Why correct sizing matters
An undersized system will not power your essential loads, leaving you frustrated and dependent on generator backup. An oversized system wastes money on equipment you do not need. The goal is to match your system capacity to your actual energy consumption — not too little, not too much.
In Nigeria, where grid supply is unpredictable, correct sizing is even more critical. You need a system that handles your daily loads comfortably while providing enough battery reserve for overnight use and outages.
Step 1: List your loads
Start by listing every appliance you want to power with solar. For each appliance, record three things: its wattage (power rating), how many hours per day you use it, and how many of that appliance you have.
Common household load example
| Appliance | Quantity | Watts Each | Total Watts | Hours/Day | Watt-Hours |
|---|---|---|---|---|---|
| LED bulb | 10 | 6W | 60W | 8 | 480 Wh |
| Ceiling fan | 3 | 75W | 225W | 8 | 1,800 Wh |
| Refrigerator | 1 | 150W | 150W | 24 | 3,600 Wh |
| Television (43″) | 1 | 100W | 100W | 6 | 600 Wh |
| Wi-Fi router | 1 | 12W | 12W | 24 | 288 Wh |
| Laptop | 1 | 65W | 65W | 5 | 325 Wh |
| Phone charging | 4 | 10W | 40W | 3 | 120 Wh |
| Water pump | 1 | 750W | 750W | 1 | 750 Wh |
| Total Daily Energy Need | 7,963 Wh | ||||
Refrigerator note
A fridge is rated at ~150W but does not run continuously. The compressor cycles on and off. A practical estimate is 36–48 hours of runtime per day divided by 24, giving roughly 1.5–2× the rated wattage as daily consumption. Always round up for refrigerators.
Step 2: Calculate total daily energy requirement
Add up all the watt-hours from your load table. This gives you your total daily energy requirement in watt-hours (Wh). For the example above, that is approximately 7,963 Wh per day, or roughly 8 kWh per day.
This is the single most important number in your solar design. Every other component — panels, inverter, batteries — is sized based on this figure.
Step 3: Determine required panel capacity
Solar panels do not produce their rated power for the full day. In Nigeria, you can expect an average of 4.5–5.5 peak sun hours per day (more in the north, slightly less in the south during rainy season). A conservative estimate uses 4.5 hours.
Panel capacity formula
Required Panel Watts = Daily Energy (Wh) ÷ Peak Sun Hours ÷ System Efficiency Factor
System efficiency accounts for losses from wiring, inverter conversion, dust, heat and battery charging. A typical factor is 0.70–0.80.
Using our example:
- Daily energy: 7,963 Wh
- Peak sun hours: 4.5
- System efficiency factor: 0.75
- Required panel watts: 7,963 ÷ 4.5 ÷ 0.75 = 2,360W ≈ 2.4 kW of panels
With 550W panels, you would need approximately 5 panels (2,750W total) to comfortably cover this load profile.
Step 4: Size your battery bank
Batteries must store enough energy to power your loads during non-solar hours — typically overnight and during cloudy periods. For Nigerian conditions where grid backup is unreliable, a good rule is to size batteries for at least one full day of autonomy (24 hours of usage without any solar or grid input).
Battery capacity formula
Battery Capacity (Wh) = Daily Energy (Wh) × Days of Autonomy ÷ Depth of Discharge
Depth of discharge (DoD) depends on battery type:
- Lead-acid: 50% DoD (do not discharge below 50%)
- Lithium-ion: 80% DoD
- LiFePO4: 90–100% DoD
Using our example with LiFePO4 batteries (90% DoD) and 1 day autonomy:
- Battery capacity: 7,963 ÷ 0.90 = 8,848 Wh ≈ 8.8 kWh
- With a 51.2V system: 8,848 ÷ 51.2 = 173 Ah
- Common option: 2× 100Ah 51.2V batteries = 200Ah total (10,240 Wh)
Step 5: Choose the right inverter
Your inverter must handle two things: the total simultaneous wattage of all loads running at once (peak load), and the system voltage of your battery bank.
- Continuous power rating — must exceed your maximum simultaneous load. Add a 20–30% safety margin.
- System voltage — must match your battery bank (48V systems are standard for residential installations above 3kW).
- Surge capacity — must handle the startup surge of motor-driven loads like refrigerators, pumps and air conditioners.
For our example, with a peak simultaneous load of around 1,400W (fridge + fans + lights + TV), a 3kVA/3.5kVA hybrid inverter on a 48V system would be appropriate, providing headroom for surge loads and future expansion.
Oversizing tip
Always design your system with 15–25% more panel and battery capacity than your current needs require. Your energy consumption will likely increase over time as you add appliances or as your household grows. It is far cheaper to install the extra capacity upfront than to retrofit later.
Quick sizing reference
| Home Profile | Typical Daily Use | Panel Size | Battery | Inverter |
|---|---|---|---|---|
| Basic (lights, fan, TV, fridge) | 3–5 kWh | 1.2–2 kW | 4–6 kWh | 2–3 kVA |
| Standard (+ AC, pump) | 6–10 kWh | 2.5–4 kW | 8–12 kWh | 3.5–5 kVA |
| Heavy (+ 2 ACs, washer) | 10–18 kWh | 4–7 kW | 12–20 kWh | 5–8 kVA |
| Large home (+ pool, 3+ ACs) | 18–30+ kWh | 7–12 kW | 20–40 kWh | 8–15 kVA |
Common sizing mistakes in Nigeria
- Ignoring the refrigerator — fridges consume more energy than almost any other household appliance because they run 24/7. Never underestimate fridge consumption.
- Oversizing the inverter, undersizing the panels — a large inverter with too few panels means you drain batteries faster than they can recharge.
- Using a single day's sunshine as the benchmark — size for at least 1 day of battery autonomy to handle cloudy days and short rainy-season sunlight.
- Ignoring motor startup surges — refrigerators, pumps and ACs draw 3–5× their running wattage for a few seconds at startup. Your inverter must handle this.
- Not planning for expansion — add 15–25% extra capacity now to avoid expensive upgrades later.
Where to go from here
These calculations give you a solid starting point, but every property is unique. A professional site survey considers roof orientation, shading, local weather patterns and wiring runs that a simple calculation cannot capture.
- Review our Solar Energy Basics if you need to understand the fundamentals
- Learn about panel choices in our Panel Selection Guide
- Get a professional system design by contacting UgoTech Energy for a free site survey