Solar Panel, Battery and MPPT Sizing Worksheet

A reliable off-grid design follows a fixed dependency order: calculate the load first, size the battery for the required autonomy, estimate the PV array from design-month solar energy, and then check that the MPPT controller can safely handle the array and charge the battery.

The calculations must be treated as one coordinated worksheet. Changing the number of cameras, overnight heating period or autonomy target affects the battery, array, controller and cabling—not just one component. Always verify design-month solar data for the actual site and use current datasheets for voltage, current, temperature and charging limits.

Create the load and daily energy worksheet

Start by listing every device powered by the system, including supporting equipment that is easy to overlook. For a remote surveillance or wireless site, that may include:

Cameras, illuminators and heaters
PoE or Ethernet switches
Wireless bridges, access points or mobile routers
Recording or edge-computing equipment
Relays, sensors and access-control devices
DC converters and inverter standby consumption
Cabinet fans, heaters or environmental controls
The controller’s own operating consumption

For each item, record its normal power, peak power and operating hours. Do not rely solely on a device’s headline rating where measured or manufacturer-documented operating consumption is available.

The basic daily-energy formula is:

Daily energy in Wh = quantity × operating power in W × operating hours

A hypothetical worksheet might look like this:

Load	Quantity	Operating power each	Hours per day	Daily energy
IP cameras	4	7W	24	672Wh
Wireless radios	2	6W	24	288Wh
Network switch overhead	1	8W	24	192Wh
Cabinet heater	1	20W	4	80Wh
Monitoring and control load	1	2W	24	48Wh
Subtotal				1,280Wh
Distribution and conversion allowance, 15%				192Wh
Worksheet design load				1,472Wh/day

This is an example calculation, not a recommended system specification. The heater operating time, camera consumption and wireless radio duty cycle must be replaced with figures appropriate to the site.

Separate average energy from peak power

Daily Wh determines how much energy the system must store and replace. Peak W and peak current determine whether the wiring, outputs, converter and controller can support the equipment at any instant.

Record at least three load values:

Continuous load: the normal combined operating power.
Short-duration peak: start-up, infrared switching, radio transmission or heater activation.
Worst concurrent load: the maximum credible combination of devices operating together.

Where PoE conversion is involved, be clear about the measurement boundary. If the switch input power already includes endpoint consumption and conversion losses, adding each powered device again would double-count the load. For a surveillance-specific starting point, see the guide to sizing an off-grid solar power system for CCTV.

Size battery storage from autonomy assumptions

Battery sizing begins with the daily load and the number of days the site is expected to operate without sufficient solar recharge. Autonomy is a design assumption, not a guaranteed runtime.

A transparent nominal storage formula is:

Required nominal battery Wh = daily load × autonomy days ÷ usable fraction ÷ temperature factor ÷ discharge-path efficiency

Using the hypothetical 1,472Wh/day load:

Autonomy target: 2 days
Planned usable fraction: 80%, or 0.80
Temperature factor: 90%, or 0.90
Discharge-path efficiency: 95%, or 0.95

The result is:

1,472 × 2 ÷ 0.80 ÷ 0.90 ÷ 0.95 = approximately 4,304Wh nominal storage

For a nominal 12.8V battery:

Required Ah = 4,304Wh ÷ 12.8V = approximately 336Ah

Round a practical selection upwards rather than treating the calculated value as an exact product size. Then confirm that the selected battery can provide the required continuous and peak discharge current.

Nominal Wh is not delivered energy

A battery marked 768Wh does not necessarily deliver 768Wh to the loads. Delivered energy depends on:

Permitted depth of discharge
Battery management system cut-off behaviour
Battery temperature
Cable and connector losses
DC converter or inverter efficiency
Battery age and condition
Load current and duty cycle
Low-voltage disconnect settings

LiFePO4 batteries also have specific charging-temperature and voltage requirements. Check the battery’s permitted charging range, maximum continuous charge current, discharge limits and protection behaviour. Where low-temperature charging is restricted, the system may need a suitable control strategy rather than simply a larger panel.

Choose autonomy with recharge capacity in mind

Increasing battery capacity without increasing the array may produce a system that runs longer during poor weather but cannot recharge promptly afterwards. That can leave the battery in a persistently low state of charge.

