Solar and Off-Grid Power

This reference provides sizing calculations, equipment specifications, and selection criteria for powering IT infrastructure in locations without reliable grid electricity. Use the power consumption tables to calculate load requirements, then apply the sizing formulas to specify solar panels, batteries, and supporting equipment.

Power Consumption Reference

IT equipment power consumption determines all downstream sizing decisions. The values below represent typical consumption under operational load, not manufacturer peak ratings or idle states.

Endpoint Devices

Networking Equipment

Server and Storage

Environmental Control

Load Calculation Methodology

Total daily energy is the sum of all equipment consumption in watt-hours (Wh), calculated as power (W) multiplied by hours of operation. This figure drives all subsequent sizing.

Worked Example: Small Field Office

A field office with 5 staff requires power for endpoints, networking, and a local server:

Endpoints:
  5x Laptops (45W × 8h)         = 1,800 Wh
  5x External monitors (25W × 8h) = 1,000 Wh
  1x Printer (500W × 0.1h)      =    50 Wh

Networking:
  1x Wireless AP (12W × 24h)    =   288 Wh
  1x Router (10W × 24h)         =   240 Wh
  1x 8-port switch (5W × 24h)   =   120 Wh
  1x VSAT terminal (40W × 24h)  =   960 Wh

Server/Storage:
  1x Mini PC server (25W × 24h) =   600 Wh
  1x NAS 2-bay (25W × 24h)      =   600 Wh

Lighting/Environmental:
  4x LED fixtures (15W × 8h)    =   480 Wh
  2x Desk fans (30W × 8h)       =   480 Wh

Total Daily Energy             = 6,618 Wh
                               ≈ 6.6 kWh/day

System Losses and Safety Margin

Raw consumption figures require adjustment for system inefficiencies. Each power conversion stage introduces losses.

The adjusted daily energy requirement applies these factors:

Adjusted Energy = (Total Daily Energy ÷ Inverter Efficiency ÷ Battery Efficiency) × Safety Margin

For the example above:
Adjusted Energy = (6,618 ÷ 0.90 ÷ 0.85) × 1.25
                = (6,618 ÷ 0.765) × 1.25
                = 8,651 × 1.25
                = 10,814 Wh
                ≈ 10.8 kWh/day

This adjusted figure accounts for energy lost during conversion and storage, plus a 25% margin for equipment additions, degradation, and suboptimal conditions.

Solar Panel Sizing

Solar panels convert sunlight to DC electricity. Panel output depends on rated capacity, solar irradiance, and temperature. The peak sun hours value for a location indicates how many hours of 1,000 W/m² equivalent irradiance occur daily on average.

Peak Sun Hours by Region

Panel Capacity Calculation

Required panel capacity derives from adjusted daily energy divided by peak sun hours, further adjusted for temperature and soiling:

Panel Capacity (Wp) = Adjusted Daily Energy ÷ (Peak Sun Hours × Temperature Factor × Soiling Factor)

Temperature factor accounts for panel efficiency loss in hot climates. Panels lose approximately 0.4% efficiency per degree Celsius above 25°C. At 45°C cell temperature (common in tropical climates), the factor is 0.92.

Soiling factor accounts for dust, dirt, and debris accumulation between cleanings. In dusty environments with monthly cleaning, use 0.90. In low-dust environments with regular rain, use 0.97.

Worked Example: Panel Sizing

For the 10.8 kWh/day field office in East Africa during wet season:

Peak Sun Hours (wet season): 4.0 hours
Temperature Factor (35°C ambient): 0.90
Soiling Factor (dusty, monthly cleaning): 0.90

Panel Capacity = 10,814 ÷ (4.0 × 0.90 × 0.90)
               = 10,814 ÷ 3.24
               = 3,338 Wp
               ≈ 3.4 kWp array

Using 400Wp panels, this requires 9 panels (3,600 Wp total).

Panel Specifications

Monocrystalline panels deliver the highest output per unit area, making them preferred where roof or ground space is limited. Thin-film panels tolerate high temperatures better and perform well in diffuse light, but require substantially more area.

Battery Storage

Batteries store energy for use during periods without sunlight. Depth of discharge (DoD) indicates what percentage of rated capacity can be used without accelerating degradation. Cycle life measures how many charge-discharge cycles the battery endures before capacity drops below 80%.

