Solar Panel, Battery, MPPT and Load Sizing Guide

A reliable solar panel, battery, MPPT and load calculation must size the entire system in dependency order: calculate the load first, choose the required battery storage, determine the PV array needed during the design month, and then verify every MPPT controller limit. Selecting each component separately can produce a system that stores enough energy but cannot recharge quickly enough, or an array whose voltage exceeds the controller limit.

For an off-grid site in Malaysia, use verified equipment consumption, design-month solar data and current component datasheets. The formulas below provide a transparent worksheet, but the assumptions must be replaced with actual site and product values before procurement.

Create the load and daily energy worksheet

Begin with the devices that must operate, their measured or datasheet power, operating hours and simultaneous peak demand. Do not start from the preferred panel or battery size.

For each load:

Daily energy (Wh/day) = Quantity × Input power (W) × Operating hours per day

A hypothetical surveillance and wireless site might look like this:

Load	Quantity	Input power each	Hours/day	Daily energy
IP cameras	2	8W	24	384Wh
Wireless radio	1	12W	24	288Wh
Network switch	1	10W	24	240Wh
Connected-load total		38W		912Wh/day

These figures are examples only. Actual camera consumption can change when infrared illumination, heaters, motors or other functions operate. Radio and switch consumption may also depend on traffic, PoE output and supply voltage.

Next, account for the power conversion path:

For direct DC loads, include DC-DC converter and cable losses.
For PoE equipment, include the switch or injector input consumption, not only the powered device load.
For AC equipment, include inverter efficiency and its standby consumption.
Include scheduled loads according to their true operating hours.
Check startup or surge current separately from daily energy.

If a preliminary design allowance of 15% is used for conversion and wiring losses:

Design daily energy = 912Wh × 1.15 = 1,049Wh/day

The 15% is a hypothetical planning value, not a universal rule. Replace it with efficiency data for the selected controller, converters, inverter and cabling. Avoid adding the same loss twice if it is already included in measured input power.

Also record peak demand:

Continuous peak load = Sum of all loads operating simultaneously

In this example, that is at least 38W before any startup surge or future expansion. For a more CCTV-specific workflow, see this guide to sizing an off-grid solar power system for CCTV.

Size battery storage from autonomy assumptions

Battery capacity depends on daily energy, required autonomy, allowable depth of discharge, discharge-path efficiency, temperature and ageing allowance. Nominal watt-hours alone do not represent the energy that will reach the equipment.

A practical preliminary formula is:

Required nominal battery energy (Wh) = Daily energy × Autonomy days ÷ (Design depth of discharge × Discharge-path efficiency)

Using the hypothetical load above:

Daily design energy: 1,049Wh
Required autonomy: 2 days
Design depth of discharge: 80%, or 0.80
Discharge-path efficiency: 90%, or 0.90

Battery energy = 1,049 × 2 ÷ (0.80 × 0.90) = 2,914Wh

For a nominal 12.8V battery:

Battery capacity (Ah) = 2,914Wh ÷ 12.8V = approximately 228Ah

This does not mean every project should use 80% depth of discharge or assume 90% efficiency. The selected battery manufacturer’s recommendations, battery management system limits, operating temperature and intended service conditions take priority.

Choose autonomy deliberately

More autonomy increases storage cost and physical size, but it does not solve insufficient solar generation. A large battery paired with a small array may take several favourable days to recover after cloudy weather.

Define autonomy according to:

Whether the site is safety-critical or convenience-based
Seasonal cloud and rain patterns
Acceptable load shedding
Maintenance access
Whether a backup supply is available
The consequences of a low-voltage shutdown
Battery charging restrictions at the expected site temperature

Where appropriate, classify loads as essential and non-essential. During low-energy conditions, a system may preserve communications while disconnecting lighting or secondary devices. This requires compatible control and output arrangements; it should not be assumed from battery capacity alone.

Check battery voltage throughout discharge

A “12V” load does not necessarily accept the full voltage range of a 12.8V LiFePO4 battery. Verify:

Maximum battery charging voltage
Typical operating voltage
Low-voltage disconnect threshold
Battery management system cut-off
Load minimum and maximum input voltage
Cable voltage drop at peak current
DC-DC converter input and output range

Battery amp-hours are meaningful only when the nominal voltage is also known. For system-level comparison, watt-hours provide a clearer starting point.

Estimate PV energy for the design month

The PV array must support the daily load and restore energy removed from the battery. Use solar irradiation or peak sun hours for the least favourable design month, not an annual average.

