Do I Need a Solar Charge Controller?
Yes, with one narrow exception. A panel connected straight to a battery will happily push it past a safe voltage and wreck it, so something has to sit in between. The more interesting question is which kind — because on a mild day that choice is worth about a fifth of your panel's output. It's worth less in hot sun, and more in the cold.
What a controller is actually for
A solar panel's output voltage rises and falls with the light, and in decent sun a "12 V" panel sits closer to 18–22 V. A 12 V battery wants to be charged at a controlled voltage — for lithium, somewhere around 14.2–14.6 V, then held there and stopped.
Wire the two directly together and there's nothing regulating that. In poor light you'd undercharge; in good light you'd push the battery well past where it should be held, which for lead-acid means boiling off electrolyte and for lithium means the battery's own protection board disconnecting to save itself. A charge controller sits between the two and does three jobs: caps the voltage, runs the correct charge profile for your battery chemistry, and stops current flowing backwards out of the battery into the panel after dark.
The narrow exception: a small 5–10 W dashboard trickle panel, used only to keep a full-size lead-acid starter battery topped up. It's too weak to overcharge a battery that size, and most have a built-in diode to stop night-time drain. Never wire any panel straight to a lithium battery or a small battery. Anything you'd actually camp with needs a controller. If in doubt, you need one.
You might already have one
Plenty of people buy a controller they didn't need because it was already in the box:
- Portable power stations — the all-in-one battery boxes with an input for solar have a controller built in. Plug the panel straight in — within the station's input voltage limit.
- Most folding panels and blankets ship with one attached to the back or supplied in the bag. Check before buying another.
- Many DC-DC chargers have a solar input. If yours does, that input is a solar controller — usually an MPPT one — and running a second controller in front of it usually costs you charge. If your blanket has its own controller, check whether the maker says to bypass it for a DC-DC charger.
- Battery boxes and some caravan management systems often include one.
PWM or MPPT — the choice that's worth real money
Two types, and the difference is bigger than the price gap suggests.
PWM works by connecting the panel to the battery and switching that connection on and off rapidly. Simple and cheap — but while connected, it drags the panel down to whatever the battery is sitting at. Your panel wants to work near 18 V; the battery is at about 13. That gap doesn't go into the battery. It's simply lost.
MPPT converts instead of connecting. It lets the panel work at its best voltage, then converts the surplus voltage into extra current on the battery side. Roughly the same energy in, considerably more of it arriving.
| PWM | MPPT | |
|---|---|---|
| Typical extra delivered | baseline | +20–25% on a mild day · +10–15% in hot sun · 30%+ cold and clear |
| Cold weather | Same as usual | Gains the most |
| Works with higher-voltage panels | No | Yes |
| Cost | Low | Higher |
On any panel worth carrying, MPPT is the sensible default. The rough guide people use: below about 100 W the price difference can outweigh the gain, and above it MPPT pays for itself — often inside a season if you're out regularly. It's also the only option if you want to run panels in series at higher voltage, which is how you keep losses down over a long cable run.
Sizing it
Controllers are rated in amps on the battery side. Work out roughly what your panels can produce:
Amps ≈ total panel watts ÷ battery voltage. So 400 W on a 12 V system is about 33 A — pick a 40 A controller and you've got sensible headroom.
Leave margin rather than sizing exactly. Cold bright days and light reflecting off snow or water can briefly push a panel above its rated output, and a controller running at its limit runs hot and derates.
Also check the controller's maximum input voltage if you're wiring panels in series — it's the specification people overlook. Panel voltage rises as it gets colder, and peaks at first light on a frosty morning. So add up the open-circuit voltage (Voc) on each panel's label, then allow about 15% extra if you'll ever camp below freezing, and 20% for serious cold. That total must stay under the controller's limit — going over it damages the controller rather than just limiting it.
What a controller can't do
Worth being straight about this, because it's where the money stops helping. A controller can only work with the energy the panel hands it. It cannot recover the output a shadow just took — about a third on an MPPT when one cell group is shaded, and more on a PWM or a blanket — it cannot make a flat panel behave like a well-positioned one in winter, and no amount of spending on one will beat five minutes spent putting the panel somewhere better.
Once the wiring is right — the right controller, a fuse between the controller and the battery, and the battery connected before the panel — positioning is the biggest lever left, and it's what keeps the fridge cold and the lights on for longer out there. A panel set up properly for the hours you need can out-deliver the same panel left flat by a margin far larger than the PWM-to-MPPT gap.
That's what Solar Spotter is for. It works out where the sun will be for your exact spot and today's date, and tells you the two things that decide your charge: which way to face the panel and how far to stand it up.
However you're running the day — set it once and get on with it, shift it after lunch, or grab a few hours between fronts — it gives you the position for that timeframe. Free covers now, +3h and +6h; Pro adds the rolling best charging window plus a window and duration you set.
It runs with no reception, because sun position is astronomy rather than a lookup — camping, vanlife, overlanding or boondocking, any latitude, either hemisphere. Your data stays on your phone: no account, no tracking. One-off purchase.
Start with the setup guide — it's the one that turns the gear you've just bought into charge in the battery.
Common questions
Can I connect a solar panel directly to a battery?
Not safely, other than a very small 5–10 W trickle panel keeping a full-size lead-acid starter battery topped up — and never a lithium battery. Anything bigger will push the battery past a safe voltage in good sun, which cooks a lead-acid battery and trips a lithium battery's protection board. It also lets current drain backwards out of the battery into the panel overnight. A charge controller prevents all three.
Is MPPT worth the extra over PWM?
For anything you'd actually camp with, yes. A PWM controller pulls the panel down to battery voltage and loses the difference between the panel's working voltage of around 18 V and the battery's 13 V or so — roughly 20–25% of your panel on a mild day, less in hot sun and more in the cold, until the battery is nearly full and both types slow down. MPPT converts that surplus into extra current instead. Below about 100 W the price difference can outweigh the gain; above it, MPPT usually pays for itself quickly.
What size solar charge controller do I need?
Divide your total panel watts by your battery voltage to get the approximate current, then choose the next size up. 400 W on a 12 V system is about 33 A, so a 40 A controller. Leave headroom — cold bright conditions and reflected light can briefly push panels above their rating. If you're wiring panels in series, add up each panel's open-circuit voltage (Voc), allow 15–20% extra for freezing mornings, and stay under the controller's input limit.
Do portable solar panels come with a controller?
Most folding panels and blankets do, either attached to the back or supplied in the bag. Portable power stations have one built into the solar input, and many DC-DC chargers include an MPPT solar input as well. Check what you already have before buying another — running a second controller in front of an existing one usually costs you charge.
Will a better charge controller fix poor solar output?
Only if a poor controller was the problem. Moving from PWM to MPPT is worth roughly 20–25% on a mild day, which is real — but a controller can only work with what the panel gives it. If a shadow is clipping a cell group you've already lost about a third before the controller sees anything — more with PWM or a blanket — and a flat panel under a low winter sun is losing far more than any controller can recover. Positioning is the bigger lever.
Get the most out of the gear you've picked
Once the wiring's sorted, positioning is the biggest lever left — the difference between rationing power and staying out longer. Solar Spotter tells you where to put the panel and when to be set up. Free to download.
