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Off-Grid Solar System Sizing Calculator

Size an off-grid solar array, battery bank, MPPT or PWM controller and inverter from daily loads, sunless days and worst-month peak sun hours.

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Published 28. October 2026 · version 1

#autonomy #battery bank #inverter sizing #load audit #MPPT #off-grid PV #peak sun hours #PWM

Use this off grid solar calculator to turn a daily load audit into a preliminary battery bank, solar array, charge controller and inverter specification. It suits cabins, workshops, remote equipment and classroom design exercises. The defaults describe a small system with lighting, refrigeration, electronics, a pump and direct DC loads. Replace them with measured consumption and the electrical data for your equipment.

Start with the load list. Enter the combined running watts for each row, including the number of appliances, and its equivalent full-power hours per day. A refrigerator connected for 24 hours may run its compressor for only part of that time. Add direct DC loads separately. The audit table shows each contribution; the battery-terminal total also includes inverter conversion losses and any additional idle draw.

Choose the number of sunless days the bank should support, its nominal voltage, allowed depth of discharge and retained capacity at the design temperature and age. This solar battery bank calculator gives both the theoretical minimum and a whole-battery arrangement. Series units establish voltage; parallel strings establish ampere-hours. Confirm that the selected batteries and their management system permit that arrangement and can deliver the reported running and starting currents.

For the array, enter peak sun hours from the worst design month at the actual panel orientation and tilt. This is daily irradiation expressed as equivalent hours at full reference irradiance, not the length of the day. Account for local shading. Enter efficiencies as decimal fractions and avoid counting a loss twice. The solar panel calculator rounds the required array up to complete equal strings and plots how the continuous requirement changes with sun availability.

MPPT and PWM use different sizing paths. MPPT works from energy and panel watts; PWM works from ampere-hours and an estimate of module operating current. A PWM array must match the battery's nominal voltage. Check cold open-circuit voltage, hot operating voltage and the controller's permitted PV short-circuit current. The reported controller current is a minimum design target, not a product recommendation.

The inverter must satisfy continuous watts, continuous VA and starting VA for the loads that can operate together. Enter additional motor starting demand above its normal running demand. Check the manufacturer's permitted surge duration and thermal derating.

This is a daily energy-balance sizing method. It conservatively sends all load energy through storage; it does not simulate hourly weather, charge taper, battery chemistry, detailed IV curves or outage probability. The recovery estimate assumes repeated design-average sunshine and unlimited charge acceptance. Cable sizes, protective devices, earthing, structural support and local compliance require separate design. Installation work belongs to a qualified person; check against the standard in force and the equipment instructions.

How it is calculated

Daily AC energy is $\sum P_i h_i$ . Battery-terminal daily energy is $E_b=E_{AC}/\eta_{inv}+E_{DC}+P_{idle}h_{inv}$ . Every listed AC load is assumed capable of running together. Continuous inverter targets are $k_{inv}P_{AC}$ and $k_{inv}P_{AC}/PF$ ; starting VA includes the entered incremental starting demand. The battery starting-current envelope treats starting VA as W, conservatively. Inverter idle power enters instantaneous current targets only when energized hours are positive.

Battery capacity follows VeraSol (2024), pp.27–33: $E_{nom}=E_b N/(DOD\,f_{cap})$ and $C_{nom}=E_{nom}/V_{bank}$ . Divide by individual battery Ah and round parallel strings upward; series units must give the requested bank voltage. Battery energy is quoted at battery terminals, so round-trip efficiency is not applied again to this capacity calculation.

MPPT array sizing follows VeraSol pp.57–59: $P_{req}=k_{PV}E_b/(H f_{PV}\eta_{wire}\eta_{CC}\eta_{b,Wh})$ . PWM follows pp.53–56: $I_{req}=k_{PV}Q_{day}/(H\eta_{b,Ah}\eta_{wire})$ . Estimate PWM module current as the midpoint of Isc and Imp, then apply its current derating. Imp is computed from rated watts / Vmp. Round up complete parallel PV strings. Controller output rating uses the whole installed array: MPPT $k_I P_{array}/V_{low}$ ; PWM $k_I I_{sc,array}$ . Check the separate factored PV short-circuit current against the permitted input limit. Victron (2020), pp.1–4 explains the voltage-to-current conversion distinction.

