Solar Panel, Battery, MPPT and Load Calculation Guide

A reliable off-grid solar design must be calculated in this order: load, daily energy, battery storage, PV array, then MPPT controller. Each result becomes an input for the next step. Choosing a panel or battery first can leave the system unable to support the actual load, recover after poor weather, or remain within the controller’s electrical limits.

A practical solar panel battery MPPT load calculation therefore needs one coordinated worksheet. Record every assumption, use design-month solar data for the specific Philippine site, and verify all voltage, current, temperature and charging limits against current equipment datasheets.

Create the load and daily energy worksheet

Begin with the equipment that must operate, not with the advertised wattage of a solar kit. List cameras, radios, switches, routers, lighting, heaters, cooling fans and conversion equipment separately.

For each load, calculate:

Daily energy in Wh = Quantity × Operating watts × Hours per day

Use measured power where possible. A nameplate maximum may overstate normal consumption, while an estimated “typical” figure may miss start-up power, infrared illumination, radio transmit peaks or processor-heavy operation.

Load	Quantity	Power each	Operating time	Daily energy
IP camera	2	8W	24 hours	384Wh
Wireless radio	1	10W	24 hours	240Wh
Network switch	1	12W	24 hours	288Wh
Total connected load				912Wh/day

This is a hypothetical example, not a product rating. Actual camera power can rise when infrared LEDs, heaters, pan-tilt motors or other features activate.

Add conversion and distribution losses

The connected load is not necessarily the energy that must leave the battery. Account for losses in:

DC-DC converters
PoE switches or injectors
Inverters, where AC equipment is used
Wiring and connectors
Output protection devices
Equipment operating in standby
The controller’s own consumption

A simple preliminary calculation is:

Adjusted daily energy = Connected-load energy ÷ Distribution efficiency

If the 912Wh/day load is supplied through a path estimated at 87% efficiency:

912Wh ÷ 0.87 = 1,048Wh/day

Alternatively, a designer may apply a transparent loss allowance:

912Wh × 1.15 = 1,049Wh/day

Do not apply both methods unless they represent different, clearly identified losses.

Check peak power as well as daily energy

Daily Wh determines energy capacity, but instantaneous watts and amperes determine whether the wiring, battery, controller and output circuits can carry the load.

Record at least three figures:

Continuous load: normal combined operating power.
Peak load: simultaneous maximum demand.
Start-up or surge load: brief current required by motors, radios, illuminators or converters.

For a 12V output circuit:

Load current = Load power ÷ Actual output voltage

A 60W load at 12V draws approximately 5A, before allowing for conversion losses. Do not assume that an output labelled DC12V can supply any 12V device. Confirm its regulated voltage range, per-port current, total output current, connector type and polarity.

For a CCTV-specific walkthrough, see this guide to sizing an off-grid solar power system for CCTV.

Size battery storage from autonomy assumptions

Battery capacity depends on the adjusted daily energy, required autonomy, permitted depth of discharge, temperature, ageing allowance and power-conversion losses.

A useful worksheet formula is:

Required nominal battery energy = Daily energy × Autonomy days ÷ Combined usable factor

The combined usable factor may include:

Permitted usable depth of discharge
Battery efficiency
Temperature derating
Design allowance for ageing
Conversion efficiency not already included in the load calculation

Keep each factor visible rather than hiding everything inside one unexplained margin.

Hypothetical battery calculation

Using the example daily requirement of 1,049Wh:

Required autonomy: 2 days
Planned usable fraction: 80%, or 0.80
Temperature factor: 90%, or 0.90
Ageing design factor: 85%, or 0.85

Required battery energy = 1,049 × 2 ÷ (0.80 × 0.90 × 0.85)

Required battery energy = approximately 3,428Wh nominal

For a nominal 12.8V battery:

Required Ah = 3,428Wh ÷ 12.8V = approximately 268Ah

This does not mean every project should use the example factors. Obtain the permitted depth of discharge, charge and discharge limits, temperature restrictions and expected capacity behaviour from the battery datasheet.

Nominal Wh is not delivered Wh

Battery energy is commonly presented as:

Nominal Wh = Nominal voltage × Rated Ah

For example, 12.8V × 60Ah equals 768Wh. However, 768Wh is not a promise that a 100W load will operate for 7.68 hours. Delivered runtime is reduced or affected by:

Battery management system cut-off limits
Permitted depth of discharge
Load and converter efficiency
Cable voltage drop
Battery temperature
Battery condition
High discharge current
Equipment low-voltage thresholds

The Maxsol CS12060 120W and 60Ah solar system documents a 120W monocrystalline panel, 12.8V 60Ah 768Wh LiFePO4 battery, MPPT controller and three DC12V outputs. These are documented parts of one system example, not a recommendation to recreate the system from interchangeable components.

