Does my battery capacity actually matter if my solar can't keep up on cloudy days?

Running a 200Ah Fogstar Drift LiFePO4 and two 200W Renogy panels on my van roof. In summer it's great, but we're heading into autumn and I've noticed even a full battery is drained by morning after a couple of grey days in a row.

The way I see it — if the panels can't replace what I'm using (running a 12V compressor fridge, some LED lighting, and charging laptops), then having a bigger battery just delays the inevitable. I've been told to upgrade to 400Ah but I'm not sure throwing money at storage fixes the fundamental input problem.

Is the advice usually to tackle generation first (more panels, or a DC-DC charger off the alternator) before upgrading the bank? Or does extra capacity genuinely help in ways I'm not thinking about?

RachelCooper | 847 posts

@NotAnElectrician Yes, battery capacity absolutely still matters, but you've identified the real issue — in autumn/winter you're not replenishing what you're using. Your 200Ah bank gives you storage, but if you're pulling say 80Ah overnight and only generating 40Ah the next grey day, you're in a slow death spiral regardless of bank size.

A few things worth considering:

• Reduce consumption first — LED lighting, 12V appliances, anything that cuts overnight draw
• Shore power top-ups when you can get them
• Adding a panel helps but cloudy days still limit you significantly
• Consider whether a DC-DC charger from your alternator makes sense for van life

What's your overnight usage roughly? That'll tell you whether more capacity, more generation, or less consumption is the priority. Could be all three honestly! 😄

@NotAnElectrician the missing piece nobody's mentioned yet is days of autonomy — how many cloudy days can you survive without any meaningful solar input?

Your 200Ah Fogstar is decent but if you're running a compressor fridge, lighting, phone charging etc. you might be pulling 60-80Ah daily. That's only 2-3 days of reserve at best.

Two things I'd prioritise:

• Audit your actual consumption first — shunt meter like a Victron BMV-712 will show you exactly what's being eaten overnight
• Consider whether another 100Ah battery is more cost-effective than chasing more panels (panels can't help you at 3am regardless)

Running a similar Fogstar setup myself. Winter changed my thinking completely — it's less about generation and more about rationing until the sun comes back.