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Solar panel sizing

Size a panel and a battery for a load that runs continuously. The figures are given twice: once at typical sun hours, and once at the worst case, which is the one that matters.

Load

The node's average draw at the system voltage, across the whole day.

Hours per day at full sun equivalent, not hours of daylight.

Uyo's rainy season sits nearer 2.5 to 3 for weeks at a time.

Everything lost between the panel face and the battery.

How long it must run with no useful sun at all.

Depth of discharge presets

Panel, sized for the worst case

23W

At 4 peak sun hours you would only need 14 W. Size for the bad week, not the good one.

Battery

11Ah

Panel, typical sun

14W

Worst case factor

1.6x

Daily energy

34.6Wh

Daily charge

2.88Ah

  • Caution

    Size for the bad week, not the annual average

    The binding constraint on an off grid node is the worst consecutive run of overcast days, not the yearly mean. A panel sized for 4 peak sun hours needs 23 W to hold the same load through a 2.5 hour stretch, which is 1.6 times the typical figure. In Uyo the rainy season sits nearer 2.5 to 3 peak sun hours for weeks at a time.

  • Note

    What the derate covers

    The 60 % derate is not one loss, it is all of them stacked: panel ageing, dust and dirt on the glass, the output drop as the cells heat up in the sun, charge controller conversion loss, wiring loss, and a panel mounted at whatever angle the pole allowed rather than the optimum. A PWM controller loses more than an MPPT one, so it deserves a lower figure.

  • Note

    Depth of discharge decides cycle life

    At 80 % depth of discharge the battery needs 11 Ah to cover 3 days. Running a lead acid battery past about 50 % shortens its life sharply. Lithium tolerates 80 % but still lasts longer if you do not use all of it.

Why this matters

A solar node sized from an annual average works for most of the year and dies in the rainy season. The constraint is never the average, it is the worst consecutive run of overcast days, and in Uyo that run can hold at 2.5 peak sun hours for weeks. The other half of the problem is the battery: a panel that cannot keep up needs a battery big enough to ride out the gap, and the two have to be sized together. A node that goes dark every wet season is not a node, it is a seasonal experiment.

Assumptions and sources

  • Peak sun hours, not hours of daylight. One peak sun hour is one hour at 1000 W/m², so a twelve hour day might deliver four peak sun hours.
  • The default 60 % derate is a rule of thumb covering panel ageing, dirt on the glass, output loss as the cells heat, charge controller conversion loss, wiring loss, and off angle mounting. A PWM controller deserves a lower figure than an MPPT one.
  • The load is treated as constant across the full 24 hours. A node whose load is concentrated during daylight needs less battery than this shows.
  • Battery capacity is sized from the load and the depth of discharge, not from the panel, because autonomy is about riding out days with no useful sun.
  • Panel nameplate watts are measured at standard test conditions, which are cooler and brighter than a roof in Uyo. The derate is what absorbs that gap.

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