DIY photovoltaic & solar water heating. Oahu has pretty hefty insolation and we pay 30-35 cents/KWHr for HECO electricity.
Back in late 2004 spouse saw a newspaper (remember them?) classified ad (remember those too?) for PV panels. The guy was a retired engineer in his 80s who'd broken a hip and no longer wanted his hobby PV panels. We bought 20 55-watt panels (1100 watts) for $2000... including the racks & wiring... which even today is pretty cheap. The panels are at least 15 years old.
We found an electrician who taught us the code for the inspection. While we were drilling holes in a perfectly good roof and re-mounting the racks, he ran the construction and net-metering permits and installed the (brand-new, retail) inverter. We bought more wiring, conduit, and electrical hardware. Between his labor and the inverter we invested another $4800. Today that's high-end full-service retail pricing, but back then $6/watt was stupid cheap. Our sweat equity was free.
We went the same route for solar water in late 2005: two used 4x8 collectors (now over 35 years old) for $150, a used solar water heater & pump for $150, another $620 for mounting hardware, piping, valves, and the electrical controller. The collectors had been sitting in a yard for a decade but I know how to assemble a hydrostatic test rig, and they held pressure. (Today they're worth more than $150 just for the copper tubing.) Our 13-year-old daughter did most of the soldering and we split the assembly tasks. Back then retail would've been around $5000.
Up through 2007 we kept buying used PV panels from Craigslist a few at a time. One company sold us 16 factory seconds over eBay and shipped them to Oahu from Florida. They were ugly but they were still rated for full power and they were under $4/watt. We made more racks out of scrap aluminum and ended up with the south roof covered in 64 sq ft of solar water collector plus 3300 watts among 45 PV panels. Back then a 3KW PV array would've retailed for about $30K but by the time we were finished we'd spent $920 for solar hot water and $15,700 for PV.
Then we took $9375 in federal & state tax credits.
By late 2010 the system had paid for itself.
In 2011 we demolished/rebuilt our familyroom (mostly contractors, some sweat equity), modified the roofline, and added foam insulation panels to the roof. We had to take down the PV array for the roof work, so I treated myself to modern rail racks with flashed mounts and clip-on hardware. The roof insulation (and wall insulation, and new tinted windows) are so energy-efficient that we no longer use our ceiling fans. The whole house is cooled by the tradewinds and frankly it's kinda cold on winter mornings. Most months our electricity bill is $18 (the minimum net-metering charge) and in winter it climbs to about $30.
The first photo is after we finished building v1.0 of the PV & solar water. The second photo is after the renovated familyroom and v2.0 of the PV.