Malaysia’s high module temperatures, cloud cover, rain patterns and site-specific shading can materially reduce daily production. Obtain credible design-month data for the actual location and panel orientation, then apply realistic system losses.

For normal daily recovery:

PV array power (W) = Daily design energy ÷ (Design peak sun hours × PV system factor)

Assume, purely for illustration:

Daily design energy: 1,049Wh
Design-month peak sun hours: 3.5 hours
Combined PV system factor: 70%, or 0.70

PV power = 1,049 ÷ (3.5 × 0.70) = approximately 428W

The system factor may account for module temperature, dirt, mismatch, wiring, controller conversion and imperfect orientation. It should be based on project conditions rather than copied automatically.

Include battery recovery after poor weather

An array that merely covers average daily consumption may not restore the battery promptly after an autonomy event.

Suppose the battery has supplied one additional day of energy, equal to 1,049Wh, and the target is to recover that deficit over two suitable solar days:

Additional recharge power = Energy deficit ÷ (Recovery days × Peak sun hours × System factor)

Additional recharge power = 1,049 ÷ (2 × 3.5 × 0.70) = approximately 214W

The coordinated preliminary array would therefore be:

428W for the daily load + 214W for recovery = 642W

This example demonstrates why battery autonomy and PV sizing must be evaluated together. A designer may choose a different recovery period, but it should be documented. The controller and battery must also be able to accept the resulting charge current.

Do not treat peak sun hours as guaranteed sunshine duration. They represent equivalent solar energy, while actual output varies throughout the day. Partial shading, panel orientation, module temperature and prolonged cloud can all alter results.

Check MPPT voltage, current and power limits

An MPPT controller must pass four separate checks:

Maximum PV open-circuit voltage
Maximum PV input current or short-circuit current
Maximum output charge current
Permitted PV power for the selected battery voltage

Passing only the wattage check is not sufficient.

Calculate cold-weather open-circuit voltage

PV module open-circuit voltage rises as cell temperature falls. Even in Malaysia, the array must be checked against the lowest credible site temperature, especially at elevated locations or during cool, clear mornings.

A simplified calculation is:

Corrected module Voc = Voc at STC × [1 + |Voc temperature coefficient| × (25°C − Minimum cell temperature)]

If a hypothetical module has:

Voc at STC: 49.5V
Voc temperature coefficient: −0.28%/°C, expressed as 0.0028
Minimum design cell temperature: 10°C

Then:

Corrected Voc = 49.5 × [1 + 0.0028 × (25 − 10)] = approximately 51.6V

For two modules in series:

String Voc = 2 × 51.6V = approximately 103.2V

The result must remain below the controller’s absolute PV voltage limit, including any margin required by its manufacturer. Use the module’s current datasheet temperature coefficient and a defensible minimum cell temperature. Ambient temperature and cell temperature are not always identical.

Check operating voltage in hot conditions

Cold Voc protects against controller overvoltage, but hot-weather operating voltage is also important. As modules heat up, maximum power voltage falls. The string must still provide enough voltage for the MPPT controller to charge the battery across its required charging range.

Confirm:

Minimum MPPT operating voltage
Controller start-up voltage
Hot-condition array Vmp
Battery maximum charging voltage
Any required voltage difference between PV input and battery

Check PV input current

Series connections increase voltage, while parallel strings increase current.

Array short-circuit current = Module Isc × Number of parallel strings

Apply any required current factor specified by the controller manufacturer or applicable system design rules. For example, two parallel strings using hypothetical 11A Isc modules would produce 22A before an additional design factor is applied.

Check both the controller’s PV input-current limit and the rating of connectors, isolators, fuses and cables. A controller may allow some PV power oversizing while still imposing a strict input-current limit.

Check controller charge current

For a preliminary estimate:

Maximum charge current ≈ PV array power ÷ Lowest relevant battery charging voltage

A 640W array connected to a nominal 12V-class system could potentially require more than 50A of controller output:

640W ÷ 12V = approximately 53A

Actual output depends on solar conditions, controller efficiency, programmed limits and battery voltage. Confirm the controller’s rated charge current and its permitted array wattage at the chosen battery voltage.

MPPT and PWM controllers behave differently, particularly when array voltage differs from battery voltage. This MPPT versus PWM controller comparison explains the practical distinctions.