Battery Technology Comparison

Battery Capacity Calculation

Days of autonomy determines how many days the system operates without solar input. Remote locations with difficult access or extended cloudy periods require more autonomy.

Battery capacity calculation accounts for depth of discharge and autonomy:

Battery Capacity (Wh) = (Daily Energy Requirement × Days of Autonomy) ÷ Depth of Discharge

Worked Example: Battery Sizing

For the 10.8 kWh/day field office with 3 days autonomy using LFP batteries:

Battery Capacity = (10,814 × 3) ÷ 0.80
                 = 32,442 ÷ 0.80
                 = 40,553 Wh
                 ≈ 40.5 kWh

At 48V system voltage, this requires 845 Ah capacity. A configuration of 8× 100Ah 48V LFP batteries provides 38.4 kWh usable capacity (48 kWh nominal × 0.80 DoD), meeting the requirement with slight margin.

For lead-acid batteries at 50% DoD:

Battery Capacity = (10,814 × 3) ÷ 0.50
                 = 32,442 ÷ 0.50
                 = 64,884 Wh
                 ≈ 64.9 kWh

This requires 1,352 Ah at 48V, achievable with 8 parallel strings of 4× 12V 200Ah batteries in series (32 batteries total, weighing approximately 1,900 kg versus 400 kg for the LFP option).

System Voltage Selection

Higher system voltages reduce current and permit smaller cable sizes for equivalent power transfer.

Most field installations use 48V for IT loads above 1 kW. This voltage is below the safety threshold requiring electrician certification in many jurisdictions while permitting practical cable sizes.

Charge Controllers

Charge controllers regulate power flow from solar panels to batteries, preventing overcharge and optimising energy harvest. Two technologies exist: Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT).

PWM controllers connect panels directly to batteries at battery voltage. Simple and inexpensive, they waste energy when panel voltage exceeds battery voltage. Suitable only when panel nominal voltage matches battery voltage precisely.

MPPT controllers convert panel output to optimal battery charging voltage, capturing energy that PWM controllers discard. Efficiency gains of 20-30% over PWM occur when panel voltage significantly exceeds battery voltage, common in systems using higher-voltage panels or series strings.

Charge Controller Sizing

Controller capacity must handle maximum panel current. For MPPT controllers, both input voltage and output current limits apply.

Maximum Panel Current = Panel Array Capacity (Wp) ÷ Panel Voltage at Maximum Power (Vmp)
Required Controller Current = Maximum Panel Current × 1.25 (safety factor)

Worked Example: Controller Sizing

For a 3.6 kWp array using 400Wp panels with Vmp of 40V, configured as 3 parallel strings of 3 panels in series:

String Voltage = 3 × 40V = 120V (input to controller)
String Current = 400W ÷ 40V = 10A per string
Total Array Current = 3 × 10A = 30A

Required Controller = 30A × 1.25 = 37.5A

An MPPT controller rated for 150V input and 60A output handles this array with margin for future expansion.

Charge Controller Specifications

Inverters

Inverters convert DC battery power to AC for standard IT equipment. Key specifications include continuous output power, surge capacity, output waveform, and efficiency.

Output Waveform

Pure sine wave inverters produce AC power equivalent to grid electricity. All IT equipment operates correctly on pure sine wave output.

Modified sine wave inverters produce a stepped approximation of sine wave. Some devices malfunction on modified sine wave: switching power supplies may overheat, audio equipment produces hum, and motor speed controls behave erratically. Modified sine wave inverters cost less but should not power sensitive IT equipment.

Inverter Sizing

Inverter continuous rating must exceed total connected load. Surge rating must handle startup currents, particularly from motors and laser printers.

Worked Example: Inverter Sizing

For the field office with steady load of 750W and a laser printer:

Continuous Load:
  5× Laptop chargers (65W): 325W
  5× Monitors (25W): 125W
  Networking equipment: 100W
  Server + NAS: 50W
  Lighting and fans: 150W
  Total Continuous: 750W

Peak Load (printer startup):
  Continuous load: 750W
  Printer surge: 1,500W
  Total Peak: 2,250W

Required Inverter:
  Continuous rating: 750W × 1.25 = 938W minimum
  Surge rating: 2,250W minimum

Selected: 1,500W continuous / 3,000W surge pure sine wave inverter

Inverter Efficiency

Inverters operate most efficiently between 50-80% of rated capacity. An oversized inverter wastes energy through higher standby consumption and lower partial-load efficiency.