Cold string voltage uses $N_s V_{oc}[1+|\beta_{Voc}|\Delta T]$ (VeraSol p.64). Hot string Vmp uses the entered retained voltage factor. Ideal recovery divides the discharged autonomy energy by the design-average daily surplus; surplus at or below 1e-9 of daily battery-terminal energy is treated as numerical zero and reported as No surplus. The schematic shows the battery charging from and discharging into the shared DC bus feeding both load branches. This DC-coupled topology is consistent with the Victron SmartSolar installation manual, section 4.6. The drawing is an energy-flow schematic, not a wiring plan.

Assumptions

All daily load energy passes through the battery, including daytime consumption. This is conservative; weather and coincident daytime PV are not simulated. Enter efficiencies and retained fractions as decimals, not percentages. The retained capacity factor covers temperature, age and discharge rate; battery Wh/Ah efficiencies affect array sizing separately.

Default load powers and use times, LFP voltages, 100 Ah battery units, 3 peak sun hours, 0.90 retained battery capacity, 0.85 retained hot Vmp, inverter factors and controller limits are illustrative design inputs, not certified product specifications or local climate data. Default PV module electrical data and Voc coefficient are from VeraSol p.56. The 1.20 array factor suits an illustrative lithium design; use at least the guidance's 1.30 for lead acid unless your documented design supports another allowance.

PWM assumes crystalline modules with a suitable voltage/current plateau; the midpoint-current estimate requires checking against IV curves. PV current factor 1.25 is an editable design allowance. No standard table or jurisdiction-specific protective-device rule is implemented. Temperatures are entered as a Kelvin difference below STC; the field help gives Celsius and Fahrenheit conversions. Electrical quantities use W, Wh, V, A, Ah and VA in both metric and US display modes.

Where it applies

Preliminary sizing of DC-coupled, fixed-load, off-grid PV/battery systems. Results are continuous minimum targets plus whole-component rounding, not a validated bill of materials. Confirm controller power limits, per-tracker current and voltage windows, startup requirements, allowable PV oversizing, connector ratings and thermal derating using actual equipment documentation. Cold Voc must remain below the controller's absolute limit, with any manufacturer-required tolerance margin.

Confirm battery maximum charge/discharge currents, surge duration, BMS compatibility, temperature charging restrictions and allowed series/parallel counts. Larger individual units are preferable to many parallel strings. Lead-acid discharge-rate effects require manufacturer capacity curves; this tool has no Peukert or electrochemical model. Surge VA and watts alone do not establish motor starting success. Recovery can take longer due to absorption/taper and charging-current limits.

Worst-month mean sunshine does not guarantee reliability through an extreme cloudy spell. Use time-series simulation for critical service and assess backup generation separately. No grid interconnection, conductor sizing, overcurrent protection, earthing, lightning, structural, regulatory or cost calculation is included. Installation work belongs to a qualified person. Check against the standard in force and manufacturer instructions.

References

VeraSol / Schatz Energy Research Center / GSES, Component-Based Off-Grid Solar Energy Systems: System Design Guidelines (2024), pp.27–35, 50–64. https://verasol.org/wp-content/uploads/2022/06/Design-Guidelines_Component-based-Off-GridSolarEnergySystems_241010.pdf
Victron Energy, Which solar charge controller: PWM or MPPT? (20 January 2020), pp.1–4. https://www.victronenergy.com/upload/documents/Technical-Information-Which-solar-charge-controller-PWM-or-MPPT.pdf
Victron Energy, SmartSolar MPPT 75/10 up to 100/20, installation manual, section 4.6: DC-coupled PV, battery and load connections. https://www.victronenergy.com/media/pg/Manual_SmartSolar_MPPT_75-10_up_to_100-20/en/installation.html