The larger Maxsol CS400150 400W and 150Ah solar system documents two 200W modules, a 12.8V 150Ah 1920Wh LiFePO4 battery, MPPT control and four DC12V outputs. Its suitability still depends on measured load, required autonomy, design-month sunlight and verified electrical limits.

Estimate PV energy for the design month

Size the array for the most demanding operating period, not for an annual average or the best sunny month. In the Philippines, extended cloud and rain can materially reduce solar production. The relevant design month depends on the exact site, local weather pattern, horizon and shading.

Obtain credible monthly solar resource data for the installation coordinates. Then confirm conditions on site, including:

Roofs, trees, poles and terrain that cast shadows
Panel orientation and tilt
Salt, dust, leaves or bird fouling
Heat and limited rear ventilation
Cable distance between the array and controller
Seasonal weather and extended cloudy periods

A preliminary formula is:

Required PV watts = Daily battery energy ÷ (Design peak-sun-hours × Solar system factor)

The solar system factor accounts for practical losses such as panel temperature, wiring, controller conversion, mismatch, dirt and imperfect orientation.

Hypothetical PV calculation

Assume:

Adjusted daily energy: 1,049Wh
Verified design-month resource: 3.5 peak-sun-hours per day
Combined solar system factor: 0.72

PV size = 1,049Wh ÷ (3.5 × 0.72)

PV size = approximately 416W

This calculation replaces the average daily load but does not necessarily recover a deeply discharged battery quickly.

Include recharge capacity after poor weather

Suppose the system consumes two days of stored energy during poor solar conditions and should recover that deficit over the next two suitable solar days.

Energy deficit = 1,049Wh × 2 = 2,098Wh

Daily recovery energy = 2,098Wh ÷ 2 days = 1,049Wh/day

The array must then provide:

1,049Wh daily load + 1,049Wh recovery = 2,098Wh/day

Using the same 3.5 peak-sun-hours and 0.72 system factor:

Recovery-sized PV array = 2,098 ÷ (3.5 × 0.72)

Recovery-sized PV array = approximately 833W

This shows why panel sizing and battery autonomy cannot be treated separately. More battery capacity may increase autonomy, but without enough array power and controller capacity, the system can take too long to recharge.

The recovery target should be realistic. Designing for full recovery in one day may require a much larger array and controller, while accepting a long recovery period can leave the system vulnerable to another cloudy interval.

Check MPPT voltage, current and power limits

An MPPT controller must satisfy several limits at the same time. An acceptable wattage rating alone does not prove compatibility.

Verify the following on the current controller and module datasheets:

Maximum PV open-circuit voltage
MPPT operating voltage range
Minimum voltage needed to begin charging
Maximum PV input current
Maximum short-circuit current, if specified separately
Maximum charging output current
Permitted PV power for the selected battery voltage
Supported battery chemistry and charge profile
Operating and charging temperature limits

For a practical comparison of controller approaches, review MPPT versus PWM for outdoor solar charging.

Check cold-weather open-circuit voltage

Panel open-circuit voltage rises as cell temperature falls. The controller must remain below its maximum PV input voltage under the coldest credible site condition, not only at the module’s standard test rating.

A preliminary calculation is:

Cold string Voc = Modules in series × Module Voc × [1 + |Voc temperature coefficient| × (25°C − Minimum cell temperature)]

Suppose a hypothetical panel has:

Voc: 41V
Voc temperature coefficient: −0.28% per °C
Two modules in series
Minimum design cell temperature: 10°C

Convert 0.28% to 0.0028:

Cold string Voc = 2 × 41 × [1 + 0.0028 × (25 − 10)]

Cold string Voc = approximately 85.4V

The controller’s maximum PV voltage must exceed the calculated value with an appropriate design margin. Use the module manufacturer’s exact coefficient and a defensible minimum temperature for the actual site. Cooler elevated locations may require a different assumption from warm lowland sites.

Check array input current

Series connections increase voltage; parallel strings increase current.

For a preliminary check:

Array Isc = Number of parallel strings × Module Isc

Apply any required current design factor or safety margin relevant to the installation standard and controller instructions. Compare the result with the controller’s maximum PV input or short-circuit current rating.

