This is the working reference we use when sizing off-grid solar systems for monitoring, security, agriculture and telecom sites. It is deliberately conservative: every factor below assumes real site conditions — dust, heat, ageing and imperfect installation — rather than laboratory values.
Core formulas
| Quantity | Formula | Notes |
|---|---|---|
| Daily energy | E = Σ (P × h × n) | P = device watts, h = hours per day, n = quantity |
| Array size | Wp = E × 1.25 / (PSH × 0.70) | 1.25 design margin, 0.70 system efficiency |
| Battery capacity | Wh = E × autonomy days / DoD | DoD = usable depth of discharge |
| Battery capacity (Ah) | Ah = Wh / system voltage | Use nominal voltage, not charge voltage |
| Controller current | I = (Wp / V) × 1.25 | Then round up to a standard rating |
| Voltage drop | ΔV = 2 × L × I × 0.0175 / A | L in metres one-way, A in mm², copper |
| Cold Voc | Voc_cold = Voc_STC × (1 + β × (T_min − 25)) | β typically −0.0025 to −0.0035 per °C |
Typical device loads
Use these as a starting point only — measure the actual site where possible. Labels on equipment are usually peak, not average.
| Device | Typical continuous draw | Notes |
|---|---|---|
| Fixed CCTV camera (day) | 3-6 W | IR illumination can double this at night |
| PTZ camera | 15-40 W | Only while moving; higher in cold weather with heaters |
| NVR / recorder | 10-25 W | Scales with disk count and channels |
| 4G / LTE router | 2-8 W average | Transmit bursts reach 1-2.5 A at 12 V |
| Radar water level sensor | 5-12 W while measuring | Duty-cycled, seconds per reading |
| Data logger / RTU | 0.1-1 W | Dominant load on many monitoring sites |
| GNSS receiver | 1-4 W | Continuous, sensitive to brownout |
| PoE switch (4 port) | 8-20 W | Plus the PoE budget of attached devices |
| Gate motor (sliding) | 5-15 A surge | Seconds per cycle; battery must carry surge |
| Weather station (full) | 1-5 W | Heated sensors add significantly |
System efficiency breakdown
The 0.70 figure in the array formula is not arbitrary. It is the product of the losses below. If any of them is worse than typical, reduce the factor.
| Loss | Typical value | Worse case |
|---|---|---|
| Soiling (dust on glass) | 3-8% | 20-35% in dry dusty seasons |
| Cell temperature | 8-12% | 15-20% at 70°C cell temperature |
| Controller conversion | 3-6% (MPPT) | 15-25% with PWM on a high-voltage array |
| Cable and connection loss | 2-4% | 8-10% with undersized cable |
| Battery round-trip | 5-10% (lithium) | 15-20% (lead-acid) |
| Mismatch and ageing | 2-5% | 8-10% at end of module life |
Autonomy selection
| Site situation | Autonomy | Reasoning |
|---|---|---|
| Weekly access, reliable roads | 2-3 days | Short outage is recoverable |
| Monthly access | 4-5 days | Must ride out typical cloud runs |
| Monsoon / harmattan climate | 5-7 days | Extended low-yield periods are seasonal |
| Inaccessible in the wet season | 7-10 days | Access and yield fail at the same time |
| Safety or security critical | 7 days plus redundancy | Outage cost exceeds equipment cost |
System voltage selection
| Array size | Recommended voltage | Reason |
|---|---|---|
| Under 300 Wp | 12 V | Lower component cost, short cable runs |
| 300-800 Wp | 24 V | Halves current, keeps cable practical |
| 800-3000 Wp | 48 V | Controller and cable limits become binding above this |
| Over 3000 Wp | 48 V or high-voltage DC | Specialist design required |
Voltage drop limits
Keep total DC voltage drop under 3% for the array circuit and under 2% for the load circuit. On a 12 V system, 3% is only 0.36 V — which is why 12 V installations are so sensitive to cable length and why 24 V is the practical minimum for anything beyond a few metres.
| Cross-section | Current at 12 V, 3% drop, 10 m | Current at 24 V, 3% drop, 10 m |
|---|---|---|
| 2.5 mm² | ~3.5 A | ~7 A |
| 4 mm² | ~5.6 A | ~11 A |
| 6 mm² | ~8.4 A | ~17 A |
| 10 mm² | ~14 A | ~28 A |
| 16 mm² | ~22 A | ~45 A |
Controller matching
- MPPT is worth the premium whenever array voltage exceeds battery voltage by more than about 20%
- Check the controller’s maximum PV input voltage against cold-morning open-circuit voltage, not datasheet Voc at 25 °C
- Check the controller’s continuous output current against the load peak, not its maximum PV current
- Leave 20-25% current headroom for array expansion and hot-weather derating
Battery depth of discharge
| Chemistry | Design DoD | Cycle life impact |
|---|---|---|
| LiFePO4 | 80% | 3,000-6,000 cycles at this depth |
| Gel lead-acid | 50% | Deeper cycling halves life quickly |
| AGM lead-acid | 50% | Slightly lower cycle life than gel |
| Flooded lead-acid | 30-50% | Requires watering and ventilation |
Use the sizing calculator to apply these formulas to your own load list, or send us the site details and we will return a sized configuration.