Step 1: The Energy Audit — What Does Your Load Actually Need?

Every system design starts with the daily energy load in watt-hours (Wh/day). This is the total electrical energy your home or facility consumes in 24 hours. Get this number wrong and every other calculation in your design is built on a false foundation.

How to Calculate Your Load

For each electrical appliance:

Energy (Wh/day) = Power (W) × Hours Used Per Day

Sum all appliances to get your total daily energy requirement. Here is a worked example for a typical 3-bedroom home in Nairobi, Kenya:

Appliance	Qty	Power (W)	Hours/Day	Energy (Wh/day)
LED Lights (15W)	8	120	5	600
TV (LED 40″)	1	80	4	320
Laptop	2	65	6	780
Phone Chargers	4	10	2	80
Refrigerator (efficient 150L)	1	60	12*	720
WiFi Router	1	10	16	160
Fan (ceiling, efficient)	2	50	8	800
Water Pump (1/2 HP, intermittent)	1	373	1	373
Security Lights (exterior LED)	4	15	8	480
TOTAL				4,313 Wh/day

Add a Safety Margin

Real-world consumption always exceeds theoretical calculations. Apply a 10–20% design margin to account for:

• Appliances running slightly longer than estimated
• Future load additions (new appliances, extra lighting)
• Inverter inefficiency (~92–96% efficiency = 4–8% energy lost as heat)
• Wiring losses (~2–3%)

Design Load = 4,313 Wh/day × 1.20 = 5,176 Wh/day ≈ 5.2 kWh/day

AC vs DC Loads

Most household appliances are AC (alternating current) and run through the inverter. Some systems include 12V or 24V DC loads (USB chargers, LED strips, DC fans) that run directly from the battery — bypassing the inverter and eliminating its 4–8% conversion loss. Where possible, use DC-rated appliances for high-use, low-power loads.

Step 2: Panel Array Sizing

With your daily energy load established, you can now calculate the required solar panel array size.

The Core Formula

Array Size (Wp) = Daily Load (Wh) ÷ Peak Sun Hours (h) ÷ System Efficiency

Where System Efficiency accounts for real-world losses in the array itself:

• Temperature derating: Panels lose ~0.4%/°C above 25°C. In Nairobi (average cell temp ~45°C) = ~8% loss
• Dust and soiling: 2–5% loss (varies with cleaning frequency and location)
• Wiring and connection losses: 2–3%
• MPPT efficiency: ~97% (3% loss)
• Combined system efficiency factor: typically 0.75–0.82

Using our Nairobi example with worst-month PSH of 4.2 hours (July) and a system efficiency of 0.80:

Array Size = 5,200 Wh ÷ 4.2 h ÷ 0.80 = 1,548 Wp

Using 400W panels: 1,548 ÷ 400 = 3.87 panels → round up to 4 panels (1,600 Wp)

String Configuration

Panels must be wired to match the charge controller’s input voltage range. For a 24V system with panels having Voc = 48V and Vmp = 40V:

• 2 panels in series: Vmp = 80V (exceeds typical MPPT range for 24V system)
• 2 panels in parallel: Vmp = 40V, current doubles → suitable for 24V MPPT
• Best for this example: 2S×2P (2 strings of 2 panels in series) = 80V, current doubled — check MPPT specs

Rule: Always check that your string Voc (open circuit voltage) does not exceed the charge controller’s maximum input voltage, especially in cold morning conditions when Voc is highest.

System Voltage	Typical MPPT Input Range	Recommended Vmp per String
12V	12–50V	17–18V (1 panel)
24V	24–100V	34–72V (1–2 panels)
48V	48–150V	68–120V (2–3 panels)

Step 3: Battery Bank Sizing

The battery bank stores energy to power loads at night and during cloudy days. Sizing is driven by two factors: days of autonomy (how many cloudy days the system must survive without sun) and depth of discharge (DoD) limits.

The Battery Sizing Formula

Battery Capacity (Ah) = (Load Wh/day × Autonomy Days) ÷ (System Voltage × DoD)

Days of Autonomy

For most residential off-grid systems in Africa: 2–3 days autonomy is standard. This covers typical cloudy spells without requiring an enormous (and expensive) battery bank. Critical systems (clinics, cold storage, telecom) may require 4–5 days.