Do not confuse these different values:

Imp: current at maximum power
Isc: short-circuit current
Maximum PV input current: controller-side limit
Maximum charge current: battery-side output limit

Check controller charge-current capacity

A rough battery-side estimate is:

Charge current ≈ PV power × Controller efficiency ÷ Battery charging voltage

For a hypothetical 833W array, 95% controller conversion efficiency and 14.2V charging voltage:

Charge current ≈ 833 × 0.95 ÷ 14.2

Charge current ≈ 55.7A

This estimate must be compared with both:

The controller’s maximum output current.
The battery’s permitted charge current.

If the controller clips at a lower current, the array may not deliver its full potential. If the battery accepts less current than the controller can provide, controller settings or array sizing must protect the battery.

Coordinate battery charging and load outputs

The controller and battery must agree on more than nominal system voltage. A “12V” label does not define the complete charging profile of a 12.8V LiFePO4 battery.

Confirm:

Bulk or absorption charging voltage
Float behaviour, if recommended
Low-temperature charge protection
High- and low-voltage cut-offs
Maximum continuous charge and discharge current
Battery management system limits
Whether equalisation must be disabled
Controller restart behaviour after protection events

LiFePO4 charging can be restricted below a specified battery temperature. Verify whether low-temperature protection is provided by the battery, controller or another control device. Do not assume that warm daytime air means the battery itself is within its permitted charging range.

Verify the load-output path

Some solar systems provide direct DC load outputs, while others require separate distribution, voltage conversion or PoE equipment. Check:

Total output power across all ports
Maximum current per output
Whether outputs are independently protected
Output voltage across the battery’s charge and discharge range
Cable size and voltage drop
Connector locking and polarity
Load disconnect and reconnect thresholds
Behaviour when the battery reaches a protection limit

For long cable runs, calculate voltage drop in both conductors:

Voltage drop = Current × Total circuit resistance

A device can restart repeatedly if the voltage at its input falls below its threshold during radio transmission, infrared activation or another peak load, even when the battery still contains energy.

Where cameras or access points require PoE, verify the entire chain: battery output, DC converter, PoE switch or injector, cable distance, PoE standard, port allocation and total power budget.

Stress-test and document the final design

A worksheet should show how the system behaves outside the ideal daily average. Test at least the following conditions before final selection.

Design condition	What to verify
Normal design-month day	Array replaces the daily load
Consecutive cloudy days	Battery meets the selected autonomy target
Recovery after discharge	Array and MPPT can serve the load while recharging
Cold clear morning	PV open-circuit voltage remains below the controller limit
Hot panel conditions	Reduced array voltage remains inside the MPPT operating range
Maximum simultaneous load	Outputs, cables and battery remain within current limits
Lowest battery voltage	Connected equipment still receives acceptable input voltage
Battery temperature limit	Charging is blocked or controlled when required
One failed or shaded module	Series or parallel arrangement behaves as expected
Future equipment addition	Revised load remains within every system limit

Document the final inputs and sources, including:

Equipment quantity, measured watts and operating schedule
Peak and start-up current
Daily Wh before and after losses
Autonomy days and usable battery assumptions
Battery charge and discharge limits
Design-month peak-sun-hours
Array orientation, tilt and shading
Module Voc, Isc, Imp and temperature coefficients
Series and parallel configuration
Cold-corrected string Voc
MPPT input voltage, current, power and charge-current limits
Output-port and wiring limits
Fuse, breaker, disconnect and surge-protection selections
Recovery-time assumption
Date and revision of every datasheet used

Common calculation mistakes

Avoid these frequent design errors:

Treating nominal battery Wh as fully deliverable energy
Sizing the panel from annual-average sunlight
Ignoring recharge time after the autonomy period
Comparing array watts only with the controller’s wattage rating
Using module operating voltage instead of cold-corrected Voc
Ignoring Isc when checking parallel strings
Dividing PV watts by nominal battery voltage to select the controller
Assuming every DC12V output is regulated identically
Omitting PoE conversion and cable losses
Adding batteries without increasing recharge capacity
Adding panels without rechecking MPPT voltage and current
Combining components solely because their nominal voltages appear similar

The final design should have traceable assumptions rather than a single oversized safety percentage. If the load, autonomy or design-month solar input changes, recalculate the battery, array and MPPT controller together.

As a practical next step, compare the documented CS12060 and CS400150 systems with your verified daily load and recovery requirement. Also check port count, cable distance, connected-device power, environmental conditions and any network management requirements before selecting equipment; neither system’s panel and battery ratings alone establish runtime or site compatibility.