Inputs

Input	Symbol	Unit	Default
Lighting: total running power Combined power of every appliance in this row. Include quantities here; set unused rows to zero.	`P_1`	W	40
Lighting: equivalent full-power hours/day For cycling appliances use equivalent running hours, not elapsed connection time.	`h_1`	h	5
Refrigerator: total running power Combined power of every appliance in this row. Include quantities here; set unused rows to zero.	`P_2`	W	100
Refrigerator: equivalent full-power hours/day For cycling appliances use equivalent running hours, not elapsed connection time.	`h_2`	h	8
Electronics: total running power Combined power of every appliance in this row. Include quantities here; set unused rows to zero.	`P_3`	W	100
Electronics: equivalent full-power hours/day For cycling appliances use equivalent running hours, not elapsed connection time.	`h_3`	h	4
Pump / other AC: total running power Combined power of every appliance in this row. Include quantities here; set unused rows to zero.	`P_4`	W	500
Pump / other AC: equivalent full-power hours/day For cycling appliances use equivalent running hours, not elapsed connection time.	`h_4`	h	0.5
Direct DC loads: total running power Loads on the battery bus. Include any DC converter losses.	`P_dc`	W	10
DC loads: equivalent hours/day Set power to zero if there are no DC loads.	`h_dc`	h	24
Inverter energy efficiency Decimal fraction at the expected load; 0.92 means 92%.	`eta_inv`		0.92
Additional inverter idle draw DC-side overhead not already included in the stated efficiency; avoid counting it twice.	`P_idle`	W	8
Inverter energized hours/day Must cover each AC row's running hours.	`h_inv`	h	24
Aggregate running power factor Running W / VA. Check actual appliance VA, especially motors.	`pf_ac`		0.85
Inverter continuous sizing factor Design allowance, not a regulatory factor.	`k_inv`		1.25
Additional simultaneous starting demand Starting VA ABOVE running VA. Sum increments for motors that can start together; use measured/nameplate values.	`S_extra`	VA	1500
Battery bank nominal voltage Use actual chemistry voltage, e.g. 25.6 V for an 8-cell LFP bank.	`V_bank`	V	25.6
Minimum operating battery voltage Under-load voltage at the lower operating limit. Used for conservative controller and discharge current ratings.	`V_low`	V	22.4
Maximum battery charging voltage Manufacturer's charge setting, including temperature compensation where relevant.	`V_charge`	V	28.8
Days of autonomy Number of days with no solar input, at the entered daily loads.	`t_auto`	d	2
Allowed depth of discharge Decimal fraction of nominal capacity. Follow battery manufacturer limits.	`dod_max`		0.8
Retained battery capacity Combined cold, ageing and discharge-rate capacity fraction. 0.90 leaves 90% available before the DoD limit. This is not round-trip efficiency.	`f_cap`		0.9
Individual battery nominal voltage Must divide the bank voltage into a whole number of series units. Confirm the battery permits series connection.	`V_unit`	V	12.8
Individual battery rated capacity Rated ampere-hours at the relevant discharge rate. Confirm permitted parallel strings.	`C_unit`	Ah	100
Worst design month's peak sun hours/day Daily plane-of-array irradiation divided by 1 kW/m². Use local tilt, orientation and shading; not daylight hours.	`H_sun`	h	3
Array oversizing factor Reserve for ageing and recovery. VeraSol uses at least 1.30 for lead acid and 1.10 for lithium. This does not guarantee a recovery deadline.	`k_pv`		1.2
PV-side wiring efficiency Decimal fraction. In PWM mode represents current transmission allowance.	`eta_wire`		0.97
Charge controller type PWM requires a nominal-voltage matched array. MPPT converts excess PV voltage into charging current.	`controller`		MPPT · size from watts and energy
MPPT: retained module STC power Combined operating-temperature, dirt and tolerance derating; exclude wiring, controller and battery losses entered separately.	`f_pv`		0.8
MPPT conversion efficiency Decimal fraction; used in the energy yield, not as a reduction of required controller current rating.	`eta_cc`		0.98
MPPT: battery round-trip energy efficiency Wh delivered / Wh charged. All daily loads conservatively pass through storage.	`eta_bwh`		0.92
PWM: retained module current Dirt and tolerance current derating. Default is 0.95 × 0.97, VeraSol Example 20.	`f_current`		0.9215
PWM: battery coulombic efficiency Ah delivered / Ah charged; different from Wh efficiency.	`eta_bah`		0.9
Module rated STC power Default module electrical data: VeraSol Example 20, p. 56. Replace with your module datasheet.	`P_mod`	W	330
Module STC maximum-power voltage Default 37.8 V from VeraSol Example 20.	`V_mp`	V	37.8
Module STC open-circuit voltage Default 46.2 V from VeraSol Example 20.	`V_oc`	V	46.2
Module STC short-circuit current Default 9.27 A from VeraSol Example 20.	`I_sc`	A	9.27
PV modules in each series string The calculator rounds up the number of parallel strings, keeping complete equal strings.	`n_series`		2
Cold design temperature below 25 °C Temperature DIFFERENCE: 25 °C minus minimum design cell temperature. −10 °C gives 35 K; 20 °C gives 5 K. In Fahrenheit: (77 − Tmin°F) / 1.8.	`dT_cold`	K	35
Fractional Voc rise per kelvin of cooling Enter the POSITIVE magnitude: a datasheet coefficient of −0.29%/°C becomes 0.0029/K.	`beta_oc`	1/K	0.0029
Retained Vmp at hottest design cells Vmp(hot) / Vmp(STC), from temperature coefficients or IV curves. Default 0.85 is an illustrative design allowance; verify for your site.	`f_hot`		0.85
Controller absolute maximum PV voltage Cold string Voc must remain strictly BELOW this rating. Use the actual controller datasheet.	`V_limit`	V	150
Controller maximum permitted PV short-circuit current Compare with factored whole-array Isc. Limits for connectors, reverse polarity and individual trackers may differ.	`I_limit`	A	50
MPPT voltage headroom above charging voltage Use manufacturer's required hot operating/startup allowance. PWM uses a fixed 0.5 V allowance here.	`V_head`	V	5
Controller current sizing factor User design allowance applied to PV Isc and controller current. Not an NEC or IEC conductor/OCPD calculation.	`k_current`		1.25