For UK outdoor sites, the design month will often be a winter month with low irradiation and short daylight hours. The appropriate autonomy target depends on the consequence of an outage, seasonal access, shading and whether another charging source is available. No finite battery and PV combination can promise uninterrupted operation through every weather sequence.

Estimate PV energy for the design month

The array must support the daily load during the selected design month and, where required, provide additional energy to recover from a battery deficit.

A useful first-pass formula is:

Required PV power = daily energy ÷ design-month peak sun hours ÷ system yield

“Peak sun hours” can be derived from daily plane-of-array irradiation: for example, 1.2kWh/m²/day is treated as approximately 1.2 peak sun hours for this sizing calculation. Obtain data for the site’s location, orientation and design month from a reputable solar database or a qualified site assessment.

If the example site receives 1.2 peak sun hours and the assumed overall PV-to-load yield is 70%:

1,472Wh ÷ 1.2h ÷ 0.70 = approximately 1,752W of PV

This deliberately demanding hypothetical example illustrates why a modest-looking continuous load can require a large winter array. It must not be applied to a site without verifying local data.

The system-yield factor may account for:

Module temperature
Non-ideal orientation
MPPT conversion losses
Cable losses
Module mismatch and soiling
Battery charging losses
Typical variations around the solar estimate

Avoid counting the same loss twice. Document which losses are already included in the load worksheet and which are included in the PV yield factor.

Include recovery energy where necessary

A design that merely replaces an average day’s load may not restore a depleted battery. If the system must recover a 2,000Wh deficit over two suitable solar days, add 1,000Wh of recovery energy to each day’s requirement:

PV power = (1,472Wh + 1,000Wh) ÷ 1.2h ÷ 0.70

PV power = approximately 2,943W

A larger recovery target can quickly make a design impractical because of space, cost or controller limits. Possible responses include reducing the load, scheduling non-essential equipment, extending the recovery period, increasing the system voltage or providing a verified supplementary charging source.

Check physical site conditions

Solar data alone does not account for every obstruction. Review:

Winter shading from buildings, trees and masts
Low solar elevation and the chosen module tilt
Dirt, leaves, snow and bird fouling
Module orientation and available mounting area
Cable distance between array, controller and battery
Wind exposure and the suitability of the mounting structure

Use the worst relevant operating season rather than an annual-average irradiation figure when year-round operation is required.

Check MPPT voltage, current and power limits

An MPPT controller cannot be selected from panel wattage alone. Check PV open-circuit voltage, input current, permitted array power and controller output current separately. For a practical explanation of the technology choice, read the comparison of MPPT and PWM outdoor solar charge controllers.

Calculate cold-weather open-circuit voltage

PV open-circuit voltage rises as cells become colder. The series-string voltage in cold, bright conditions must remain below the controller’s absolute maximum PV input voltage.

A simplified calculation is:

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

For example, assume:

Module Voc at STC: 40V
Voc temperature coefficient: −0.28%/°C, expressed as 0.0028
Design minimum cell temperature: −10°C
Two modules in series

The cold module voltage would be:

40 × [1 + 0.0028 × (25 − −10)] = 43.92V

The two-module string would therefore be:

43.92V × 2 = 87.84V

This is only a method demonstration. Use the actual module coefficient, controller limit and a defensible site temperature. The calculated voltage must remain below the controller’s absolute maximum, including any additional margin required by its manufacturer or the applicable design method.

Check array current

Modules in series increase voltage, while parallel strings increase current. A first-pass array short-circuit current calculation is:

Array Isc = module Isc × number of parallel strings

If a module has an Isc of 11A and the design uses two parallel strings, the array Isc is 22A before applying any required design factor.

Confirm all of the following:

Maximum controller PV short-circuit current
Maximum normal MPPT input current
Connector and cable current ratings
Combiner, isolation and overcurrent-protection requirements
Reverse-current implications for parallel strings
Manufacturer-specified current multipliers or safety factors

Do not assume the controller will simply limit any excess input safely. Some limits are operating limits; others are absolute limits that must never be exceeded.

Check controller output current

The controller’s battery-side current can be much higher than its PV-side current because it converts higher PV voltage to lower battery voltage.

A useful approximation is:

Maximum charge current ≈ array power × controller efficiency ÷ lowest relevant battery charging voltage

For an illustrative 800W array, 95% conversion efficiency and a 12V calculation voltage:

800W × 0.95 ÷ 12V = approximately 63A

The selected controller must support the resulting output current. The battery and battery management system must also accept it. Use the controller’s published sizing rules because permitted PV power may change with battery-system voltage.

