How to Size Solar Power for an Outdoor PoE Device

To calculate solar power requirements for an outdoor PoE device, first measure the complete system’s power draw, multiply it by operating hours to obtain daily watt-hours, then account for cable, converter and auxiliary losses. Battery capacity is based on daily energy, required autonomy and usable storage assumptions; solar array size is based on the site’s design-month peak sun hours and expected system losses.

Keep energy sizing separate from PoE compatibility. A solar system may provide enough watt-hours yet still fail if its converter has the wrong voltage, polarity, pinout, protocol or continuous power rating.

Define the complete PoE load and duty cycle

Begin at the powered device, but do not stop there. The complete load may include:

• One or more outdoor cameras
• Wireless access points or point-to-point radios
• A PoE switch, injector or DC-to-PoE converter
• A cellular router or fibre media converter
• Heaters, infrared illuminators or pan-tilt-zoom motors
• Cabinet fans, controllers, relays or monitoring equipment
• Night-time lighting or other DC accessories

The most dependable input is measured power under representative operating conditions. Nameplate ratings are useful for initial screening, but they may not describe typical consumption, start-up demand or short periods of higher load.

For each device, record:

Load	Measured or specified power	Hours per day	Daily energy
PoE device 1	(P_1) W	(t_1) h	(P_1 \times t_1) Wh
PoE device 2	(P_2) W	(t_2) h	(P_2 \times t_2) Wh
Network equipment	(P_3) W	(t_3) h	(P_3 \times t_3) Wh
Auxiliary equipment	(P_4) W	(t_4) h	(P_4 \times t_4) Wh

The basic device-energy calculation is:

[ E_{\text{devices}} = \sum(P_i \times t_i) ]

where:

• (E_{\text{devices}}) is daily device energy in Wh/day
• (P_i) is the power of each load in watts
• (t_i) is its daily operating time in hours

For a continuously operating device, use 24 hours. For scheduled equipment, use its actual duty cycle. A radio that transmits intermittently may still remain powered all day, so its energy calculation should reflect total input power rather than transmission time alone.

Allow for changing operating states

Outdoor devices often have more than one power state. A surveillance camera, for example, may draw more power when infrared illumination, analytics, a heater or a motor is active.

Where practical, log power over a complete day and include the operating conditions most relevant to the design season. If logging is not possible, calculate each state separately:

[ E_{\text{device}} = (P_{\text{day}} \times t_{\text{day}}) + (P_{\text{night}} \times t_{\text{night}}) ]

For multi-camera systems, determine both energy consumption and the instantaneous PoE budget. The latter establishes whether the switch or injector can supply all connected devices at once. The process is covered separately in this guide to calculating a PoE power budget for security cameras.

Check peak power as well as average energy

Daily watt-hours determine storage and solar generation, but converters and wiring must also support the highest simultaneous load.

Record:

• Normal continuous power
• Highest sustained operating power
• Start-up or inrush demand, if documented
• Whether multiple devices can reach peak demand together

Do not size a converter from average watts alone. A system that averages 15W could still require a converter rated for a higher continuous or transient load.

Add converter, cable and auxiliary losses

Power is lost between the solar panel, battery and remote PoE device. These losses must be entered as explicit design assumptions rather than hidden inside an arbitrary margin.

Relevant losses may include:

• DC-to-PoE conversion
• Voltage drop in low-voltage DC wiring
• Ethernet cable resistance
• Charge-controller conversion
• Battery charge and discharge losses
• Always-on cabinet or network equipment
• Self-consumption by controllers and converters

There are two practical calculation methods.

Method 1: Measure total input at the battery or DC bus

This is usually the simplest method for an assembled system. Connect the intended device, converter and cable, operate them under representative conditions, and measure the DC input power.

If the complete system draws (P_{\text{bus}}) watts for (t) hours:

[ E_{\text{daily}} = P_{\text{bus}} \times t ]

Because the measurement is taken before the converter and PoE cable, their losses are already included. Do not add the same losses again.

For variable loads, use logged energy over a suitable period rather than one instantaneous reading.

Method 2: Calculate each loss separately

When the installation cannot be assembled in advance, work backwards from the powered-device load:

[ E_{\text{source}} = \frac{E_{\text{PD}}} {\eta_{\text{cable}} \times \eta_{\text{converter}}} + E_{\text{auxiliary}} ]

where:

• (E_{\text{PD}}) is energy delivered to the powered device
• (\eta_{\text{cable}}) is the assumed cable-delivery efficiency
• (\eta_{\text{converter}}) is the assumed converter efficiency
• (E_{\text{auxiliary}}) covers loads not included in the PoE path

Use efficiency values applicable to the actual input voltage and load point. A published maximum efficiency should not automatically be used as the design efficiency.

