How do I know if a crimp is good?

Pull on it firmly — a proper crimp will not budge, and a bad one comes apart in your hand, which is exactly what you want to discover on the bench. Visually, the conductor should be fully captured with no stray strands outside the barrel. For a definitive check, compare resistance across the connection against an equivalent length of plain cable.

Why did my system work fine and then start underperforming?

The usual suspects, in order: degrading crimps or connectors raising resistance, UV damage to wiring or ties, corrosion at terminations, new shading from vegetation, and soiling. Panel degradation is slow and predictable, so a noticeable drop over months is almost always a connection or a shading problem rather than the panels ageing.

Can I use household electrical connectors for solar DC wiring?

No. PV circuits need connectors rated for DC at the array's voltage, for outdoor UV exposure and for the temperature range — MC4 and equivalents exist for exactly this. Household wire nuts and terminal blocks are not rated for sustained outdoor DC and are a genuine fire risk in that application.

Common Wiring Mistakes That Cause DIY Solar Systems to Fail

Bad crimps, undersized battery cable and connectors left in the sun cause more failures than anything else — and they show up months after everything worked fine.

Wiring faults have a particular character: the system works perfectly at commissioning and degrades quietly months or years later. That delay is what makes these mistakes worth avoiding up front, because by the time output drops or something gets hot, the cause is buried under a finished installation.

1. Bad crimps

The most common fault in DIY solar and the hardest to spot. A crimp that is loose, made with pliers instead of a proper tool, or made with the wrong die creates electrical resistance. Resistance at current creates heat. Heat degrades the connection, which raises resistance further — a feedback loop that ends at a melted connector or a fire.

Use the crimping tool matched to your connectors, and test every one with a firm tug before it goes anywhere. A connection that pulls apart on the bench would have failed on the roof in year three.

2. Undersized battery-to-inverter cable

This run carries the highest current anywhere in the system — a 3,000W inverter on a 12V bank draws around 250A — and it is the run most often undersized because the cable is expensive and awkward. It should be as short and fat as physically possible, and it must be fused close to the battery terminal.

The full sizing method is in sizing solar cabling and voltage drop, and it is the one calculation in the build genuinely worth doing twice.

3. Reversed polarity

Connecting positive to negative destroys charge controllers and inverters instantly, and it is not usually covered by warranty. Some equipment has reverse-polarity protection; plenty does not, and none of it should be relied on.

Check every connection with a multimeter before energising anything. Colour coding helps, but colours get swapped by whoever made up the cable — the meter is the authority.

4. Ordinary wire and connectors outdoors

Standard indoor THHN degrades under UV; the insulation cracks and eventually exposes conductor. Exterior DC runs need sunlight-resistant PV wire or protection in conduit. The same applies to cable ties — ordinary nylon ties become brittle and snap within a couple of years on a roof, dropping wiring onto shingles where it chafes. Use UV-rated ties or stainless clips.

5. Mismatched panels in one string

A series string carries the same current throughout, so it performs like its weakest member. Putting a 300W panel in a string of 400W panels drags the whole string toward the 300W panel's current, and you lose the difference across every module. Different-age or different-model panels have the same effect to a lesser degree.

Keep strings electrically identical. If you must mix, put each type on its own string with its own MPPT input.

6. No strain relief

Cable hanging from a connector puts its weight on the electrical connection rather than on a mechanical support. Add wind movement and thermal expansion over twenty-five years and the connection works loose. Every run should be supported so the connectors carry no load, with a drip loop so water runs off rather than tracking into an enclosure.

7. Working on a live array

A panel produces voltage whenever light hits it — there is no off switch. Pulling an MC4 connector apart on a producing array draws a sustained DC arc that damages the connector and can injure you. Cover panels with an opaque tarp, work in low light, and open the disconnects properly. See disconnects and what code requires.

The habit that prevents most of this

Test as you build rather than at the end. Check polarity and voltage at every stage, tug-test every crimp, and photograph connections before they are covered. A fault found on the bench costs minutes; the same fault found after the array is mounted and the conduit is run costs a weekend.

And have a licensed electrician look over the DC side before you close anything up. An hour of their time catches undersized conductors, missing bonds and poor terminations in minutes — the reasoning is in hiring an electrician for the final hookup.