Dometic MT-PP Series

Daylight costs nothing; a solar system runs silently; and apart from keeping the panels clean, it needs no maintenance. It also boosts your available battery power, helps the battery last longer, and lets you stay off hook-up for longer without other energy sources - though how much it produces depends on the weather.

There's no single right answer - what counts as enough power varies hugely from one person to the next, from a fully equipped motorhome to a basic camper with little more than a water pump and an oil lamp. Dometic's approach is to recommend a system based on what each owner actually needs, rather than simply the biggest one.

Size the panel to how much energy (in Ah) you use from your leisure battery each day. For example, a 45W TV used for 2 hours uses 90Wh, which on a 12V battery is 7.5Ah. Dometic gives two ways to estimate the panel size from that: a quick rule of thumb of daily Ah x 3 (7.5 x 3 = 22.5Wp, watt peak), or daily Ah x 12.6V ÷ 4 (7.5 x 12.6 ÷ 4 = 23.6Wp) - the panel output Dometic's experience suggests will cover daily use between spring and autumn.

A typical day's use, for example: TV 45W (3.75A) for 1.5 hours = 5.6Ah; lights 16W (1.30A) for 5 hours = 6.6Ah; water pump 25W (2.00A) for 0.5 hours = 1.0Ah; radio 15W (1.25A) for 2 hours = 2.5Ah - 15.7Ah in total. Some vehicles simply don't have the roof space for the panel they'd need; in that case, fit the most solar you can and make up the shortfall with extra battery capacity.

Dometic's guide (figures are roughly the energy produced per day, in Wh/d): 20/40Wp (80/160 Wh/d) - keeps a motorhome's battery maintained all year, and on a caravan is enough to recharge the leisure battery used while manoeuvring. 55/60Wp (220/240 Wh/d) - smaller motorhomes without TV/satellite, touring spring to autumn, running lights, water pump and radio. 80/90Wp (320/360 Wh/d) - small to medium motorhomes, spring to autumn, adding 2-3 hours of TV/satellite.

From 120Wp (480 Wh/d) - medium to large motorhomes, spring to autumn, with lights, pump, radio and TV/satellite. From 160Wp (640 Wh/d) - medium to large motorhomes used all year (weather and battery capacity permitting), adding a compressor fridge. From 280Wp (1,120 Wh/d) - large motorhomes used all year, with the same equipment. Most makers quote a panel's rating in Wp; some use Wh/d.

The cell type determines how efficient a panel is. Polycrystalline cells (blue) are cheaper and simpler to make but less efficient, so they suit homes with plenty of roof area. Monocrystalline cells (black) cost more but are more efficient, which suits the smaller space on a vehicle roof. Dometic's Büttner panels all use monocrystalline cells.

Panels made for houses aren't suitable for vehicles: they aren't built to cope with vibration, they lose output when shaded, and they have fewer cells over a bigger area. Dometic's Büttner panels are rigid and built for use on the move, cope better with shade, and pack more and better cells into a smaller area. There are two mounting types across three panel families, each in several sizes.

Bracket-mounted panels suit the flat aluminium or GRP roofs most motorhomes and caravans have: they bond on with brackets fairly easily, the glass surface makes them robust and long-lasting, and the gap underneath lets air cool them - important, as panels lose efficiency as they heat up. The extra height is usually irrelevant, since roof vents already stand taller.

Of these, Black Line is the best seller: 72 high-performance monocrystalline cells (rather than 36) give a strong daily yield even in poor weather or shade, it's the best value, and there's an unusually narrow 33 cm version. Power Line has the highest output for its size, using 40 premium monocrystalline cells (rather than 36) with a special charge controller - ideal where roof space is tight.

CDS Power Line is designed for partial shade and gives the best daily output of any of them when part of the panel is shaded for much of the day (by a satellite dish, for example). Direct-mount panels (for campervans) stick straight onto the roof rather than sitting on spoilers, can be walked on when on a flat surface, and can follow slightly curved roofs (bend radius 3 cm lengthways).

Flat Light and Flat Light Q are the options for this: just 2 cm thick and 3 kg, they keep the vehicle's height down for low entrances, and because they're bonded on they're hard to steal. The Flat Light Q, with 42 high-performance cells, suits pop-top and high-top campervans, panel vans and camper buses. The drawback is that they have to stay on the roof if you sell the vehicle.

A solar remote display lets you check at a glance that the system is working and when the battery is full; it plugs into any of Dometic's complete solar kits. Dometic's Solar Remote Display III (MT iQ range) shows the voltage (V), the charging current (A) and charging mode, plus solar output as a bar graph and percentage, and also counts the energy delivered to the battery (in Ah and Wh).

The Solar Pro (also MT iQ) is a universal display that works with any solar system: it shows voltage (V), charging current (A) and solar output (bar graph and %), and counts Ah delivered. It's designed to be built in, but a surface-mount housing comes in the box.

It's a battery charger like any other (the alternator, or a mains hook-up charger) - the difference is that the energy comes from daylight, so how much it charges depends on the light available. Panels are rated in Wp (watt peak): their maximum output under standardised test conditions. As a rule of thumb, to keep a 95Ah leisure battery topped up, plan on a system of about 100W.

Each leisure battery type has its own design, lifespan, capacity, pros and cons. Lead-acid (the most common): cheapest, but heavy, needs topping up with fluid, self-discharges faster so needs regular recharging to avoid sulphation, loses a lot of capacity in the cold, must be installed upright in a ventilated space outside the living area, and only gives its full capacity at low discharge currents, with an uncertain cycle life.

AGM (Absorbent Glass Mat - an advanced lead-acid type): better for repeated cycling and running an inverter, mid-priced, maintenance-free, can go inside the living area and be mounted on its side, but heavy; Dometic gives its usable capacity as 50% of the total (50Ah from a 100Ah battery). Gel (silica is used to turn the acid into a gel): also better for cycling and inverter use, maintenance-free and can be fitted inside or on its side, priced a little above AGM, heavier than AGM, with about 50% usable capacity.

Lithium iron phosphate (LiFePO4/LFP): the most advanced - about a third of the weight and half the space of a lead-acid battery, maintenance-free, best installed inside (side mounting possible), up to 100% usable capacity, roughly 3-4 times the cycle life of lead-acid, and no need to be stored fully charged; the drawback is the highest price.

There's no simple answer. Side by side with an AGM or gel battery of the same size, a LiFePO4 battery is lighter and can deliver far more current, and there's a lot in its favour. The downsides are a higher price and possibly some changes to your charging equipment. Whether that's worth it depends on how you tour: if you want to travel without worrying whether there's a hook-up, and to run a hairdryer on the inverter without counting the minutes, having the battery's full capacity available makes lithium a worthwhile investment.

Dometic offers two routes. Its Lithium Power Units add a lithium battery to your existing system with your current AGM or gel battery kept as a back-up; you can keep your current charger, and a battery computer manages everything. Its plug-and-play Power Sets replace your leisure battery with lithium entirely: a built-in battery control booster (BCB), connected between the starter battery and the LiFePO4 battery, makes sure the lithium battery charges properly both while driving and when parked.