Sizing a Solar Home System for Rural Electrification

Step one: build the daily load profile

List every appliance, its power in watts and the hours per day it actually runs — not the hours it might run. Multiply and add. A worked domestic example for a single household:

Two cautions. First, adding a small refrigerator changes everything: a 60 W DC fridge running about a third of the day adds roughly 480 Wh and would more than double the system. Second, if the household already runs a television and a fridge, the appliances are usually alternating-current types and an inverter enters the design, with its own losses.

Step two: worst-month sunshine, not annual average

Peak sun hours express how many hours of full-strength sun the site receives on an average day, and the figure to design on is the worst month’s. Across much of the world this is between about 3 and 5.5 hours, with monsoon and cloudy seasons at the low end; a site with 5.5 hours in the dry season may fall to 3.2 in the wet season. Designing on the annual average is the most common cause of systems that work for eight months and fail in the other four.

Step three: size the array

Assumptions: a 410 Wh daily load, a battery charge/discharge efficiency of 90%, wiring losses of 5%, a controller efficiency of 90% (slightly conservative for a good MPPT unit), worst-month peak sun hours of 3.6, and a further 0.85 factor for panel temperature and dust.

• Energy that must be delivered by the array: 410 ÷ (0.90 × 0.95 × 0.90) = 533 Wh per day.
• Panel rating: 533 ÷ (3.6 × 0.85) = 174 Wp. Specify 200 Wp, which gives practical margin for ageing, a dirty panel and a slightly worse month than the data suggests.

Use an MPPT controller if the panel’s maximum-power voltage sits well above the battery voltage — a 200 Wp module is almost always a higher-voltage type, and a PWM controller would waste a large share of its output. The comparison is set out in MPPT versus PWM controllers.

Step four: size the battery

Assume two days of autonomy and a lithium iron phosphate pack at 12.8 V with 80% depth of discharge and a 0.9 temperature factor. The battery must hold 410 × 2 = 820 Wh; dividing by 0.8 gives 1,025 Wh of nameplate energy; dividing again by 0.9 gives 1,139 Wh, or about 89 Ah at 12.8 V. A 100 Ah pack covers it with a little headroom. A lead-acid pack at 50% depth of discharge would need 820 ÷ 0.5 ÷ 0.9 = 1,822 Wh, about 152 Ah at 12 V — two 100 Ah blocks, or one 150 Ah block, at three times the weight. The trade-offs are covered in LiFePO4 versus lead-acid in hot climates, and the autonomy arithmetic in more detail in battery autonomy for cloudy weeks.

DC appliances or an inverter?

DC appliances avoid conversion losses and are usually more efficient, but they are harder to source and repair in remote areas, and their range is narrower. An inverter opens the system to ordinary appliances at the cost of roughly 10–15% conversion loss plus standby consumption. Standby is the trap: a 1 kW inverter drawing 15 W at idle consumes 360 Wh over 24 hours, which is nearly as much as this household’s entire daytime-plus-evening load of 410 Wh. If an inverter is unavoidable, size it close to the real peak load rather than generously, and switch it off when not in use; that single habit can change the array from 200 Wp to 150 Wp.

Distribution and protection

• Fuses or circuit breakers on the array, battery and load circuits, close to the battery, rated for DC and for the cable’s current-carrying capacity.
• Cable sized for voltage drop, not just for current. On a 12 V or 24 V DC system a 3% drop is a reasonable ceiling; on a long run this pushes the cross-section up sharply, which is one argument for choosing a 24 V or 48 V system on larger installations.
• A low-voltage disconnect to protect the battery from being drained flat, and separate over-current protection for each load circuit so a fault in one takes out only that circuit.
• Lockable battery enclosure, ventilated if lead-acid is used, with the terminals covered and the wiring labelled.
• Earthing arrangement appropriate to the site and to local practice, with surge protection where lightning is common.

Plan maintenance and replacement from day one

A solar home system is bought once and maintained for years, so the business case only balances if the operating cost is planned. Budget for battery replacement within the system’s life — commonly every two to three years for lead-acid in a hot climate, longer for lithium — and set aside that amount from the start rather than treating the first replacement as an emergency. Train at least one person locally to clean the panel, check terminals for corrosion and tightness, read the controller and confirm that the low-voltage disconnect is not tripping regularly, because repeated tripping means the system is undersized in practice. Keep a small stock of spare fuses, a spare charge controller and one replacement battery per group of households. The equipment side of the same discussion is on our solar home systems page.

How we can help

We are a China-based trading and project sourcing company, not a manufacturer. We review specifications and BOQs, match a product and configuration to your site conditions, source from qualified manufacturers, and manage quality through to pre-shipment inspection. If you are preparing a tender or specification and want a second opinion, contact us.