Coordinate battery charging and load outputs

A valid electrical design must coordinate the battery’s charge limits with the controller’s output. A large array does not help if the controller clips charge current continuously or the battery management system rejects the charging profile.

Verify the following against current datasheets:

Battery chemistry and nominal voltage
Recommended bulk, absorption and float settings, where applicable
Maximum continuous and peak charge current
Maximum discharge current
Low-voltage disconnect and reconnect levels
Charging and discharging temperature ranges
Battery management system protection behaviour
Controller temperature compensation requirements
Whether equalisation must be disabled for the selected battery
Controller output rating and per-output limits

LiFePO4 settings should follow the battery supplier’s requirements. Do not reuse lead-acid charging values or enable equalisation unless specifically permitted.

Check every load output and cable run

For systems with multiple DC outputs, verify both the combined load and each output’s individual current. Three low-power devices may be acceptable in total but still overload one port if they are connected incorrectly.

Cable voltage drop can be estimated using:

Voltage drop = 2 × Cable length × Current × Conductor resistivity ÷ Conductor area

Use the full outgoing and return path. At low DC voltages, even a modest voltage drop can cause remote equipment to restart before the battery reaches its intended low-voltage threshold.

Record:

Longest cable run
Cable material and cross-sectional area
Continuous and peak current
Voltage at the source
Minimum voltage expected at the load
Connector, fuse and surge-protection ratings

If the site uses PoE, include the DC-to-PoE conversion stage and confirm the required PoE standard, port power, voltage and distance. Do not assume a 12V DC output can directly power every network device.

Use documented systems as comparison points, not formulas

Maxsol documents two complete solar system examples that can be compared with the worksheet:

Documented system	PV configuration	Battery	Controller	DC12V outputs
CS12060 120W 60Ah 768Wh solar system	120W monocrystalline panel	12.8V 60Ah, 768Wh LiFePO4	MPPT	3
CS400150 400W 150Ah 1920Wh solar system	Two 200W modules, 400W total	12.8V 150Ah, 1920Wh LiFePO4	MPPT	4

These are documented system examples, not interchangeable component prescriptions. Their nominal battery watt-hours are not the same as usable energy delivered to a load. Runtime and recharge performance depend on verified consumption, allowable discharge, solar input, losses and site conditions.

Before specifying either system, verify the supplied controller configuration, PV open-circuit voltage and current limits, battery charging profile, charging temperatures and protection behaviour.

Stress-test and document the final design

A worksheet is not complete until it has been tested against combinations of unfavourable conditions. A system that works under separate average assumptions may fail when high load, low solar input and a partly discharged battery occur together.

Run at least these scenarios:

Scenario	What to verify
Maximum continuous load	Output, converter and cable ratings remain adequate
Startup or surge event	Protection does not trip and voltage remains acceptable
Design-month low solar	Daily generation supports the planned energy budget
Autonomy event	Essential loads operate for the specified period
Post-autonomy recovery	Array and controller restore the battery within the target time
Cold module condition	Array Voc stays below the controller’s absolute limit
Hot module condition	Array Vmp remains adequate for battery charging
Low battery voltage	Remote loads still receive their minimum input voltage
High battery state of charge	Charging voltage remains within the battery profile
Restricted charging temperature	Controller and battery protection respond as intended
Partial shading or dirt	Reduced production does not invalidate critical assumptions

Keep a design record

The final worksheet should document:

Device quantities, measured power and operating schedules
Daily Wh and simultaneous peak W
Conversion, cable and standby losses
Autonomy target and load-shedding plan
Battery voltage, nominal Wh and design depth of discharge
Design-month solar input and its source
PV derating assumptions
Battery recovery period
Module Voc, Vmp, Isc and temperature coefficients
Series and parallel module arrangement
Controller voltage, current and power limits
Battery charging and temperature settings
Cable sizes, protection devices and voltage-drop results
Spare capacity and any planned future loads

Common mistakes include using annual-average sunlight, calculating battery amp-hours without voltage, ignoring controller charge-current limits, treating nominal battery energy as fully usable, and assuming a larger battery automatically gives reliable autonomy. Another frequent error is adding panels without recalculating series Voc and parallel-string current.

The final design should be reviewed whenever the load list, panel arrangement, battery, controller, cable distance or installation environment changes. As a practical next step, compare the linked Maxsol system examples against the project’s port count, verified load, cable distance, power budget, environmental conditions and management requirements, then confirm every relevant value in the current product and component datasheets.