The Maxsol POE24V adjustable DC-to-PoE converter accepts 12–36V DC and provides an adjustable 12–57V PoE output, Gigabit Ethernet and an LCD voltage display. Its documented continuous load for 12V-to-48V conversion is no more than 30W. The stated conversion efficiency of up to 94% is a maximum, not a guaranteed value for every operating point, so use a measured or appropriately conservative input figure in the final energy calculation.

Estimate cable loss from actual installation details

Ethernet cable loss depends on conductor resistance, cable length, current, connector quality and the conductors used to carry power. Lower PoE voltage normally means more current for the same delivered power, increasing resistive loss.

For a known circuit resistance:

[ P_{\text{cable loss}} = I^2R ]

where:

• (I) is current in amperes
• (R) is total resistance of the power loop in ohms

Voltage measured at the source and at the powered-device end under load provides a useful commissioning check. Do not rely only on no-load voltage, because voltage drop appears when current is flowing.

Convert power into daily energy demand

Solar and battery systems are sized in watt-hours, not watts alone. Watts describe the instantaneous rate of consumption; watt-hours describe how much energy is consumed over time.

For a constant load:

[ E_{\text{daily}} = P_{\text{total}} \times 24 ]

For variable or scheduled loads:

[ E_{\text{daily}} = \sum(P_i \times t_i) ]

As a calculation example, if a fully assembled system is measured at an average DC-bus input of (X) watts over continuous operation, its daily demand is:

[ E_{\text{daily}} = X \times 24 \text{ Wh/day} ]

If a separate auxiliary load uses (Y) watts for (Z) hours:

[ E_{\text{daily,total}} = (X \times 24) + (Y \times Z) ]

Replace (X), (Y) and (Z) with measurements from the proposed installation. This approach avoids implying that every camera, radio or converter has the same power profile.

Build a transparent energy worksheet

A useful design worksheet should show:

1. Device power at each operating state
2. Hours per day in each state
3. PoE cable-loss assumption
4. Converter-efficiency assumption
5. DC cable loss
6. Auxiliary energy
7. Charge and battery delivery assumptions
8. Total daily Wh at the battery bus

Keep the original figures beside the adjusted figures. This makes it easier to identify whether a later change—such as adding a camera or extending a cable—affects converter capacity, daily energy, or both.

Select autonomy and usable storage assumptions

Autonomy is the number of days the system must operate without meaningful solar recharge. It is a design choice based on site conditions, service criticality and acceptable outage risk.

Nominal battery capacity can be estimated with:

[ C_{\text{nominal}} = \frac{E_{\text{daily}} \times D_{\text{autonomy}}} {U_{\text{battery}} \times \eta_{\text{delivery}}} ]

where:

• (C_{\text{nominal}}) is required nominal battery capacity in Wh
• (E_{\text{daily}}) is daily energy demand in Wh/day
• (D_{\text{autonomy}}) is the selected number of days
• (U_{\text{battery}}) is the chosen usable fraction of nominal capacity
• (\eta_{\text{delivery}}) is the assumed efficiency from battery to loads

Do not treat nominal battery watt-hours as fully usable delivered energy. Battery management limits, temperature, ageing, discharge rate and conversion losses can all affect available energy. Confirm the battery supplier’s operating limits for the intended environment rather than applying a universal usable percentage.

Compare nominal storage without claiming runtime

The Maxsol CS8040 80W and 576Wh solar system documents an 80W monocrystalline panel, a 12.8V 45Ah 576Wh LiFePO4 battery, an MPPT controller and three DC12V outputs.

The Maxsol CS12060 120W and 768Wh solar system documents a 120W monocrystalline panel, a 12.8V 60Ah 768Wh LiFePO4 battery, an MPPT controller and three DC12V outputs.

These nominal figures can be inserted into a design worksheet as follows:

Calculation point	CS8040	CS12060
Documented panel rating	80W	120W
Nominal battery energy	576Wh	768Wh
Estimated usable stored energy	(576 \times U)	(768 \times U)
Estimated daily array energy before losses	(80 \times H)	(120 \times H)

In the table:

• (U) is the project’s confirmed usable-storage factor
• (H) is the site’s design-month peak sun hours

The table is a calculation framework, not a runtime statement. These products and a PoE converter should also be treated as comparison examples rather than a tested compatible bundle.

For a broader CCTV design involving several loads, the same storage principles are explained in the guide to sizing an off-grid solar power system for CCTV.