Depth of Discharge by Battery Type

Battery Type	Recommended Max DoD	Cycle Life at Recommended DoD	Notes
Flooded Lead-Acid (FLA)	50%	300–500 cycles	Cheapest upfront; requires maintenance (water top-up); vents hydrogen gas
Sealed AGM Lead-Acid	50–60%	400–600 cycles	Maintenance-free; better vibration resistance; moderate cost
Gel Lead-Acid	50–70%	500–800 cycles	Deep discharge tolerant; sensitive to overcharging
LiFePO4 Lithium	80–90%	3,000–6,000 cycles	Best value over lifetime; safest lithium chemistry; preferred for new installations

Worked Example — 48V LiFePO4 Bank

Using our Nairobi 3-bedroom home: Load = 5,200 Wh/day, 2 days autonomy, 48V system, 85% DoD (LiFePO4):

Capacity (Ah) = (5,200 × 2) ÷ (48V × 0.85) = 10,400 ÷ 40.8 = 255 Ah at 48V

Use 2× 200Ah 48V LiFePO4 batteries in parallel (400Ah total) — this gives 2.8 days true autonomy at 85% DoD, with buffer for efficiency losses.

Alternatively: 16× 200Ah 3.2V LiFePO4 cells wired 16S (in series) = 51.2V nominal, which is the standard LiFePO4 48V bank configuration.

Step 4: Charge Controller Sizing

The charge controller sits between the solar array and the battery bank, regulating the charging process and protecting batteries from overcharge.

MPPT vs PWM — Choosing the Right Type

Feature	PWM (Pulse Width Modulation)	MPPT (Maximum Power Point Tracking)
Efficiency	70–80%	93–97%
Panel voltage flexibility	Must match battery voltage ±	Accepts wide voltage range (up to 150V+)
Performance in partial shade	Poor	Good (tracks best power point)
Performance in cold/cloudy	Poor (loses panel voltage benefit)	Excellent
Cost	Low	Higher (but pays back quickly)
Best for	Small 12V systems <400W; tight budgets	Any system above 400W; all 24V/48V systems

Diaspora Solar recommendation: Always use MPPT for systems above 400W. The efficiency gain of 15–25% over PWM pays for the cost difference within 12–18 months in African conditions.

Sizing the MPPT Controller

The required charge controller current rating:

MPPT Current (A) = Array Power (Wp) ÷ Battery Bank Voltage (V) × 1.25 (safety factor)

For our example: 1,600Wp ÷ 48V × 1.25 = 41.7A → use a 60A MPPT controller

Also verify the controller’s maximum PV input voltage is not exceeded by your string Voc (including cold temperature boost of ~10–15%).

Step 5: Inverter Sizing

The inverter converts DC battery power to AC for household appliances. Its size is determined by peak power demand — not average consumption.

Inverter Sizing Formula

Inverter Size (VA or W) = Sum of Simultaneously Running Loads × 1.25

Identify which loads will run at the same time at peak demand. In our Nairobi home, the realistic simultaneous peak load is:

• Fridge: 60W (running)
• Lights: 120W (all on)
• TV: 80W
• Laptops × 2: 130W
• Fan × 2: 100W
• Water pump start surge: 373W × 3 (surge = ~1,120W for 2–3 seconds)

Steady-state peak: 60 + 120 + 80 + 130 + 100 + 373 = 863W continuous
Surge: 863W − 373W + 1,120W = ~1,610W surge (2–3 seconds)

Inverter selection: 1,500W continuous / 3,000W surge inverter (most manufacturers rate surge at 2× continuous) — this covers our load with comfortable margin.

Inverter Types

Type	Output Wave	Cost	Suitable For
Modified Sine Wave	Stepped approximation	Low	Basic resistive loads only (lights, simple fans). Damages motor windings, sensitive electronics — avoid for modern appliances
Pure Sine Wave	True sine wave	Moderate	All household appliances. Required for motors, refrigerators, laptops, medical equipment
Hybrid Inverter-Charger	Pure sine wave	Higher	Combines inverter + MPPT charger + grid/generator transfer switch in one unit. Best for most residential systems
Multi-Mode (Grid-tied with battery)	Pure sine wave	High	Grid-tied with battery backup; export to grid; maximises solar self-consumption

Diaspora Solar recommendation: For off-grid and hybrid residential systems, use a Pure Sine Wave Hybrid Inverter-Charger (brands: Victron Multiplus, Growatt, Deye, Sunsynk). These simplify wiring, enable generator integration, and protect battery health with sophisticated charging algorithms.

Step 6: Wire Sizing & Voltage Drop

Undersized wiring is both dangerous (fire risk) and wasteful (energy lost as heat). Every cable in a solar system must be sized for two criteria simultaneously: current-carrying capacity (ampacity) and voltage drop.

