This Isn't a Standard A vs B Comparison
I took over purchasing for a 400-person company in 2020. I process about 60–80 equipment orders a year across eight vendors, and I report to both operations and finance. When our operations manager asked me to compare backup power options, I assumed the choice was a 175 kW Perkins backup generator or a 3kW solar generator.
It wasn't that simple. They solve different problems. After dozens of supplier calls and some painful lessons, here's how I frame the comparison. I look at three things: power profile, lifecycle cost, and reliability. If you're an admin buyer like me, that order works.
(Should mention: I'm not an engineer. I'm a buyer. I know enough to ask hard questions, but an electrical engineer needs to do the real sizing. If a supplier says "just buy this," that's a red flag.)
Our operations manager wanted to like the solar option. Quieter, cleaner, no fuel logistics. I wanted to like it too. But backup power is not about what you like. It's about what keeps people safe and the business running.
Power Profile: The 175 kW Gap
A 175 kW Perkins backup generator is building-sized equipment. It can support a small factory, a supermarket's refrigeration circuits, or a multi-floor office with HVAC and lifts. A 3kW solar generator is more like a large uninterruptible power supply. It can keep a few circuits running: network equipment, a sump pump, a handful of lights.
The first thing that surprised me was that I wanted to compare them as different sizes of the same product. They're not. The diesel generator keeps producing power as long as fuel is delivered. The solar generator produces power until its battery is empty, then it depends on the sun. For a two-hour outage, a 3kW solar generator is a clean, quiet option. For a two-day winter outage, it won't support a facility that needs a 175kW load.
To be fair, solar generator technology has improved a lot in the last five years. I get why people want to avoid diesel noise and emissions. But "improved" doesn't mean "scaled up by fifty times." You can't bolt together fifty 3kW solar generators and expect the same performance as one 175kW Perkins unit. The transfer switching, load sharing, and fuel logistics would all be worse.
So on this dimension, the conclusion is clear: for whole-building backup, the Perkins generator wins. For keeping a few critical circuits alive, the solar unit is arguably better. They aren't substitutes.
Cost: The Part That Confused Me
This is where "cheap" gets complicated. The ticket price of a 3kW solar generator is much lower than a 175kW diesel generator. But I don't buy on ticket price. I learned that in 2024, when a vendor couldn't provide a proper invoice and finance rejected a $2,400 expense. Now I compare total cost and vendor capability.
For the 175kW Perkins, the cost includes the machine, automatic transfer switch, installation, commissioning, concrete pad, fuel tank, and a maintenance plan. For the solar generator, the main cost is the battery. Batteries last roughly five to ten years depending on chemistry and how deeply they're discharged. Replacing them is a real line item. Someone has to track that, and in a busy company, "tracking that" often becomes your job.
When I looked at Perkins generator suppliers UK, I didn't only ask for a quote. I asked for a service schedule and parts availability. I asked if they stock the Delphi FS0120 fuel pump strainer for units with Delphi fuel pumps. A cheap strainer can stop a generator, and a supplier who can't answer a parts question is a box shifter, not a partner.
"We sell the Perkins generator, but we don't design backup power systems. For that, you should use an electrical engineer."
That answer, from one UK supplier, earned my trust. I'd rather work with a specialist who knows his limits than a generalist who says yes to everything. That philosophy shaped our 2024 vendor consolidation project: fewer vendors, but ones that could talk about parts, not just machines.
I also asked where to buy AC contactor replacements for the transfer switch. A worn contactor is a common failure point, and in an outage you don't want to search the internet. The supplier who could answer that question got the order.
And here's the surprising part: for our load, one 175 kW Perkins generator was the lower total-cost path. My gut resisted because the upfront number was bigger. Every spreadsheet said the single diesel unit was less complex than a pile of small solar units. I went with the spreadsheet. When I saw one machine installed instead of dozens of little boxes, I knew it was right.
Reliability: Both Sides Require More Work Than You Think
Diesel generators are reliable only if you maintain them. The Perkins engine itself is proven, but the failure modes are usually fuel quality, a dead starter battery, or a contactor that welded shut. A 175 kW Perkins backup generator needs periodic load tests, oil changes, and filter changes. That's not "zero maintenance." Any supplier who says otherwise doesn't understand generators. NFPA 110, the standard for emergency and standby power systems, has a testing schedule for a reason.
Solar generators look simpler, and in one sense they are—no oil, no fuel, no exhaust. But batteries have their own demands. State of charge, temperature, cycle count, and eventual replacement all matter. Leave a solar generator for six months and the battery may not forgive you.
The part that surprised me was that for a small, always-on load—say, a network closet or a gate controller—a 3kW solar generator can be more reliable than a diesel that nobody exercises. The diesel needs routine attention; the solar system just sits there. But for the loads that matter in a long outage, the diesel wins.
Oh, and learn where the contactor is before you need it. I now ask every supplier where to buy AC contactor replacements, and whether they carry the Delphi FS0120 fuel pump strainer. The best generator in the world can sit dead for a week waiting on a $30 part.
What I'd Actually Buy
If you need whole-building backup with big starting loads, the 175 kW Perkins backup generator is the practical answer. If you only need to keep a few critical loads up for short outages, a 3kW solar generator is a legitimate solution. If you need both, that's not unusual. Buy the diesel for the building and the solar unit for portable emergency power.
- Define the load. What has to keep running, and for how long?
- Get an engineer to size it. If your supplier avoids that, be careful.
- Vet the supplier before the model. Ask about parts like the Delphi FS0120 fuel pump strainer and where to buy AC contactor.
- Plan the maintenance honestly. Diesel needs monthly exercise and annual service. Solar needs battery state-of-charge checks.
Even after we approved the 175kW Perkins, I kept second-guessing. What if two 80kW units would have been more flexible? I didn't relax until the full-load test passed. I think some post-decision doubt is normal with a purchase this size.
This was accurate as of Q1 2025. Generator pricing, supplier stock, and battery technology change fast, so verify current specs before you budget.
My experience is based on about 200 equipment orders for a multi-site company. If you're buying for a single retail shop or a site with very different loads, your numbers may look different.