For loads that vary substantially, a dual-inverter configuration with automatic transfer provides better efficiency: a small inverter for light loads (nights, weekends) and a larger inverter for peak periods.

Hybrid System Architecture

Stand-alone solar systems sized for worst-case conditions (wet season, extended clouds) result in substantial excess capacity during optimal conditions. Hybrid systems integrating generators reduce battery and panel requirements while maintaining reliability.

+------------------------------------------------------------------+
|                      HYBRID POWER SYSTEM                         |
+------------------------------------------------------------------+
|                                                                  |
|  +-------------------+                                           |
|  |   SOLAR PANELS    |                                           |
|  |   (3.6 kWp)       |                                           |
|  +--------+----------+                                           |
|           |                                                      |
|           | DC (120V)                                            |
|           v                                                      |
|  +--------+----------+         +-----------------+               |
|  |  MPPT CHARGE      |         |   GENERATOR     |               |
|  |  CONTROLLER       |         |   (5 kVA)       |               |
|  |  (60A)            |         +---------+-------+               |
|  +--------+----------+                   |                       |
|           |                              | AC (230V)             |
|           | DC (48V)                     |                       |
|           v                              v                       |
|  +--------+----------+         +---------+-------+               |
|  |   BATTERY BANK    |         |  HYBRID         |               |
|  |   (40 kWh LFP)    +-------->|  INVERTER/      |               |
|  |                   |<--------+  CHARGER        |               |
|  +-------------------+         |  (3 kW)         |               |
|                                +---------+-------+               |
|                                          |                       |
|                                          | AC (230V)             |
|                                          v                       |
|                                +---------+-------+               |
|                                | DISTRIBUTION    |               |
|                                | PANEL           |               |
|                                +-----------------+               |
|                                          |                       |
+------------------------------------------------------------------+
                                           |
                                           v
                                    [IT EQUIPMENT]

Figure 1: Hybrid solar-generator system architecture

Generator Integration

A hybrid inverter/charger accepts AC input from a generator and uses it to charge batteries and power loads simultaneously. When generator AC is present, the inverter synchronises and passes power through. When generator stops, the inverter seamlessly transitions to battery power (transfer time under 20ms for quality units, acceptable for IT equipment with power supplies that bridge short interruptions).

Generator sizing for battery charging:

Minimum Generator Size = Battery Charger Rating + Continuous Load

For a 60A × 48V = 2,880W charger and 750W load:
Minimum Generator = 2,880 + 750 = 3,630W
Recommended (80% loading): 4,500W = 4.5 kVA

Generator Runtime Calculation

Generator fuel consumption depends on loading percentage. Running generators at low load (under 30%) causes carbon buildup and premature wear; optimal loading is 60-80%.

Worked Example: Hybrid System

Resizing the field office system with 2 days autonomy and generator backup for extended cloudy periods:

Reduced Battery Requirement:
  Battery Capacity = (10,814 × 2) ÷ 0.80 = 27,035 Wh ≈ 27 kWh
  Reduction: 40.5 kWh → 27 kWh (33% smaller)

Generator Scenario:
  3 consecutive cloudy days with 50% solar production
  Daily shortfall: 10,814 × 0.50 = 5,407 Wh
  Total shortfall: 3 × 5,407 = 16,221 Wh

  Generator charging at 2,500W net (after load):
  Runtime needed: 16,221 ÷ 2,500 = 6.5 hours over 3 days
  Fuel required: 6.5 × 1.4 = 9.1 litres

Generator backup permits smaller battery banks while maintaining reliability. The trade-off is fuel logistics, noise, and maintenance requirements.

UPS Integration

Uninterruptible power supplies provide bridge power during transitions between power sources. In solar installations, the inverter/charger provides this function, but separate UPS units protect individual critical equipment from power quality issues.

UPS Topology for Solar Systems

Line-interactive UPS units suit solar installations. They pass inverter AC through while conditioning voltage and switching to battery during outages. The internal battery provides 5-15 minutes of runtime, sufficient to bridge any transfer delays.

Online/double-conversion UPS units continuously convert AC-DC-AC, which wastes energy in a system already performing DC-AC conversion. Their isolation benefits are unnecessary when a quality inverter produces clean sine wave output.

