Anyone else running a small van system off a single 100Ah lithium and a cheap PWM controller — worth upgrading the controller first?

So I've been running my little Peugeot Partner setup for about eight months now and I'm trying to work out where my next £50–80 is best spent. At the moment I've got one 100W panel on the roof, a cheap 10A PWM controller I grabbed off eBay for about £12, and a 100Ah lithium (LiFePO4) I bought secondhand from a bloke on here actually. It mostly does the job for a weekend away — running a 12V coolbox, phone charging, and a small LED strip — but I keep reading that PWM controllers waste a fair chunk of potential from the panel, especially in dull UK weather.

The thing is, a decent 20A MPPT controller seems to come in around £35–55 on Amazon (Victron SmartSolar 75/15 is about £55 new, or there are the generic ones for less). I'm wondering whether that jump to MPPT would actually make a noticeable real-world difference on a single 100W panel, or whether the gains are basically theoretical and I'd barely feel it in day-to-day use through a British winter.

Has anyone actually swapped out a PWM for an MPPT on a similarly modest setup and measured the difference? I'm not after massive numbers, just genuinely curious whether it shifted anything meaningful — like getting to 80% state of charge noticeably earlier in the day, or managing a full charge on a grey November afternoon where before it just wouldn't quite get there.

@MeganStevens the controller upgrade is genuinely the right call first. A PWM on a lithium battery is a poor match — you're losing somewhere between 20–30% of your panel's potential output because PWM can't properly harvest the panel's maximum power point. An MPPT like the Victron SmartSolar 75/15 sits right in your budget at around £65–70, and crucially it'll communicate properly with a lithium's charge profile rather than just hammering it with whatever voltage it fancies.

I ran a near-identical setup in my Transit conversion before upgrading — same frustration, same budget constraint. Swapped the controller first and immediately saw better state-of-charge by early afternoon on overcast days, which in the UK is basically the only kind of day we get.

Second panel can wait. Sort the harvesting efficiency before adding more generation capacity — otherwise you're just wasting more efficiently.

Hey @MeganStevens! Just to add to what @WD40Wizard78 was getting at — the key thing with MPPT over PWM on a 100W panel is you're typically recovering 20-30% more usable charge, which on a small system genuinely matters day-to-day. A decent Victron SmartSolar 75/15 sits right in your budget at around £55-65 and the Bluetooth monitoring alone is worth it — you'll actually see what your panel is doing rather than guessing.

One thing worth checking first though: what's the Voc of your panel? If it's a standard 12V nominal panel with Voc around 22V you're absolutely fine, just want to make sure before you buy.

Short answer — yes, controller first, then consider a second panel once you've got the MPPT in place. Good order of upgrades! 👍

@MeganStevens one thing worth adding — panel voltage matters here. If your 100W panel has a Voc around 22V you're losing a fair chunk to PWM inefficiency, especially on dull UK days when the panel's already struggling. An MPPT will squeeze more out of marginal light conditions, which is exactly what we deal with most of the time.

For your budget, the Victron SmartSolar 75/15 is around £60–65 and genuinely worth it over the cheap no-name units. Done it myself on the motorhome and the Bluetooth monitoring alone is worth half the price — you can actually see what's going in rather than guessing.

Second panel can come later. Get the controller sorted first, @WD40Wizard78 and @CraigLamb are right on that.

@MeganStevens a Victron 75/15 MPPT is about £60 and will also stop your lithium getting charged like it's a 2009 leisure battery from Halfords.

@BatteryJason that Victron 75/15 is the one I'd go for too — picked one up a few months back and the Bluetooth monitoring alone is worth it for peace of mind. One thing nobody's mentioned yet: does your PWM controller even have a proper lithium charge profile, @MeganStevens? A lot of the cheap ones don't, and you could be slightly undercharging or — worse — not cutting off correctly at the top end. That 100Ah battery deserves to be charged properly. What brand is the lithium? If it's a Fogstar or similar with a decent BMS it'll protect itself to a point, but you really want the controller doing its job correctly too.

Just to add something nobody's mentioned yet — with a single 100W panel you're not going to see massive gains from MPPT in terms of raw harvest, maybe 10–20% in good conditions. But @BatteryJason's point about lithium charging profiles is arguably the bigger win here. A PWM controller genuinely doesn't know how to treat a lithium properly, and over time that matters more than the efficiency difference. That said, £60–80 is tight for a Victron — have a look at the Renogy Wanderer or EPever Tracer 1210AN as cheaper alternatives if the Victron's pushing your budget. The EPever in particular handles lithium profiles reasonably well for the price. What PWM controller are you running currently? Some of the newer budget ones have a lithium setting that buys you a bit more time before upgrading.

@MXM_OffGrid is right on the physics but there's a practical win people miss — a proper MPPT like the Victron 75/15 will actually let you set lithium charge parameters correctly. PWM controllers either don't have the profile options or do it badly. Running LiFePO4 to the wrong voltage ceiling for eight months is the bit that'd bother me more than harvest efficiency.

That said, @MeganStevens — if your PWM has no lithium profile at all, that's your answer right there. Fogstar cells especially don't love being treated like AGM.

Really good thread this. One thing worth adding to what @PanelGraham and @MXM_OffGrid have touched on — if you're running lithium specifically, a decent MPPT will have a proper lithium charge profile built in, whereas most cheap PWM units are still optimised around lead-acid voltages. You might actually be slightly undercharging your battery right now without realising it. Over eight months that adds up. So even beyond the panel efficiency argument, getting the charge profile right for your actual battery chemistry is a genuine reason to upgrade the controller first before anything else.

Been in almost the exact same boat — single 100W panel, PWM, one 100Ah lithium (Fogstar Drift in my case). Honestly the thing that made the biggest difference before I touched the controller was checking my wiring losses. Undersized cable between panel and controller was robbing me of more than the PWM inefficiency ever was. Worth grabbing a cheap clamp meter and seeing what you're actually getting at the battery terminals versus what the controller claims. Might find some easy wins there before spending anything. What PWM are you running at the moment — some of the really cheap ones also have dodgy low-voltage cutoffs that can stress lithium cells.