Optimate Solar Duo 20W review | Off-grid motorcycle charger

Date reviewed: August 2025 | Tested by: John Milbank | Price: £164.90 (RRP) | www.optimate.co.uk

When Bennetts BikeSocial member and Kawasaki Z1000SX owner Roger Carlisle asked for advice about keeping his bike’s battery charged in a shed that has no mains power, it was a great opportunity to finally find out if a quality solar charger can keep a motorcycle’s battery topped up in Britain’s very changeable weather. I thoroughly tested the Optimate Solar Duo 20W on review here before passing it to Roger, and this is why I think it’s worth the money…

Pros & Cons

Optimate Solar Duo Features

The Optimate Solar Duo is much more than just a solar panel that attaches to your bike’s battery…

Polycrystalline solar panel. This isn’t some cheap sun-catcher – the Optimate’s panel (which is also available in 10W, 40W, 60W and 80W versions) uses a polycrystalline structure that’s better at producing energy in all weathers than a monocrystalline, which is designed for set-ups that can track the sun. It’s housed in a tough, weatherproof aluminium frame with fixing points at each corner and is designed to be screwed in place, but a car kit with OBD charging plug includes suction cups (also available separately) to stick the panel inside a car’s windscreen.

Larger panels can of course generate more power, but 20W should be big enough for most home users.

Smart Controller. Featuring the same tech as Optimate’s excellent mains-powered chargers, the solar panel plugs into a compact charging and monitoring unit that delivers a controlled power output when there’s energy coming in from the panel, then switches to a monitoring mode at night.

Lead-acid and lithium compatible. The Optimate charge controller is compatible with standard 12V lead-acid, GEL, and AGM motorcycle batteries between 5 and 80AH, as well as Lithium packs as long as they’re the most common LFP (LiFePO4, also known as Lithium Iron Phosphate).

Weatherproof. Not only is the solar panel waterproof, the controller and hence entire unit can be used in all weathers (just don’t submerge it), at temperatures down to -20°C.

Optimate Solar Duo connections

The Optimate Solar Duo 20W tested here comes with a pair of crocodile clips terminating in an SAE connector, as well as a fly-lead that can be permanently wired to your bike’s battery.

The panel itself has a 2m cable that plugs into the charge controller, which has a 1m cable of its own. You can buy extensions if needs be, but this was more than enough for Roger’s set-up.

It would be possible to plug the solar panel direct to the crocodile clips or fly-lead, leaving the controller out of the circuit, but DO NOT DO THIS as the panel outputs a much higher voltage than should be used to charge a bike battery. In my testing, the panel outputs 21.11V in the sun and 18.97V in the shade.

In use

Setting up the Solar Duo is a simple matter of plugging the panel into the controller and connecting that to the bike. If there’s no power coming from the panel, the controller displays the charge state of the battery, so you know the bike’s ready to go.

When there’s light – and it doesn’t have to be bright sunlight – the ‘charge’ light illuminates and the four LEDs will show the status:

Anyone who’s used Optimate chargers before will recognise this pattern. It’s important to understand that the Solar Duo is a maintenance charger, not just a basic trickle charger that delivers a low current all the time, which can be harmful to batteries.

The Optimate will bring a low battery back up to full power as safely as it can, then it’ll keep ‘watching’ it, and ensuring it’s kept at the optimum voltage to be ready for the next ride.

What the Smart Controller used in the Solar Duo can’t do is recover a sulphated battery. Some other Optimates can attempt to rescue a lead-acid pack that’s as low as 2V, but if the battery has got that bad you might be better off buying a new one anyway.

Optimate Solar Duo performance

I ran several tests of the Optimate Solar Duo, monitoring its charge rate in various conditions…

Using a halogen bulb to discharge it, I got the 9Ah bike battery down to 11.8V, then put it on charge with the solar panel outside at 8.45am. The sun was behind the neighbour’s house at that point, and it was a hazy / cloudy day throughout, but the charger was showing the yellow LED and putting out pulses of current to bring it back up to full charge.

After just two and a half hours the controller was flashing between the blue and green LEDs, indicating that it was in the final stages of checking. This is an old battery that I use for testing, and after five hours the controller was back to blue, cycling its current with a voltage of 13.4V. The battery was ready to go, but the charger could see it wasn’t reaching its target voltage of 13.6V.

