Three 100W Renogy slim panels. That’s what I put on the roof of my ProMaster in 2018, and that’s what I ran for six years.
It worked. Most of the time. The “most of the time” is what I want to tell you about.
Why I Chose 300W in 2018#
Budget and roof space. Three 100W panels fit the ProMaster roof without cutting into the vent fan clearances, and 300W was the practical ceiling for the equipment I could afford at the time. More solar meant more panels, and more panels in 2018 meant either a much tighter roof layout or spending money I didn’t have.
The math on paper looked fine. 300W × 4 peak sun hours = 1,200Wh potential on a good day. My daily load was around 700–900Wh. The math said I had margin.
Where 300W Showed Its Limits#
Colorado fall and winter. Peak sun hours drop from 5–6 in summer to 2–3 in fall, and lower still in December. A system that harvests 1,200Wh in July harvests 450–600Wh in November. If your daily load is 800Wh, you’re running a deficit every single day. After four days of that deficit, you’re at the bottom.
This happened. Not as a theory — as a real situation, multiple times. I got through it by driving more, by finding outlets, by managing loads more carefully. But I was solving a problem that didn’t have to exist.
The Pacific Northwest and Northeast. Extended overcast is the variable that 300W can’t compensate for. When there’s no sun for three days, it doesn’t matter how efficient your panels are. You’re not harvesting anything. I spent time in New York in fall and November, and the combination of low sun angles and overcast sky made solar essentially useless for stretches that lasted a week.
Summer mornings in Arizona. The counterintuitive one. Arizona summers have intense sun, but a black ProMaster interior can hit 130°F+ by mid-afternoon, and panel temperature affects output. Panels mounted flat on a dark roof get hot, and panel efficiency drops as temperature rises. Real harvest on Arizona summer afternoons was noticeably lower than the specs would suggest (Measured, rough observation — not instrumented).
What 300W Got Right#
I don’t want to overstate the problem. 300W of solar covered the majority of my days over six years. In good sun conditions — Arizona spring, California coastal, Colorado summer — it was more than enough. Many weeks I ended each day with partial charge remaining and never touched an outlet.
The Renogy slim panels held up physically. They’re still on the van. After six-plus years of vibration, temperature cycling, and desert sun exposure, all three are still producing. The build quality justified the price.
What I’d Do Today#
500–600W minimum, same roof footprint. This is the key change that modern slim panel technology makes possible. In 2018, 100W slim panels were state of the art in that form factor. In 2026, 180–200W slim panels exist in similar physical dimensions. Three panels at 180W each = 540W in the same roof layout that fit 300W before. Same mounting footprint, 80% more capacity, similar price point.
That 540W system on a 4-hour sunny day delivers ~1,600Wh — enough to cover a full daily load and bank meaningful surplus. On a 2-hour overcast day it delivers ~350Wh — still covering a meaningful portion of the day’s essentials rather than nothing.
The math changes in the bad scenarios. 540W in a 3-hour Colorado fall day delivers ~810Wh — almost enough to cover a full load without deficit. 300W in that same day delivers ~450Wh — a 350–400Wh daily deficit that compounds across a cloudy week.
Orientation flexibility matters. Portable or tiltable panels let you angle toward the sun when parked. Fixed flat roof panels lose 20–30% of potential harvest compared to optimal angle (Estimate). If you’re stationary for extended periods, a setup that lets you prop one panel at an angle while parked is worth considering.
The Honest Recommendation#
If you’re building a van system today and you have roof space for 300W of slim panels, plan the layout for 500–600W instead and add capacity as budget allows. The incremental cost of going wider while you’re already doing the mounting work is small. The cost of retrofitting later is much larger.
I ran 300W for six years. It was fine. But every time the Colorado winter or a Northeast cloudy stretch showed up, I wished I’d done more.