So­lar Power

Fit­ting a so­lar set up is quite of­ten a win­ter, or early spring, project af­ter a sum­mer of think­ing ‘I wish we had so­lar pan­els…’, so here’s a guide to help if you’re think­ing along those lines

Canal Boat - - Back Cabin - WORDS BY MAR­TYN JUDD

Our guide to help you de­cide how many and what type of so­lar pan­els to in­stall on your boat

Pro­vid­ing so­lar power to a boat has be­come sig­nif­i­cantly more cost ef­fec­tive over the last few years as so­lar panel prices have fallen and the tech­nol­ogy has be­come more main­stream. Nowa­days, so­lar pan­els are smaller and less ob­tru­sive, the elec­tron­ics are more re­li­able and nearly every­thing can be fit­ted by any­one with only ba­sic DIY skills.

Es­sen­tially, boat so­lar sys­tems are de­signed to work along­side any mains hook-up that you may use while moored, plus they give you the free­dom to moor

any­where and en­joy the peace and quiet with­out the drone of an en­gine or gen­er­a­tor to keep the bat­ter­ies charged. A ba­sic sys­tem has three

com­po­nents: so­lar panel(s), charge con­troller and bat­tery(ies). The pho­to­voltaic so­lar cells in the panel pro­duce elec­tric­ity, the con­troller regulates it, just as a mains charger would, and the bat­ter­ies store the power for later.

Typ­i­cally, a bat­tery (or bank of bat­ter­ies) is 12v, al­though 24v sys­tems are be­com­ing more pop­u­lar.


The size of your so­lar sys­tem de­pends on what you want it to do; there is no limit to what you can achieve apart from size and bud­get. You should first do a power au­dit, list­ing all of the elec­tri­cal de­vices you use reg­u­larly, their power con­sump­tion in watts ( W) and how long they are used dur­ing any given 24-hour pe­riod.

In this ex­am­ple, we have stuck to a sim­ple 12v sys­tem and as­sumed that heat­ing wa­ter is mainly done by the calori­fier.

For com­plete au­ton­omy (i.e. not hav­ing to top up the sys­tem with power from a gen­er­a­tor), you need a sys­tem that will re­place this en­ergy each and ev­ery day. In real­ity, many peo­ple find they ac­tu­ally use less power af­ter in­stalling so­lar pan­els, mainly be­cause they are more aware of where the elec­tric­ity is com­ing from; so you can try in­stalling a sys­tem that is slightly smaller than you cal­cu­late at first, but if you do that, make sure the charge con­troller is large enough to ac­com­mo­date an ex­tra panel or two in case you want it later.


So­lar pan­els pro­duce dif­fer­ent amounts of power de­pend­ing on the type of so­lar cells they use and the qual­ity of construction. There are two types of so­lar cells avail­able: poly­crys­talline and monocrys­talline. Poly­crys­talline has a more blue colour, whereas the monocrys­talline cells are al­most black.

Monocrys­talline cells are more ef­fi­cient and tend to be able to pro­duce more power in low or vari­able light con­di­tions. The higher the ef­fi­ciency, the bet­ter, as this is a sign of a good qual­ity cell and re­sults in a smaller, more com­pact so­lar panel.

There is no easy way to tell the ef­fi­ciency of a so­lar cell, un­less the man­u­fac­turer de­clares it on their datasheet (a rep­utable com­pany al­ways will). How­ever, just look­ing at the size of the panel can be quite telling. For ex­am­ple, the small­est, most ef­fi­cient 100W so­lar pan­els are about 1.0m x 0.5m, whereas the cheap­est, least ef­fi­cient 100W so­lar pan­els might be 1.2m x 0.8m in size. In terms of area, the cheap­est may well be 90% larger and where space is re­stricted on the roof of a boat this se­verely lim­its your op­tions.

In terms of power pro­duc­tion, a sin­gle 100W ul­tra-high ef­fi­ciency panel us­ing monocrys­talline cells, for ex­am­ple, will pro­duce 600-750Wh a day in sum­mer, whereas a cheap 100W poly­crys­talline panel might only be 500- 600Wh.

This dif­fer­ence is even more marked when the sky is over­cast. Sun­power so­lar cells with 21.5% ef­fi­ciency will pro­duce up to twice as much power on an over­cast day as a stan­dard B-grade poly­crys­talline panel. As a rule of thumb, to achieve year-round per­for­mance, you can es­ti­mate the daily out­put of a panel at 70% max­i­mum for five hours a day, so 100W x 0.7 x 5 = 350Wh a day. Us­ing the above ex­am­ple with a 912Wh daily load, 300- 400W of pan­els would be ideal.

Monocrys­talline pan­els are avail­able with rigid alu­minium frames and also, more re­cently, as flex­i­ble lam­i­nated pan­els which weigh sig­nif­i­cantly less. Good qual­ity glass-fronted pan­els will be fit­ted with 3.2mm or 4.0mm tough­ened glass and a 35mm or 46mm alu­minium frame.


