Sizing a Solar System for a Rural Home That Also Wants to Run a Vaccine Fridge
A household in a rural municipality of Tanahun has an unreliable grid connection and wants a solar system for six LED lamps of 9 W each used 5 hours a night, one television of 60 W used 4 hours, one fan of 45 W used 6 hours in summer, and mobile phone charging at 10 W for 3 hours. The site receives an average of 4.5 peak sun hours, and the installer assumes 70 per cent system efficiency. The family has saved Rs 60,000. A local vendor offers a package of a 150 W panel, a 12 V 100 Ah battery, a 300 W inverter and a charge controller for Rs 55,000, promising 'full day and night power'. The family also wants, later, to run a small refrigerator for a nearby health post that averages 40 W continuously, but no decision has been taken yet.
1. Calculate the daily energy demand and check whether the vendor's package can meet it. State your assumptions.
Read the worked answer
Daily energy:
- Lamps: 6 × 9 W × 5 h = 270 Wh
- Television: 60 W × 4 h = 240 Wh
- Fan: 45 W × 6 h = 270 Wh
- Phone: 10 W × 3 h = 30 Wh
- Total = 810 Wh/day
Panel: 810 ÷ (4.5 peak sun hours × 0.70) = 810 ÷ 3.15 ≈ 257 W, so about 300 W gives a safe margin. The vendor's 150 W panel would make only 150 × 4.5 × 0.7 ≈ 472 Wh/day, roughly 58 per cent of demand.
Battery: at 50 per cent depth of discharge, a 12 V 100 Ah battery gives 12 × 100 × 0.5 = 600 Wh of usable energy - less than one day's need, and forcing it deeper will shorten its life. For one day at 50 per cent DoD you need 810 ÷ (12 × 0.5) = 135 Ah; for two cloudy days, 270 Ah.
Inverter: simultaneous load is about 54 + 60 + 45 + 10 ≈ 170 W, so 300 W is adequate.
Conclusion: the package fails on generation and on storage. At Rs 55,000 it is not a bargain; ask for a 300 W panel with a 150 Ah battery (one-day autonomy) or 270 Ah (two-day autonomy).
2. The health post now wants the fridge added on the same system. Recalculate and explain any change in system voltage.
Read the worked answer
Fridge: 40 W × 24 h = 960 Wh/day. New total = 810 + 960 = 1,770 Wh/day.
- Panel: 1770 ÷ 3.15 ≈ 562 W, so a 600 W array.
- Battery for one day: 1770 ÷ (12 × 0.5) = 295 Ah (about 300 Ah); for two days, 590 Ah.
Voltage: at 12 V, 600 W means about 50 A; at 24 V it is about 25 A. Cable loss is I²R, so halving the current cuts loss to one quarter for the same cable, and thinner cable and a smaller controller become possible. A 600 W array should therefore be wired as a 24 V system with an MPPT controller rated at least 30 A.
Two professional cautions. First, 40 W is an average, not a nameplate rating; vaccine fridges cycle, so measure real 24-hour consumption before buying. Second, this is a critical load: add a temperature logger and alarm, a low-voltage disconnect that protects the fridge circuit, and a grid or generator backup, because a vaccine fridge that silently loses power destroys the vaccines.
3. The family asks: 'Can we buy the 150 W panel now and simply add more batteries later?' Answer with reasoning.
Read the worked answer
No. A battery is a store, not a source; it can only return energy that the panel has put in. With a 150 W panel supplying about 472 Wh/day against a demand of 810 Wh/day, the bank runs a deficit of about 340 Wh every day. Adding batteries spreads that same deficit over more cells, so all of them sit at a low state of charge.
Chronic undercharging causes lead sulfate to harden on the plates (sulfation), capacity falls month by month, and the extra batteries die early - the money is lost twice, first on the purchase and then on the replacement.
The energy balance must be closed at the panel first: raise generation to about 300 W, then size storage from the load and the number of autonomy days - 135 Ah for one day, 270 Ah for two, at 50 per cent depth of discharge. If the budget cannot move, the honest alternative is to cut the load, for example shorter television hours or a more efficient fan, until 472 Wh/day is genuinely enough.