spend typically falls by 80 to 95%, generator life stretches by years, and on heavy diesel users the whole system can pay for itself in under two years.
What primary generator power really costs
The fuel bill is the visible part, and on a hard-working farm it's brutal on its own. But diesel-as-primary-power costs you three more ways. Servicing is driven by run-hours, and a generator running around the clock clocks up 8,760 hours a year, so oil, filters and major services arrive relentlessly. Light loading, which is most of a farm's day, is the worst thing you can do to a diesel engine: running at a fraction of rated load causes glazing and wet stacking that shortens engine life. And every kilowatt-hour is hostage to the diesel price, delivered to a tank that needs filling, monitoring and securing.
How the hybrid approach works
The design principle is simple: meet the load with the cheapest source first, and hold the generator in reserve for the moments nothing else can cover.
- Solar sized for the shoulder months, not just summer, so it carries the daytime load for most of the year
- Battery storage sized in days of autonomy, not hours, so nights and dull days come from stored solar rather than the engine
- The generator integrated properly through the inverter system: it starts automatically only when state of charge demands it, runs at high load where diesels are efficient, charges the bank quickly, and shuts down
- Everything monitored remotely, so run-hours, fuel behaviour and battery health are visible without walking to the shed
Done right, the generator stops being your power station and becomes your insurance policy. It still earns its keep in the darkest fortnight of December, but at high load, briefly and rarely, which is the duty diesels are happiest with.
Design study: a 74kVA dairy generator in Mid Wales
Real numbers from a system we designed and quoted in 2026 for a working dairy farm. We've kept the customer anonymous, but the figures are unedited.
The starting point: a 74kVA diesel generator running 24/7 as the farm's primary power, through milking, cooling and everything else. Fuel bill: £66,000 to £80,000 a year in red diesel, before servicing.
The system we designed: 60kW of solar split to work the way the farm does, 35kW DC-coupled on the milking shed and 25kW AC-coupled on the workshop, feeding a 192kWh Pylontech battery bank (40 x US5000) through a 45kVA three-phase Victron MultiPlus-II system, with the existing generator retained and integrated as automatic backup.
The money: £71,372 ex VAT as quoted. Set against the annual fuel bill it replaces, that's payback in under two years for a VAT-registered farm business, before counting reduced servicing, longer generator life, and the £1m Annual Investment Allowance writing the capital off against tax in year one.
The rule of thumb that falls out of this and every similar job we look at: your annual diesel spend is roughly your system budget. If the fuel bill is £20,000 a year, a system around that cost will usually retire most of it. If it's £70,000, the maths gets silly in your favour.
The honest caveats
Every figure above came from a proper site survey: metered loads, roof orientations, the milking schedule, the December scenario. A system sized off a guess will either disappoint you or cost you money it didn't need to. Winter design is where off-grid systems are won or lost, and it's also where an experienced designer saves you from both under-building and gold-plating. Payback depends on your actual fuel use, so we quote against your fuel receipts, not an assumption.
What's your generator actually costing you?
Tell us your generator size and roughly what you spend on fuel, and we'll tell you honestly whether the numbers work for your site. Free survey, no obligation, and a funding summary included with every quote.
Written by Mark Street, director of Xtant Energy. The design study figures are from our own 2026 quotation for a working dairy farm. General information, not financial advice: tax treatment depends on circumstances, speak to your accountant.