Estimate design-month solar input

Solar array sizing should use the least favourable month in which the system is expected to meet its service requirement—not an annual average.

Obtain site-specific monthly solar-resource data for the proposed New Zealand location and panel orientation. Consider the actual horizon, shading, tilt and exposure rather than assuming that one peak-sun-hours figure applies nationwide.

The basic array calculation is:

[ P_{\text{array}} = \frac{E_{\text{daily}}} {H_{\text{design}} \times \eta_{\text{solar path}}} ]

where:

• (P_{\text{array}}) is required panel capacity in watts
• (E_{\text{daily}}) is energy that must be replaced each day
• (H_{\text{design}}) is design-month peak sun hours per day
• (\eta_{\text{solar path}}) represents the documented or chosen allowance for panel, controller, wiring, temperature and battery-charging losses

Do not insert an annual-average sun figure into a winter-critical design. Likewise, do not invent a generic loss percentage. Record the source and basis of every assumption.

Include recovery after low-solar periods if required

Autonomy answers how long the battery can support the load. It does not show how quickly the array can recharge the battery while continuing to power the site.

If the project requires recovery of depleted energy within (R) days:

[ E_{\text{recovery per day}} = \frac{E_{\text{depleted}}}{R} ]

Then:

[ P_{\text{array}} = \frac{E_{\text{daily}} + E_{\text{recovery per day}}} {H_{\text{design}} \times \eta_{\text{solar path}}} ]

This recovery requirement can materially increase array size. State it separately rather than burying it in an unexplained safety factor.

Account for the physical site

Before choosing a panel rating, check:

• Seasonal and permanent shading
• Panel orientation and tilt
• Vegetation growth
• Nearby structures and terrain
• Dirt, salt or other surface contamination
• Cable distance between the panel, controller and battery
• Expected temperature range
• Mounting constraints and wind exposure
• Access for inspection and cleaning

Solar calculations assume that the panel receives the solar input used in the worksheet. Poor positioning cannot be corrected by battery capacity alone.

Verify voltage, protocol and commissioning measurements

Energy capacity does not establish PoE compatibility. Before connection, verify the electrical interface of every component.

Check:

• Required powered-device voltage
• Acceptable voltage range at the device
• Output polarity
• Power pinout and pair assignment
• Active or passive PoE requirements
• IEEE detection and negotiation requirements
• Continuous converter rating
• Peak and start-up demand
• Ethernet speed and distance
• Cable conductor size and resistance
• Earthing, surge protection and enclosure requirements

For the POE24V converter, IEEE powered-device detection or negotiation is not established by the documented facts. Confirm the voltage, polarity, pinout and protocol requirements of the intended device before treating it as compatible.

Commission the system under load

After installation, measure the system rather than assuming the spreadsheet is exact. Record:

1. Battery or DC-bus voltage at rest
2. DC input current with all normal loads operating
3. Converter output voltage
4. Voltage at the remote device under load
5. Night-time consumption where cameras use infrared illumination
6. Peak demand during start-up or motor movement
7. Daily energy over a representative operating period
8. Solar charge performance under known weather conditions

Compare the measured daily Wh with the design figure. If consumption is higher, identify whether the difference comes from the device, cable, converter or auxiliary equipment before changing the array or battery.

Common sizing mistakes

Avoid these recurring errors:

• Sizing from a camera’s advertised PoE class instead of measured consumption
• Using watts and watt-hours interchangeably
• Forgetting the PoE switch, converter, router or cabinet loads
• Applying maximum converter efficiency as a guaranteed design value
• Counting conversion losses twice after measuring total DC input
• Treating nominal battery Wh as fully usable energy
• Using annual-average solar input for a design-month requirement
• Sizing the battery for autonomy but not sizing the array for recharge
• Assuming sufficient energy means the PoE protocol is compatible
• Ignoring voltage drop on long DC or Ethernet cable runs
• Adding future devices without recalculating peak power and daily energy

A defensible design keeps every assumption visible and replaces estimates with measurements as the project progresses.

Final sizing checklist

Before selecting equipment, confirm that the calculation includes:

• Complete device and network load
• Day and night operating states
• Daily duty cycle
• Peak simultaneous demand
• Converter and cable losses
• Auxiliary consumption
• Required autonomy
• Confirmed usable-storage assumption
• Design-month solar resource
• Array recovery requirement
• Voltage, polarity, pinout and PoE protocol
• Loaded commissioning measurements

As a practical next step, compare the linked Maxsol solar systems and converter against the project’s port count, measured load, cable distance, voltage and power requirements, environmental conditions, and any network management needs. Select only after both the energy worksheet and the PoE compatibility checks are complete.