Fuses vs Resettable Breakers for Off-Grid Solar: What Will Prowse Gets Right
- wwolverton
- 6 hours ago
- 5 min read
If you've spent any time in the DIY solar community, you've probably come across Will Prowse — one of the most prolific and trusted educators in the off-grid solar space. His YouTube channel and website (mobile-solarpower.com) have become go-to references for anyone designing their own system, and his content on fusing and circuit breakers is some of the most practically useful out there.
We've pulled together the key points from his research, testing, and recommendations into this summary. Whether you're mid-build or just starting to think about system protection, this is worth reading before you buy anything.
Start Here: Quality Is Non-Negotiable
Will's core message on this topic is consistent and blunt: using a low quality breaker or fuse could mean catastrophic failure of your home or vehicle. This isn't fearmongering — it's the reality of what these components are actually doing. A fuse or breaker is the last line of defense between a fault condition and a fire. The component category (fuse vs. breaker) matters less than whether it is properly rated and sourced from a trustworthy manufacturer.
The Most Important Thing Most Beginners Get Wrong: DC vs AC Breakers
One of the things Will hammers on repeatedly — and that causes real fires when ignored — is the fundamental difference between AC and DC circuit breakers.
Standard household AC breakers are designed to snuff out an arc as the current naturally crosses zero 60 times per second. DC current is constant and never crosses zero, which means a DC arc is much harder to extinguish. An AC breaker used in a DC solar application may simply fail to stop the arc — and potentially melt or catch fire in the process.
Never use AC breakers in a solar or DC system. Always verify that any breaker you buy is explicitly DC-rated at the voltage your system operates at.
Will's Testing: Do Cheap Amazon Breakers Actually Work?
Will put this to the test by running a range of common DC-rated breakers from Amazon through real-world fault testing. The results were nuanced. The majority of the cheap Chinese resettable breakers actually tripped within the expected tolerances — performing comparably to name-brand products on the bench.
But the caveats matter a lot. Some units arrived already broken. One appeared to be melting rather than tripping under load. And this is the key insight: even if 19 out of 20 cheap breakers pass, a roughly 1-in-20 failure rate is completely unacceptable for a fire-prevention device. This isn't a component you want to gamble on.
His Current Top Pick: MCCB DC Breakers
Will has become genuinely enthusiastic about Molded Case Circuit Breakers (MCCBs) specifically rated for DC use. Here's why they stand out:
Current breaking capacity comparable to or higher than a T-class fuse — capable of safely interrupting even if a large lithium battery bank dumps 25,000A of fault current.
Insulation voltage ratings of 1,000–1,500VDC — well above what most off-grid systems operate at.
Available from 100A to 800A, covering everything from small van builds to large residential off-grid systems.
Built-in arc barrier and a test button. Can handle inductive load surges without nuisance tripping.
Resettable — unlike a fuse, you don't need a spare on hand to restore power after a trip.
Will uses these to connect batteries to busbars, inverters to busbars, and solar charge controllers to a system. He recommends always verifying that your cables are sized large enough to carry the breaker's rated current to actually trip it under a fault condition.
His Fuse Hierarchy
For situations where you want the reliability of a fuse (no moving parts, no possibility of mechanical failure), Will has a clear pecking order.
Megafuse (70VDC Rated)
Will's go-to recommendation for most 12V, 24V, and 48V off-grid systems. It has a 2,500A disconnect rating — more than adequate for the vast majority of builds. His warning here is strong: do not buy the clone versions commonly found on Amazon. He's documented these fakes as genuinely dangerous and describes them as capable of burning down your house. Buy the original Littelfuse Megafuse from a reputable supplier.
Class T Fuse
Will considers these the best fuse option available — no moving parts, lowest chance of failure — but notes they are expensive and cannot be reset. Their best use case is connecting large battery banks in parallel, such as linking two stacks of server rack batteries together, where fault currents can be extreme. For systems larger than 100kWh, Will recommends stepping up to the MCCB breaker above due to its dramatically higher interrupt rating.
Terminal Fuses
For setups running multiple 12V batteries in parallel or anything in a marine environment, Will recommends bolt-on terminal fuses directly at each battery. These have an interrupt rating of 10,000A and address the lack of overcurrent protection on many budget batteries. Critical note: these are only rated for 12V systems — do not use them in 24V or 48V applications.
The AMG/Bolt-On Fuse Warning
Will has specifically tested fake AMG fuses from Amazon — and the results were alarming. He documented pulling 300A through a 150A knock-off for 15 minutes without the fuse tripping. These counterfeit fuses look identical to the real thing and are priced similarly, but are capable of causing a fire. Always buy from a verified source and check that the part number matches a genuine Buss AMG product.
How to Size Your Fuse or Breaker
Will's published sizing method is straightforward:
Take the continuous wattage rating of the device and divide by the system voltage (12V, 24V, or 48V) to get maximum current. Example: a 1,500W inverter on a 12V system = 125A.
Multiply that current by 1.25 to get your minimum fuse size. Example: 125A × 1.25 = 156.25A.
Round up to the next available fuse or breaker size — in this case, 175A or 200A.
Make sure your wire is sized to carry the full fuse rating — if the wire can't carry 200A to the fuse, the fuse can't trip to protect it.
The Bottom Line
Will's overall approach to system protection can be summed up in a few principles: use DC-rated components only, buy from reputable sources, size everything correctly, and don't cut corners on safety components to save a few dollars.
His preferred setup for most builds is a quality MCCB DC breaker at the battery for convenient resettability, combined with a proper fuse on each critical connection — a belt-and-suspenders approach that gives you both convenience and the no-moving-parts reliability of a fuse where it matters most.
At Lightharvest Solar, we stock quality fusing and breaker components and include proper overcurrent protection in every custom system we design. If you're not sure what your build needs, give us a call at 971-712-6468 (Mon–Fri, 10am–5pm) — we're happy to walk through it with you.
Will covers this in depth in the video below — we recommend watching it alongside this guide
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