An undersized system disappoints. An oversized system wastes money. Here's the engineering method for getting it right — step by step.
Everything starts with what you actually consume. Not what you think you consume, not what your electricity bill says (bills include distribution losses and meter inaccuracies), but what each appliance draws and how long it runs.
| Appliance | Rated Watts | Actual Running Watts* | Typical Daily Hours | Daily Wh |
|---|---|---|---|---|
| LED tube light (18W) | 18 | 18 | 6 | 108 |
| Ceiling fan (standard) | 75 | 70 | 14 | 980 |
| Ceiling fan (BLDC) | 28–35 | 28 | 14 | 392 |
| Refrigerator (inverter-type, 260L) | 130 (rated) | 60–80 (avg) | 24 (compressor duty ~40%) | 1,440–1,920 |
| Refrigerator (non-inverter, 260L) | 150 (rated) | 150 (on), 0 (off) | 24 (compressor duty ~50%) | 1,800 |
| Washing machine (front-load) | 500 | 300 (avg cycle) | 1 (per load) | 300 |
| TV (43" LED) | 60 | 55 | 5 | 275 |
| Laptop charger | 65 | 45 (avg) | 6 | 270 |
| WiFi router | 12 | 12 | 24 | 288 |
| Water purifier (RO) | 60 | 60 | 1 | 60 |
| Mixer-grinder | 750 | 500 (avg) | 0.25 | 125 |
| Iron (steam) | 1,200 | 600 (cycling) | 0.5 | 300 |
| AC (1.5 ton, inverter, 5-star) | 1,500 (rated) | 500–1,100 (variable) | 8 | 4,000–8,800 |
| AC (1.5 ton, non-inverter) | 1,500 (rated) | 1,500 (on), 0 (off) | 8 (compressor duty ~60%) | 7,200 |
| Water heater (geyser, 15L) | 2,000 | 2,000 | 0.5 | 1,000 |
| Induction cooktop | 2,000 | 1,200 (avg use) | 1 | 1,200 |
*Running watts differ from rated watts because of duty cycling (compressors), power factor, and inverter-type variable-speed operation.
The most common sizing mistake: treating rated watts as running watts. A 1.5-ton inverter AC rated at 1,500 W doesn't draw 1,500 W continuously — at 35°C ambient with a thermostat set to 24°C, it runs at 500–800 W most of the time, spiking briefly during compressor startup. Measure with a plug-in power meter (₹500–1,000) for accurate data.
Sum all appliance Wh/day values from your load audit. Add system losses:
Total system derating factor: multiply by 0.75–0.82 (i.e., 18–25% total loss). For a conservative estimate, use 0.78.
Example: load audit shows 12,000 Wh/day. Adjusted requirement: 12,000 ÷ 0.78 = 15,385 Wh/day of solar generation needed.
Array size (kWp) = Daily generation needed ÷ (Peak Sun Hours × 1,000)
For Bangalore (PSH = 5.0 average, 3.0 worst month):
For off-grid systems, size for worst-month PSH. The system must be self-sufficient year-round. For grid-connected systems, annual average is acceptable — the grid covers monsoon shortfalls.
Battery capacity depends on three factors:
Formula: Battery capacity (kWh) = (Daily load × Autonomy days) ÷ DoD
Example: 12 kWh/day load, 1.5 days autonomy, LFP at 85% DoD:
Battery = (12 × 1.5) ÷ 0.85 = 21.2 kWh → specify 22–24 kWh battery bank.
Same scenario with lead-acid at 50% DoD:
Battery = (12 × 1.5) ÷ 0.50 = 36 kWh → 50% more battery capacity needed, at lower cycle life.
The inverter must handle two things: the continuous running load and the peak surge load.
Continuous load: The sum of all appliances that could run simultaneously. Not the sum of all appliances (you don't run the iron and the mixer while the AC and washing machine are on) — use a realistic worst-case simultaneous load. Typically 60–75% of the total installed load.
Surge load: Motor-driven appliances (pump, AC compressor, washing machine motor, mixer) draw 3–7× rated watts for 0.5–3 seconds during startup. A 370 W water pump surges to 1,100–1,500 W. A 1,500 W AC compressor surges to 4,500–6,000 W. The inverter must handle the highest simultaneous surge without tripping.
| System Size | Typical Continuous Load | Peak Surge Requirement | Inverter Rating |
|---|---|---|---|
| Small (2–3 kWp, no AC) | 800–1,200 W | 2,500–3,500 W | 2 kW (2× surge) |
| Medium (3–5 kWp, 1 AC) | 1,500–2,500 W | 5,000–8,000 W | 3–5 kW |
| Large (5–10 kWp, 2+ ACs) | 3,000–5,000 W | 10,000–15,000 W | 5–8 kW |
Alloy Charge's FreedomVolt inverter delivers 2× rated capacity as surge for 5 seconds — enough for any motor startup. The pure sine wave output runs sensitive electronics (laptops, medical devices, BLDC fans) without issues. Low-load efficiency remains above 90% even at 10% of rated capacity — critical for nighttime loads (WiFi router + fridge = 80–100 W on a 3 kW inverter).
Each 540 W panel measures approximately 2.3 m × 1.1 m = 2.53 m². With inter-row spacing to avoid self-shading (panels tilted at 10–15° need roughly 1.5× their footprint for spacing), effective area per panel is ~3.8 m².
Array size vs roof area needed:
| Array Size (kWp) | Number of 540 W Panels | Roof Area Needed (with spacing) |
|---|---|---|
| 2.0 | 4 | ~15 m² |
| 3.0 | 6 | ~23 m² |
| 5.0 | 10 | ~38 m² |
| 8.0 | 15 | ~57 m² |
| 10.0 | 19 | ~72 m² |
If roof area is limited, consider higher-efficiency panels (TOPCon 580 W or HJT 600 W in the same frame size), bifacial panels on elevated mounting (captures reflected light from the roof surface), or ground-mounted arrays if land is available.
We run a detailed load audit, assess your location's irradiance data, and design a system sized for your actual needs — not a standard package.
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