UPS Sizing for Critical Equipment

The UPS function is bridging, not extended runtime. If the main battery bank is depleted, UPS batteries provide time for graceful shutdown rather than extended operation.

Power Distribution

Power distribution connects generation, storage, and loads safely and efficiently.

+------------------------------------------------------------------+
|                    POWER DISTRIBUTION                            |
+------------------------------------------------------------------+
|                                                                  |
|  FROM INVERTER                                                   |
|       |                                                          |
|       v                                                          |
|  +----+----+                                                     |
|  | MAIN    |                                                     |
|  | BREAKER |                                                     |
|  | (32A)   |                                                     |
|  +----+----+                                                     |
|       |                                                          |
|       v                                                          |
|  +----+----+----+----+----+----+----+                            |
|  |    |    |    |    |    |    |    |                            |
|  v    v    v    v    v    v    v    v                            |
| +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+                          |
| |16| |16| |10| |10| |10| |10| |10| |6 |                          |
| |A | |A | |A | |A | |A | |A | |A | |A |                          |
| +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+                          |
|  |    |    |    |    |    |    |    |                            |
|  v    v    v    v    v    v    v    v                            |
| NET  SRV  WS1  WS2  WS3  WS4  WS5  LTG                           |
|                                                                  |
| NET = Networking (UPS protected)                                 |
| SRV = Server/NAS (UPS protected)                                 |
| WS1-5 = Workstation circuits                                     |
| LTG = Lighting and fans                                          |
|                                                                  |
+------------------------------------------------------------------+

Figure 2: Distribution panel layout for field office

Circuit Protection

Each circuit requires appropriate overcurrent protection. The protection device rating must not exceed the cable current capacity.

Grounding and Surge Protection

All solar installations require proper grounding:

• Solar panel frames connected to ground rod
• Inverter chassis grounded
• Battery negative connected to ground (single point)
• Distribution panel grounded
• Equipment grounds via three-wire outlets

Surge protection devices (SPDs) at the distribution panel protect against lightning-induced surges. Install Type 2 SPDs at the main panel and consider Type 3 SPDs at sensitive equipment locations.

Maintenance Requirements

Solar power systems require scheduled maintenance to sustain rated performance.

Maintenance Schedule

Performance Monitoring

Record daily energy production and consumption to identify degradation or faults. Quality charge controllers and inverters provide data logging; extract logs monthly and review for anomalies.

Key metrics to track:

Climate Adjustments

Solar system performance varies substantially with climate. The following adjustments apply to base calculations.

Temperature Adjustments

Altitude Adjustments

At high altitude, increased UV intensity slightly improves panel output while reduced air density decreases generator output.

Seasonal Planning

Design for worst-case conditions but verify adequate drainage of excess production during peak periods. Batteries left at 100% state of charge for extended periods degrade faster than those cycled regularly.

For locations with extreme seasonal variation (monsoon regions, high latitudes), consider:

• Wet season: Reduce non-essential loads; schedule generator charging
• Dry season: Verify charge controllers limit production to prevent overcharging
• Shoulder seasons: Optimal for maintenance and capacity testing

Procurement Specifications

When procuring solar power equipment, specifications should include the following minimum requirements.

Solar Panels

• IEC 61215 certification (crystalline) or IEC 61646 (thin-film)
• IEC 61730 safety certification
• 25-year linear power warranty with degradation under 0.7%/year
• Manufacturer’s temperature coefficient documentation
• Fire rating appropriate for installation context

Batteries

• Cycle life specification at stated depth of discharge
• Operating temperature range covering site conditions
• Battery management system (BMS) for lithium chemistry, including:
  • Cell balancing
  • Over-temperature protection
  • Over-current protection
  • Communication interface for monitoring
• UN 38.3 transport certification for lithium batteries

Charge Controllers

• MPPT topology for systems over 200W
• Input voltage range exceeding panel open-circuit voltage at lowest temperature
• Temperature compensation with remote sensor
• Data logging capability with export function
• Modbus or similar protocol for integration

Inverters

• Pure sine wave output with THD under 3%
• Efficiency over 92% at 50-100% load
• Transfer time under 20ms for hybrid models
• Operating temperature range covering site conditions
• Warranty of 5 years minimum

See Also

• Field Hardware Selection
• Ruggedised and Environmental
• Field Site IT Establishment
• Satellite Connectivity
• Backup Systems