Solar System Sizing
From Load List to Fully Sized System
Step-by-step solar system sizing using real engineering — not guesswork. The same method used by Maina Mumbi across East and West Africa for over 20 years.
“Most solar systems in East and West Africa are installed using guesswork — not engineering. Vendors sell kits that look impressive on paper but fail under real household loads.” — Maina Mumbi, Solar Prescription Initiative
Step 0
Choose Your System Type First
Before touching a calculator, match your situation to the right system type. This decision changes everything downstream.
System Type
Best For
Watch Out For
| On-Grid | Reliable grid areas, lower budgets | No backup during outages |
| Off-Grid ✅ | Rural areas, no grid access, full independence | Higher upfront cost, more maintenance |
| Hybrid | Unreliable grid + need backup | Moderate cost, more complex |
Step 1
Count Your Loads
Write down every device you plan to run. Find the wattage on the label or in the manual. Be honest — do not leave anything out.
Running Load
370W
(blender excluded)
Peak Load
1,370W
(blender included)
Step 2
Calculate Daily Energy Consumption
Multiply each device’s wattage by the hours you use it per day. This gives you Watt-hours (Wh) — the currency of solar sizing.
Formula
Device Wattage × Hours Per Day = Watt-hours (Wh)
Bulbs: 90W × 6 hrs
= 540 Wh
Fans: 30W × 8 hrs
= 240 Wh
TV: 50W × 6 hrs
= 300 Wh
Freezer: 200W × 10 hrs
= 2,000 Wh
Step 3
Select the Right Inverter
Your inverter must handle the startup surge of your largest appliance. A fridge or pump draws 2–3× its running wattage at startup.
Peak load = 1,370W → Add 25% safety margin:
→ Recommended: 2 kVA – 3 kVA pure sine wave inverter
⚠️ CRITICAL: Do NOT oversize your inverter
✗ High idle power drain
Drains your battery even with nothing connected.
✗ Low-load inefficiency
At partial load, efficiency drops — you waste energy as heat.
✗ Battery damage
Voltage drops and deep cycling shorten battery lifespan.
✗ Less money for batteries
Bigger inverter = less budget for batteries = shorter backup.
✅ Rule: Match your inverter to your actual peak load.
Step 4
Size Your Solar Panels
Formula
Panel Power = Daily Energy ÷ Peak Sun Hours ÷ System Efficiency
3,080 Wh ÷ 5 PSH ÷ 0.80 = 770W minimum
Peak Sun Hours by Location
6.5
Lodwar, Kenya
5.5
Nairobi / Colorado
5.2
Mombasa / Dar es Salaam
4.8
Eldoret, Kenya
6.5
Southwest US (AZ/NV)
5.0
Southeast US (TX/FL)
4.2
Midwest/Northeast US
3.5
Seattle / Pacific NW
Step 5
Select Your Charge Controller
Always use MPPT — never PWM for systems above 400W
✓ MPPT (Recommended)
- Extracts up to 30% more power
- Converts excess voltage into current
- Required for any LiFePO₄ setup
- Best for any system over 400W
PWM (Small Systems Only)
- Simpler and cheaper
- Reduces panel output to battery voltage
- Fine for systems under 400W
- Less efficient in low-light
Step 6
Size Your Battery Bank
Your battery bank must carry you through cloudy days without solar. Design for 1–2 days of autonomy minimum.
Formula
Battery Ah = (Daily Energy × Days) ÷ (Voltage × DoD × Efficiency)
Example (1 day, 24V, 80% DoD LiFePO₄, 90% eff.):
= 3,080 × 1 ÷ (24 × 0.80 × 0.90) = 178 Ah @ 24V
→ Round up to: 200Ah @ 24V LiFePO₄ OR 2 × 200Ah @ 12V in series
80–90%
LiFePO₄ Usable Depth
50%
Lead-Acid Max Usable Depth
3,000+
LiFePO₄ Cycle Life
Full System Summary
Your Complete Sized System
| Component | Recommended Size |
|---|---|
| Solar Panels | 800W – 1,000W (4 × 250W) |
| Inverter | 2 kVA – 3 kVA Pure Sine Wave |
| Charge Controller | 40A – 60A MPPT |
| Battery Bank | 200Ah @ 24V LiFePO₄ |
| Daily Energy | 3,080 Wh (≈ 3.1 kWh/day) |
| System Voltage | 24V DC |
“Solar system sizing is not guesswork. It is engineering. Every undersized battery, every oversized inverter, every wrong cable choice — has a victim.” — Maina Mumbi
Quick Reference
System Size by Use Case
Want Maina to Size Your System Live?
Join the Solar Prescription Session
45 minutes. Real calculations. Maina walks you through a live sizing exercise using your specific location data — not a generic template.