Now you need to se­lect the right so­lar charge con­troller and size it for your in­stal­la­tion. The two main types are PWM (Pulse Width Mo­du­la­tion) and MPPT (Max­i­mum Power Point Track­ing). Un­til quite re­cently, good qual­ity MPPT con­trollers were very ex­pen­sive, but prices are com­ing down quite quickly. Some start at less than £100, so most so­lar sys­tems are now us­ing MPPT tech­nol­ogy.

With PWM con­trollers some power is lost as the volt­age is re­duced from, typ­i­cally, 17v from the panel, to 14v for charg­ing the bat­ter­ies. In 95% of cases this is ab­so­lutely fine be­cause the cost of these con­trollers is much lower.

An MPPT con­troller al­lows the volt­age of the pan­els and the bat­ter­ies to be independent (so long as bat­tery volt­age is less than panel volt­age). In a nor­mal in­stal­la­tion, us­ing a typ­i­cal 17v panel, you can ex­pect 10-15% more charge cur­rent from an MPPT con­troller. With pan­els that have much higher volt­ages, MPPT con­trollers are es­sen­tial. You can also choose con­trollers with built-in dis­plays, re­mote pan­els or even the abil­ity to split the charge (eg, 10% to the starter bat­tery and 90% to the leisure

bat­ter­ies). The size of the con­troller is de­clared in amps (A). Each panel has a ‘short cir­cuit cur­rent’ value and these should be added to­gether to se­lect the con­troller. The 100W DC So­lar panel for ex­am­ple has a short cir­cuit cur­rent of 5.92A so four pan­els re­quire a con­troller of 24 amps or more. Fit­ting a con­troller that has ad­di­tional ca­pac­ity al­lows you to fit more pan­els later. The fi­nal part of the equa­tion is bat­ter­ies. As a gen­eral rule, the size of your bat­tery(ies) should al­low two days with­out any so­lar in­put, with­out go­ing be­low 50% state of charge. Us­ing our ex­am­ple of 912Wh, that sug­gests about 300Ah of bat­ter­ies (3 x 100Ah con­nected to­gether in ‘par­al­lel’). The cal­cu­la­tion is: Wh ÷ 0.5 x [num­ber of de­sired days of au­ton­omy] ÷ volt­age (912 ÷ 0.5 x 2 ÷ 12 = 304Ah)

As al­ways, never use stan­dard car bat­ter­ies for your leisure bank only ever use deep cy­cle ones, and when it comes to bat­tery stor­age ca­pac­ity, you can never have too much. There are how­ever, a few im­por­tant tips to con­sider to main­tain a healthy bat­tery bank and get the long­est life­time: When con­nect­ing bat­ter­ies in par­al­lel (ie, pos­i­tive to pos­i­tive, neg­a­tive to neg­a­tive), never use more than six in a string, other­wise you will strug­gle to keep the bat­ter­ies bal­anced Make sure that all bat­tery-to-bat­tery ca­bles are the same thick­ness and length and are as straight as pos­si­ble; this helps keep the bal­ance Never mix bat­ter­ies of dif­fer­ent sizes, makes or sig­nif­i­cantly dif­fer­ent ages Al­ways dou­ble check that the op­ti­mum charg­ing volt­ages for your spe­cific bat­tery type are com­pat­i­ble with the charge set­tings of the so­lar charge con­troller Use an ac­cu­rate bat­tery mon­i­tor so that you have as much in­for­ma­tion as pos­si­ble about the state of your bank Al­ways con­nect to the bat­ter­ies on a neg­a­tive ter­mi­nal at one end of the bank and a pos­i­tive at the other end; never con­nect to pos­i­tive and neg­a­tive ter­mi­nals at one end of the bank, as this will ef­fec­tively kill the bat­ter­ies very quickly


To add 240v you need an in­verter which con­nects di­rectly to the bat­ter­ies. This can be ei­ther ‘pure/true’ sine wave, or ‘mod­i­fied’ sine wave. Pure sine wave in­vert­ers are much more ef­fi­cient and will op­er­ate any de­vice; there are some lim­i­ta­tions to mod­i­fied sine wave in­vert­ers, al­though they are a lot cheaper.

One of the most im­por­tant things to bear in mind when con­sid­er­ing so­lar power, is that you tend to get what you pay for. That does not mean pay­ing over the odds, but the in­dus­try has ma­tured to the point where prod­ucts can­not sim­ply be sold cheaper; they can only be made more cheaply and sold for less.

‘When it comes to bat­tery stor­age you can never have too much. There are, how­ever, a few im­por­tant tips to con­sider’

POLY­CRYS­TALLINE 100W (1240 X 795mm)

MONOCRYS­TALLINE 100W (1340 X 540mm)

DC SO­LAR 100W (1038 X 527mm)

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