Maximum Allowable Voltage Drop

• Panel to charge controller: ≤3% voltage drop
• Battery to inverter: ≤1% voltage drop (high current, short run)
• Inverter to distribution board: ≤2% voltage drop

Voltage Drop Formula

Cable CSA (mm²) = (2 × Length (m) × Current (A)) ÷ (Conductivity × Allowable Vdrop (V))

Where copper conductivity = 56 (S·m/mm²). Or use the simplified practical rule:

mm² = (Current (A) × Cable Length (m) × 0.04) ÷ Allowable % Voltage Drop

Worked Example — Battery to Inverter Cable

Inverter: 1,500W at 48V = 31.25A continuous. Cable run: 2m (battery to inverter). Allowable Vdrop: 1% of 48V = 0.48V

CSA = (2 × 2 × 31.25) ÷ (56 × 0.48) = 125 ÷ 26.9 = 4.6 mm² → use 6mm² cable (next standard size up)

System Section	Current (A)	Run Length	Min Cable Size	Recommended
Panel strings (1,600Wp, 4 panels, 2S×2P)	~10A per string	10m	4mm²	6mm² solar DC cable
Array combiner to MPPT (DC)	~20A	5m	4mm²	6mm² DC cable
MPPT to Battery Bank (DC)	42A (MPPT rated)	1m	10mm²	16mm² DC cable
Battery Bank to Inverter (DC)	32A (1,500W÷48V)	2m	6mm²	10mm² DC cable
Inverter AC output to Distribution Board	7A (1,500W÷230V)	5m	2.5mm²	2.5mm² AC cable

Always use DC-rated cable for all DC circuits. Standard AC building wire is not rated for the sustained DC current and voltage characteristics of solar installations — it can arc, degrade, and cause fires in DC applications.

Step 7: Protection Devices

Every solar system requires overcurrent and fault protection. Omitting fuses and circuit breakers is one of the most dangerous mistakes in DIY installations — and a common cause of system fires.

Location	Protection Device	Rating (for our example)	Purpose
Each panel string (+ve lead)	DC String Fuse	15A	Protects string cable from reverse current faults
Array DC main	DC Isolator Switch	32A / 100VDC	Safe array disconnection for maintenance
MPPT output (to battery)	DC Circuit Breaker	63A	Overcurrent protection on charge cable
Battery main fuse (at battery terminals)	ANL or Class T Fuse	100A	Critical: first protection for the battery — must be as close to battery as possible
Inverter DC input	DC Isolator	80A / 100VDC	Inverter disconnection for maintenance
Inverter AC output	AC MCB	16A	Output overcurrent and short circuit protection

Complete System Summary — Nairobi 3-Bedroom Home

Component	Specification	Quantity
Solar Panels	400W Monocrystalline, Vmp 40V, Voc 48V	4 panels (1,600Wp total)
Panel Configuration	2 strings × 2 panels in series (2S×2P)	—
MPPT Charge Controller	60A, 48V system, max PV input 150V	1 unit
Battery Bank	48V 200Ah LiFePO4, 2 units in parallel	400Ah total (19.2 kWh usable at 85% DoD)
Inverter	Pure Sine Wave Hybrid, 1,500W cont./3,000W surge, 48V DC	1 unit
Panel-to-MPPT Cable	6mm² DC solar cable (red/black)	~25m total
MPPT-to-Battery Cable	16mm² DC cable	~3m
Battery-to-Inverter Cable	10mm² DC cable with lugs	~5m
String Fuses	15A DC blade fuse with holder (×2 strings)	2
Battery Main Fuse	100A ANL fuse + holder	1
DC Isolators	32A array / 80A inverter	2
Mounting Structure	Aluminium rail, roof clamps for metal sheet	Per roof layout

Estimated System Cost Range (Kenya, 2026)

Component	Estimated Cost (KES)
4× 400W Monocrystalline Panels	KES 48,000 – 60,000
60A MPPT Charge Controller	KES 18,000 – 28,000
2× 200Ah 48V LiFePO4 Battery	KES 120,000 – 180,000
1,500W Pure Sine Hybrid Inverter	KES 25,000 – 40,000
Cables, Fuses, Isolators, Connectors	KES 12,000 – 18,000
Mounting Structure	KES 8,000 – 15,000
Installation Labour	KES 15,000 – 25,000
TOTAL ESTIMATE	KES 246,000 – 366,000

Prices are estimates based on Kenyan market rates (2026). Actual costs vary by brand, supplier, and exchange rate. Contact Diaspora Solar for a formal quote with current pricing.

What Comes Next: Installation, Wiring & Safety

With your system fully sized on paper, the next and final article in this series covers physical installation:

• Mounting panel frames and rails safely on different roof types
• DC wiring layout: correct polarity, conduit routing, labelling
• Grounding and earthing (critical for lightning protection in Africa)
• AC distribution board connection
• System commissioning and first-charge procedure
• Maintenance schedule: cleaning, terminal checks, capacity testing

← Part 1: Why Solar Systems Fail  |  ← Part 2: Geographic Orientation & Site Assessment