You want to go off-grid. Great choice.
But here's the honest truth: the #1 reason off-grid solar systems fail in the first year is not panel quality or inverter brand. It's poor sizing.
Too few panels, batteries that run flat after one cloudy night, an inverter that trips the moment a refrigerator compressor starts — these are all sizing mistakes. And they're entirely preventable.
In this guide, we'll walk through the six critical sizing steps for a residential off-grid solar system, with real formulas and a worked example you can adapt to your own situation.
1 Conduct Your Energy Audit
Before you touch a spec sheet, you need to know exactly how much energy you consume daily. Skipping this step — or guessing — is what causes every sizing problem downstream.
List every electrical appliance in the property: lights, fans, refrigerator, television, phone chargers, water pump, and anything else that draws power. For each device, write down its rated power in Watts and an honest estimate of how many hours per day it runs.
Multiply each device's power (W) by its daily hours of use to get its daily energy in Watt-hours (Wh). Sum everything up.
• 4 × LED lights 10W × 5 h = 200 Wh
• 1 × ceiling fan 55W × 8 h = 440 Wh
• 1 × refrigerator 90W × 10 h (compressor on-time) = 900 Wh
• 1 × TV 80W × 4 h = 320 Wh
• Phone chargers and misc. = 80 Wh
→ Raw daily total: 1 940 Wh/day
Now apply a system loss factor of 1.3 (30% for cable losses, temperature derating, inverter inefficiency, and dust on panels). This gives you the energy your solar array must actually produce.
Formula
Required PV Energy (Wh/day) = Daily Consumption × 1.3
Using our example: 1 940 × 1.3 = 2 522 Wh/day required from panels.
2 Size Your Solar Panels
Now you know the target. The question is how many panels you need to hit it.
The key variable here is Peak Sun Hours (PSH) — the number of hours per day during which sunlight intensity averages 1 000 W/m². This is not the same as daylight hours. It is a standardised measure of your location's solar resource.
Algeria is exceptionally well-positioned. Most of the country receives between 5 and 6.5 PSH per day, making it one of the best solar locations in the world. The Saharan south is closer to 7–8 PSH. Use data from NASA SSE, SolarGIS, or PVsyst's Meteonorm database for your exact location.
Formula
Total Peak Power (Wp) = Required PV Energy (Wh/day) ÷ PSH
Required PV energy = 2 522 Wh/day, PSH = 5.5 h
→ Total Wp = 2 522 ÷ 5.5 = 458.5 Wp
Using 200 Wp panels: 458.5 ÷ 200 = 2.29 → round up to 3 panels × 200 Wp
Always round up — never down. A 10% oversized array is cheap insurance. A 10% undersized array means flat batteries every second evening.
3 Choose the Right Inverter
The inverter converts DC power from your batteries to the AC power your appliances need. Getting the size wrong is one of the most expensive mistakes in an off-grid build.
The inverter must handle the sum of all loads that could run simultaneously, not just the average. And critically, motors and compressors (refrigerators, pumps, air conditioners) draw 3 to 7 times their rated current at startup for a fraction of a second.
The standard rule is to size the inverter at 125–130% of the total continuous load. For any motor loads, verify the inverter's surge rating covers the startup current of that device.
Formula
Inverter Size (W) = Sum of All Loads (W) × 1.25
Total simultaneous load = 10W×4 + 55W + 90W + 80W = 265W
→ Minimum inverter size = 265 × 1.25 = 331W
→ Select at least a 500W pure sine wave inverter (standard commercial step)
→ Verify surge rating ≥ 3× refrigerator = 270W surge ✓
Pure sine wave only. Modified sine wave inverters are cheaper but damage sensitive electronics (LED drivers, variable speed motors, some chargers). For a home system, always specify pure sine wave.
4 Size Your Battery Bank
The battery bank is the heart of an off-grid system — and the most expensive component to replace if sized incorrectly.
Two key parameters govern battery sizing:
- Autonomy days: how many consecutive days without sun must the system survive? For a residential home in Algeria, 2–3 days is the standard design target.
- Depth of Discharge (DoD): for lead-acid batteries, limit discharge to 50% of capacity to preserve cycle life. For lithium iron phosphate (LiFePO4), 80% DoD is acceptable.
Formula (lead-acid, 12V bank)
Capacity (Ah) = (Daily Wh × Autonomy Days) ÷ (0.85 × 0.5 × Battery Voltage)
Daily consumption = 1 940 Wh, Autonomy = 3 days, Voltage = 24V
→ Capacity = (1 940 × 3) ÷ (0.85 × 0.5 × 24) = 5 820 ÷ 10.2 = 570.6 Ah @ 24V
→ Select: 4 × 150 Ah batteries in series-parallel = 600 Ah @ 24V
The 0.85 factor accounts for battery charging efficiency. The 0.5 is the DoD limit. If you use LiFePO4 batteries, replace 0.5 with 0.8 — you will need significantly fewer cells for the same autonomy.
5 Select the Charge Controller
The charge controller sits between the panels and the battery bank. Its job is to prevent overcharging and deep discharge, two of the fastest ways to destroy a battery bank.
There are two technologies: PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).
PWM is simpler and cheaper, but it only works efficiently when the panel string voltage closely matches the battery voltage. For any system with more than two panels, or with a significant mismatch between string voltage and battery voltage, MPPT is the correct choice. MPPT can recover 20–30% more energy from the same panels.
Sizing an MPPT Controller
For MPPT, size by power rather than current. The charge current delivered to the battery equals the array power divided by the battery voltage, with a 1.25 safety margin.
MPPT Formula
Charge Current (A) = Total Array Wp ÷ Battery Voltage × 1.25
Array = 3 × 200 Wp = 600 Wp, Battery voltage = 24V
→ Charge current = 600 ÷ 24 × 1.25 = 31.25A
→ Select a 40A MPPT controller rated for 24V
→ Verify Voc of the string is below the controller's max input voltage
6 Putting It All Together
Once you have the five components sized, perform a final cross-check:
- Does the daily panel production (Wp × PSH) exceed the daily consumption including losses? ✓
- Does the battery capacity provide the required autonomy at the chosen DoD? ✓
- Is the inverter surge rating ≥ 3× the largest motor load? ✓
- Is the MPPT controller's max input voltage above the array Voc at minimum temperature? ✓
- Are all cable cross-sections sized for the maximum current with less than 2% voltage drop? ✓
Temperature matters. Panel output drops by roughly 0.4% per °C above 25°C. In summer in central Algeria, panels can reach 65–70°C. At 65°C that's a 16% output reduction. Always verify your array produces enough in the hottest month, not just on a standard test-condition spec sheet.
Is the first attempt always perfect? No. Real installations always surface surprises — a neighbour's wall casting partial shade, a new appliance added after commissioning, load growth over two years.
But with these six steps, you will avoid the most expensive mistakes — the ones that mean replacing an entire battery bank a year after installation.
And if you want a qualified engineer to review your calculations before you purchase anything, that is exactly what the team at Diresk is here for.
Want an expert to review your sizing?
Our engineers verify your calculations and give you a precise component list adapted to your actual site in Algeria, Saudi Arabia, or the UAE.