Weekend van, a 100 Ah battery, a compressor fridge that draws about 30 Ah a day, plus phones and a laptop. I've been offered a folding 120 W briefcase panel secondhand and I'm trying to work out whether that meaningfully covers me in shoulder season or whether it's a summer-only toy. I'd rather buy once than build up in 60 W increments over three years.
The controller matters more than people budget for. An MPPT controller versus a cheap PWM one is a real difference on a panel this size, especially in weak light. Don't spend on the panel and then throttle it with a £15 box.
And check the controller's input voltage range against the panel's open-circuit voltage in the cold, which is when that voltage is at its highest.
Angle and shade beat wattage outright. A 120 W panel you can aim at the sun and reposition twice a day will out-produce a fixed 200 W roof panel in shoulder season. That's the genuine argument for the folding one, and it's the only argument that matters.
I'd skip the folding panel and put a fixed one on the roof. Folding kit means remembering it, deploying it, securing it against theft and stowing it again, and after one season I stopped bothering entirely. The panel you never think about is the one that actually charges your battery.
Your 100 Ah battery is the constraint that'll bite first if it's lead-acid, since you can realistically use about half of it. Worth confirming which chemistry you have before you spend anything on generation.
Secondhand folding panels: check the blocking diodes and put a meter on it in real sun before you hand over money. The hinges and the cabling die long before the cells do.
@probe_to_ground · 7mo ago · 3 replies
Do it in amp-hours rather than watts, and derate hard. 120 W nominal, real-world, on a folding panel at an imperfect angle, in shoulder season at a northern latitude — assume something like three to four useful equivalent hours, so call it 25-35 Ah on a good day and close to nothing under thick cloud. Against a 30 Ah daily draw that's break-even in good conditions and a slow drain in bad ones.
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@lowtech_lior · 7mo ago
Break-even in good conditions is a far more useful answer than any wattage figure I've been given.
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@probe_to_ground · 7mo ago
It always is. Watts sell panels, amp-hours run fridges.
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