In another test keeping the panel in heavy shade all day, the battery started charging at 12.1V, and within an hour was up to 12.8V, getting to 13.0V after about three hours. By the evening – remember that this was a pack that was far from in peak condition – the controller indicated that the battery, which I tested to be at 13.0V, could be used but still needed charging to complete.

I also tested the Optimate on the VFR800’s battery, which is in good condition having been replaced in the last year. I connected the Solar Duo in hazy sunlight and it immediately starting charging at 13.7V, with a current of 0.7A being delivered. Within an hour the controller was showing solid green and totally full.

The Optimate’s Smart Controller is claimed to deliver up to 5A, but that will depend on the input. Nothing is 100% efficient, but if we ignore losses in the circuitry then the most the 20W solar panel could deliver to the bike’s battery – via the Smart Controller – is 1.47A at 13.6V.

I measured the output of the solar panel using a halogen lamp as the load and in the sun saw 21V and 0.98A delivered, which is 20.58W so on spot-on for the company’s claims.

In the shade on the same day, the panel’s output voltage dropped to 19V, with a current of 0.25A, which is 4.75A. In these conditions, the controller (ignoring losses) could charge at up to about 0.4A at 13.6V. This is of course going to be a lot slower than in the sun, but keep in mind that the entry-level mains-powered Optimate 1 Duo has a maximum output of just 0.6A, and the Solar duo’s performance is put more into perspective.

In summary then, even during the winter the Solar Duo will easily keep a reasonably healthy motorcycle battery topped up and ready to go, no matter how long you leave it. If working correctly, your motorcycle’s charging circuit will be keeping the battery at a good voltage when you ride, so a charger’s job is to maintain it when parked up – the Optimate does that very well.

The Smart Controller is well sealed, and solidly assembled inside

Three alternatives to the Optimate Solar Duo

Many modern bikes have some degree of ‘parasitic draw’ caused by various electronic devices (and trackers) pulling a small amount of charge from the battery, and that’s before the natural self-discharge of lead-acid packs. If you ride regularly it shouldn’t be an issue, but if your bike gets left for more than a couple of weeks it’s well worth keeping it connected to a quality maintenance charger. There aren’t that many alternatives when it comes to off-grid charging, but here are some others you might have been considering…

• Optimate DC, £96.90 | We haven’t tested the Optimate DC, but it’s the one we’d be most likely to recommend for anyone needing fairly high-current off-grid charging. This is ideal if your bike’s in a lockup or somewhere remote that you can’t fit a solar panel to as it runs off an 11-16V input, so is ideally paired with a large leisure battery. It’ll charge lead-acid and lithium packs, and can recover them from as low as 2V thanks to its 2A maximum output. Here’s a link to the Optimate DC.

• CTEK CS Free, £179 (charger), £199 (solar panel) | The design of the CS Free’s solar panel really makes it more suitable for use when travelling, but it can charge the main unit, which acts as a maintenance charger or as a ‘boost’ charger that rapidly charges a flat battery. There aren’t fly-leads available for it though, so it’s not really ideal as a day-to-day maintenance charger. Read the full review of the CTEK CS Free here.

• Oxford Solariser, £44.99 | The Oxford Solariser is a trickle charger, which I’d tend to recommend against using as it’s not properly maintaining your bike’s battery. It’s claimed to deliver a current of 0.01 to 0.1A. I got hold of one and connected it to a halogen lamp to test its output, measuring 20.70V and 2.42mA in the sun, or 18.18V and 2.02mA in the shade. While extremely low current, these are high voltages, and I’d shy away from such an unregulated charging system. Here’s a link to the Oxford Solariser.

These are just three of many alternatives – you can find all the motorcycle battery chargers we’ve tested here and be sure to regularly check for the discounts available through Bikesocial membership.

Optimate Solar Duo review: Verdict

If you need something that can deliver a high current for fast charging, and that can attempt to de-sulphate badly worn batteries, the Optimate Solar Duo isn’t the best choice. But when it comes to keeping your bike’s battery in its best condition where there’s no power available, I’m really impressed with how capable it is.

So what does Roger think now he’s had it installed in his shed for a couple of months? “It’s great,” he told me. “The shed is a long way from the house and would be a pain to get power to, so this was ideal. Whenever I come to the bike it’s ready to go, which is really important to me as I’m busy with work a lot, but enjoy long tours when I get the chance, so the Kawasaki has to be ready to go as soon as I am.”

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