Sizing Solar for a Liveaboard or Overland Rig

What "sizing solar" actually means

Sizing a solar array is the process of working out how many watts of panel you need to refill your batteries each day, starting from how much energy you actually use rather than from a panel you happened to like. In practice it is a backwards calculation: define a daily energy budget in watt-hours, estimate how many usable hours of full sun your location gives, account for the losses between panel and battery, and only then arrive at an array size in watts.

For a liveaboard boat or a serious overland rig, this matters more than on a house roof. Space is limited, you cannot simply bolt on another twenty panels, and you are often anchored or parked somewhere far from a charging point. Getting the sum roughly right is the difference between waking up to full batteries and nursing a flat house bank by torchlight.

Step one: build a daily energy budget

Everything starts here. List every load, estimate how many hours per day it runs, and convert to watt-hours per day (watts multiplied by hours). The classic example is the fridge, and it is also where most people overestimate. A 12 V compressor fridge might pull around 3 A — roughly 36 W — while the compressor is actually running, but it cycles on and off rather than running flat out. If it runs about a third of the time, the average is closer to 1 A, so over 24 hours you are looking at something on the order of 250 to 350 Wh, not the 860 Wh you would get by assuming it never switches off. Hot ambient temperatures, a poorly insulated box or a freezer setting push that figure up, so budget generously for the conditions you will actually be in.

Add lighting, water pump, instruments, the NMEA 2000 network, laptop and phone charging, an inverter for occasional AC loads, and anything seasonal like a fan or a watermaker. Be honest, and budget for the season you will be out in — a summer cruising budget with the fridge working hard is very different from a winter one. If you want a structured way to do this, our power-budget planner walks through the loads systematically so nothing gets quietly left off the list.

The output of this step is a single number: your daily consumption in watt-hours.

Step two: peak-sun-hours, not daylight hours

Peak-sun-hours (PSH) is the key concept and the one most often misunderstood. It is not how long the sun is up. One peak-sun-hour equals one hour of sunlight at the standard test intensity of 1,000 W/m². A location that receives 5 PSH might have twelve hours of daylight, but only the energy equivalent of five hours at full strength once you account for morning and evening angles, atmosphere and weather.

Much of Southern Africa is genuinely good for solar, with many inland areas seeing roughly 5 to 6 PSH averaged across a clear year, dropping in winter and under coastal cloud. Coastal and offshore conditions can be lower and far more variable — fog, swell-borne spray and overcast days all cut the figure. Treat any single PSH number as a regional, seasonal average, and size for the worse months you intend to be out, not the annual best case.

A first-pass array estimate looks like this:

Array watts ≈ daily watt-hours ÷ peak-sun-hours ÷ system efficiency

Step three: derate for the real world

Panels never deliver their nameplate rating in service. You lose output to:

• Heat. Panel output falls as cell temperature rises, and panels bolted to a hot deck or metal roof run well above air temperature. This is a real, continuous loss in our climate.
• Dirt, salt and bird mess. A salt-hazed or dusty panel can lose a meaningful slice of output.
• Wiring and controller losses. Cable voltage drop and the charge controller's own conversion efficiency both take a cut.
• Battery round-trip efficiency. Energy stored and later withdrawn is never returned in full; lithium is good here, lead-acid noticeably less so.
• Mismatch and angle. Panels lying flat rather than tilted toward the sun, and slight differences between panels in a string, both cost output.

A sensible rule of thumb is to plan around an overall system efficiency of roughly 60 to 75 percent — in other words, derate the nameplate by about a quarter to two-fifths. This is approximate and conditions-dependent, but planning around it stops you building an array that looks fine on paper and disappoints in the anchorage.

Step four: size the panels, then size the controller separately

These are two distinct jobs. The array is sized to meet your daily watt-hour budget after derating, as above. The MPPT charge controller is sized to handle that array's electrical output safely, and it has its own limits to respect:

• The array's open-circuit voltage (Voc) in the coldest weather you will see must stay safely below the controller's maximum PV input voltage. Panel voltage rises as temperature falls, so the worst case is a cold, clear morning — at altitude or on a cold inland night this can be well above the rated Voc. Exceeding the controller's limit can destroy it.
• The controller's rated output current must comfortably handle the maximum charge current the array can push into your battery bank.

An MPPT controller is strongly preferred over an older PWM type for anything but the smallest systems, because it harvests more from the panels and lets you wire panels in series at higher voltage for lower cable losses on long marine runs. How you wire the array — series for voltage, parallel for shade tolerance, or a combination — is itself a sizing decision that interacts with the controller's window. Confirm the numbers against the controller datasheet, and where the stakes are high have a qualified installer check the voltage limits and the high-current battery-side wiring and fusing.

Shading: the hidden array-killer

On paper a series string of panels is elegant. In reality, partial shade on even one panel in a series string can drag the whole string's output down disproportionately, because the shaded cells limit current through the rest. This is the single biggest difference between a clean roof and a real boat or vehicle.

A liveaboard deck is a forest of shadows — boom, sails, radar arch, antennas, bimini, dinghy on the foredeck — and those shadows track across the panels all day. An overland rig parked under the only available tree faces the same problem. Practical responses include splitting the array across more than one controller or MPPT input, choosing a parallel or mixed wiring arrangement so a shaded panel hurts less, and simply placing panels where shadows fall least. Plan the layout around the shade you will actually live with, not an idealised clear sky.

Marine versus vehicle mounting

The constraints differ. On a boat, rigid panels go on arches, hard tops and coachroof; flexible or semi-flexible panels suit curved bimini and sprayhood surfaces but generally run hotter and last fewer years. Everything must survive salt, UV and motion, with marine-grade fixings and properly sealed cable entries. On an overland vehicle, the roof is flatter and more uniform but space is tighter and weight high up affects handling; panels bake against the roof, and you trade tilt for low profile and ground clearance under branches.

In both worlds, the rated panel figure is a laboratory number. Build the system around your derated, real-world expectation, size the controller to match the array honestly, and you will have a system that keeps up rather than one that merely looks good in the brochure.

Talk to PowerSol about specifying it

Sizing solar well is a chain of small, honest estimates — budget, peak-sun-hours, derating, controller limits and shade — and a weak link anywhere shows up as a flat battery. If you are planning a liveaboard or overland array, talk to PowerSol about power production and the monitoring and control that lets you verify the system is actually delivering. Tell us your loads, your cruising or travel grounds and the season you care about, and we will help you specify panels, an MPPT controller and the storage to match the build rather than guess at it.