The Solar Bible

A solar panel's nameplate watt is measured under Standard Test Conditions (1,000 W/m², 25 °C cell temperature). What matters on a roof is how many hours per day the sun delivers that reference intensity — peak sun hours (PSH). Most of Kenya enjoys 5.0–6.0 PSH, among the highest sustained figures anywhere, with the arid north (Turkana, Marsabit) higher still and the misty highlands lower. Daily energy is, to first order, simply array size × PSH × system efficiency.

The catch is the performance ratio (PR) — the slice of theoretical yield you actually keep after inverter losses, wiring, soiling, heat and mismatch. A well-built system in Kenya runs a PR of about 0.75–0.80; a dusty, hot, badly-strung one can drop below 0.65. That gap is the difference between a system that pays back and one that disappoints, and it is decided at design and install, not at purchase.

So sizing starts from the load, not the roof. Measure the daily energy demand in kWh, divide by PSH and PR, and you have the array size. Working from "how many panels fit" is how sites end up with arrays that look impressive and still fail to carry the afternoon chiller load.

Counter-intuitively, panels make less power as they get hotter — and on a Kenyan roof the cells routinely sit at 55–65 °C, far above the 25 °C test condition. Crystalline silicon loses roughly 0.35–0.45% of its output per °C above 25 °C (the module's temperature coefficient of power). At a 60 °C cell temperature that is a real-world loss of around 12–16% off the nameplate, every clear afternoon, before any other loss.

This is why mounting matters as much as the module. Panels need an air gap behind them to convect heat away; flush-mounting onto a hot iron-sheet roof with no clearance bakes them and quietly throws away yield. We design for ventilation, choose modules with a low (better) temperature coefficient for hot sites, and rate string voltages at the coldest dawn temperature — because cold mornings push voltage up toward the inverter's limit while hot afternoons pull it down.

For any site that wants power after dark or through an outage, the battery is the most expensive and most misunderstood part. Two numbers govern it: days of autonomy (how long the bank carries the load with no sun) and depth of discharge (DoD) — how much of the rated capacity you dare use each cycle. Lead-acid batteries hate deep cycling; draw them below 50% routinely and their life collapses. Lithium iron phosphate (LiFePO₄) will cycle to 80–90% DoD for thousands of cycles, which is why — despite a higher sticker price — it is usually the cheaper battery per usable kWh over its life.

Sizing the bank means inflating the energy you actually need by the usable DoD and the round-trip and inverter efficiencies. Skip that and the bank looks fine on paper but sags every cloudy week. We size for the worst realistic run of dull days at the site, not the annual average — a system that fails in the one rainy week that matters has failed, regardless of its yearly numbers.

A panel has one operating point where it delivers maximum power, and it moves constantly with sun and temperature. A maximum power point tracking (MPPT) charge controller or inverter continuously hunts that point; a cheaper PWM controller simply clamps the panel to the battery voltage and throws the difference away — often 20–30% of the harvest on a cold, bright morning. On anything beyond a tiny system, MPPT pays for itself quickly.

String design is where installs quietly go wrong. Wire too few panels in series and the string voltage never reaches the inverter's MPPT window on hot afternoons, so it under-harvests. Wire too many and the cold-morning open-circuit voltage can exceed the inverter's maximum and damage it. The string voltage must be checked at both temperature extremes, and the conductor sized so volt-drop stays under about 2–3% — long DC runs on undersized cable are a yield leak you never see on a spec sheet.

The right architecture follows the tariff and the load, not fashion. A business with a reliable grid and high daytime consumption wins most from a grid-tied system that offsets expensive daytime units — no battery, fast payback. A site with frequent outages needs a hybrid (battery-backed) system. Only a site with no grid at all — a remote lodge, a borehole, a mast — justifies a full off-grid design with the storage cost that implies.

The honest decision tool is levelised cost of energy (LCOE): total lifetime cost divided by total lifetime kWh. A well-designed Kenyan PV system delivers solar electricity at roughly KSh 8–15 per kWh over 25 years — well under both commercial grid tariffs and the KSh 40+ per kWh of diesel generation. That spread is the entire business case, and it is why we so often pair solar with an existing generator: the panels carry the day cheaply, the genset becomes true backup, and the diesel bill falls by half or more.