Coordinate battery charging and load outputs

A successful design must align the complete voltage and current path, not merely the four headline component ratings.

Interface	What to verify
PV array to MPPT	Cold Voc, operating voltage range, Isc, input current and permitted array power
MPPT to battery	Charge profile, maximum charge current, cable size, fuse or breaker and temperature restrictions
Battery to load	Continuous current, peak current, BMS limits, low-voltage behaviour and branch protection
Load output to equipment	Maximum output current, connector rating, voltage drop and equipment voltage tolerance
Complete system	Earthing approach, isolation, surge exposure, enclosure environment and service access

A 12.8V LiFePO4 battery is described by its nominal voltage, but the operating bus voltage varies during charging and discharge. Every connected load must tolerate the highest charging voltage and continue operating above the intended low-voltage disconnect point.

Do not overload controller load terminals

Some charge controllers provide dedicated load outputs, but these are not automatically suitable for the system’s full load current. Check:

Total continuous output rating
Per-output or per-connector limit
Combined peak current
Inductive or capacitive start-up current
Low-voltage disconnect and reconnect thresholds
Whether output scheduling is supported or required

If loads are connected directly to the battery through a protected distribution block, the system still needs suitable low-voltage protection. Do not assume that the battery management system should act as the routine disconnect control; it is normally a final protective boundary, subject to its documented behaviour.

Treat documented kits as complete system examples

Maxsol documents two examples that illustrate different nominal system scales:

The CS12060 120W and 60Ah solar power system includes a 120W monocrystalline panel, 12.8V 60Ah 768Wh LiFePO4 battery, MPPT controller and three DC12V outputs.
The CS400150 400W and 150Ah solar power system uses two 200W modules for a 400W panel configuration, with a 12.8V 150Ah 1920Wh LiFePO4 battery, MPPT control and four DC12V outputs.

These are documented system examples, not interchangeable component prescriptions. Their nominal battery Wh figures should not be treated as guaranteed delivered energy or runtime. Before selecting either example, verify the supplied controller configuration, PV voltage and current limits, battery charge profile, charging temperatures, output ratings and protection against the completed worksheet.

Stress-test and document the final design

Before procurement, test the calculations against conditions that are more demanding than the base case.

Run at least six design scenarios

Winter energy case: lowest credible design-month solar input with normal loads.
Cold, clear case: highest expected array open-circuit voltage.
High-load case: all credible loads operating together.
Start-up case: camera, radio, heater and converter inrush or start-up peaks.
Battery recovery case: normal loads continue while the array replaces a defined deficit.
Low-temperature battery case: charging and discharge behaviour at the minimum enclosure temperature.

Also examine cable voltage drop at both continuous and peak current. Long 12V cable runs can create substantial losses and cause equipment resets even where the battery retains adequate energy.

Record the assumptions

The final design file should include:

Device list, quantity, measured or documented power and operating schedule
Continuous, peak and daily load totals
Loss assumptions and where they are applied
Autonomy target and usable battery fraction
Minimum and maximum battery temperatures
Design-month solar input, tilt, orientation and shading assumptions
Module Voc, Isc and temperature coefficients
Series and parallel array arrangement
Controller input-voltage, input-current, power and output-current checks
Battery charge and discharge limits
Load-output and branch-circuit ratings
Cable lengths, conductor sizes and calculated voltage drop
Protection, isolation and environmental requirements
Datasheet versions and calculation date

Avoid common sizing mistakes

The most frequent errors are:

Starting with an available panel rather than calculating the load
Using average annual solar data for a winter-critical site
Equating nominal battery Wh with usable delivered energy
Ignoring recovery time after low-solar periods
Checking module operating voltage but not cold Voc
Checking PV wattage but not controller current limits
Ignoring battery charge-current acceptance
Connecting loads that cannot tolerate the charging voltage
Omitting controller, converter, heater or network overhead
Assuming an output connector can carry the battery’s full rated current
Treating a calculated autonomy period as a guarantee

The practical next step is to complete the worksheet with verified site and load data, then compare the linked Maxsol system examples against the required output or network port count, continuous and peak load, cable and wireless distance, power budget, environment and management requirements.