Photovoltaics & Storage

The design load is 2.73 kWh/day: 2.0 kWh of induction cooking, 0.26 kWh of water treatment (pasteurizer, UV-C, chlorinator), and under 0.5 kWh of accessories (lights, phones, mesh node, radio, fan, sensors). At a design solar resource of 5.5 kWh/m²/day — typical for East Africa, the Middle East, and South Asia — with a 0.78 system derate for dust, temperature, and wiring, each watt of PV delivers about 4.3 Wh/day. The load therefore needs ~635 W of array, which two standard 320 W_p modules supply with a 1% margin: 640 W × 4.29 = 2.75 kWh/day.

There is no AC inverter anywhere in the unit. The induction hob is driven by a resonant DC-fed stage directly from the 48 V bus; a 48→12 V/5 V converter feeds lights, comms, sensors, the UV-C cell, and the chlorinator. Every conversion stage removed is 3–8% of energy and one field-failure mode fewer — and 48 V is below the 60 V DC touch-safe threshold, so household maintenance does not require an electrician. The choice pays a bonus later: standard e-bike packs are 48 V, which is what makes the battery-swap economy of the extensions chapter possible without additional electronics.

Battery sizing follows the evening: 1.2 kWh for the dusk meal (the main meal in the camps this unit serves — a daylight-only cooker was rejected on cultural grounds, not technical ones), ~0.5 kWh of overnight lights, comms, and fans, and buffer. LiFePO₄ at 6,000 cycles is 16 years of daily cycling — twice the 8-year design life — and is the safest common lithium chemistry in a hot, dusty enclosure.

The design brief asked for battery or centrifugal storage, so the flywheel got a fair hearing and lost on arithmetic. A flywheel sized for the evening meal (1.5 kWh at dusk) retains only ~54% of its charge after 12 hours even at an optimistic 5%/hour standby loss — low-cost bearings do worse — and small steel flywheels cost roughly $2,000/kWh against $160/kWh for LiFePO₄: 12× the price to lose half the dinner by dawn. Flywheels make sense for seconds-to-minutes surge buffering, but the induction driver's surge is already handled by $20 of extra battery headroom. Physics, not fashion, picked the battery.

Shelter roofs — tarp and sheet metal — cannot carry 18 kg glass modules, and flexible thin-film laminates cost more per watt and degrade fast in equatorial heat. So modules live on a lockable ground rack, and the mesh network is the real lock: every module is network-registered, a panel that leaves its geofence or drops off-mesh raises an alarm, and MPPTs accept only registered serials. A panel that cannot be quietly resold is a panel not worth stealing — the network is what makes household solar insurable.