Results

With the default inputs above, the calc gives:

Result	Value
Battery-terminal daily energy	2,225 Wh
Minimum nominal battery energy	6,182 Wh
Minimum bank capacity	241.5 Ah
Selected nominal bank energy	7,680 Wh
Total individual batteries	6
Selected-bank autonomy	2.48 d
Minimum array before string rounding	1,272 W
Installed array STC power	1,320 W
Total PV modules	4
Minimum controller output rating	73.7 A
PV input short-circuit rating target	23.2 A
Cold string open-circuit voltage	101.8 V
Inverter continuous real-power target	925 W
Inverter continuous apparent-power target	1,088 VA
Inverter starting apparent-power target	2,371 VA
Bank continuous discharge target	36.7 A
Bank starting-current upper estimate	115.8 A
Ideal recovery after sunless interval	8.2 d

Checked against sources

Each case below was calculated by this calc and agrees with the value from its source (within the tolerance where one is shown).

VeraSol Example 6 · 2-day battery bank

Source: VeraSol System Design Guidelines (2024), p.30, Worked Example 6. Published energy 6306 Wh and 263 Ah are rounded. Uses 2207 Wh/day already at the battery terminals, not an AC-side load.

Inputs: Lighting: total running power = 0 W; Refrigerator: total running power = 0 W; Electronics: total running power = 0 W; Pump / other AC: total running power = 0 W; Direct DC loads: total running power = 2,207 W; DC loads: equivalent hours/day = 1 h; Additional inverter idle draw = 0 W; Additional simultaneous starting demand = 0 VA; Battery bank nominal voltage = 24 V; Individual battery nominal voltage = 12 V; Minimum operating battery voltage = 21 V; Allowed depth of discharge = 0.7; Retained battery capacity = 1; the others as above.

Result	Source	This calc
Usable energy for sunless interval	4,414 Wh	4,414 Wh
Minimum nominal battery energy	6,306 Wh	6,306 Wh
Minimum bank capacity	263 ± 0.5 Ah	262.7 Ah

VeraSol Examples 22–23 · MPPT array

Source: VeraSol pp.53,58–59, Examples 18,22–23: 1500 Wh AC / 0.9 + 112 Wh DC; 4.6 PSH; efficiencies 0.97×0.95×0.8; power derate 0.86×0.95×0.97; 1.3 reserve. Intermediate published values are rounded, so the continuous requirement differs slightly. Three series modules follow p.64.

Inputs: Lighting: total running power = 150 W; Lighting: equivalent full-power hours/day = 10 h; Refrigerator: total running power = 0 W; Electronics: total running power = 0 W; Pump / other AC: total running power = 0 W; Direct DC loads: total running power = 28 W; DC loads: equivalent hours/day = 4 h; Additional inverter idle draw = 0 W; Inverter energy efficiency = 0.9; Battery bank nominal voltage = 24 V; Individual battery nominal voltage = 12 V; Minimum operating battery voltage = 21 V; Worst design month's peak sun hours/day = 4.6 h; Array oversizing factor = 1.3; PV modules in each series string = 3; MPPT conversion efficiency = 0.95; MPPT: battery round-trip energy efficiency = 0.8; MPPT: retained module STC power = 0.7925; Cold design temperature below 25 °C = 5 K; the others as above.

Result	Source	This calc
Battery-terminal daily energy	1,779 Wh	1,779 Wh
MPPT array requirement before string rounding	861 ± 2 W	860.4 W
Total PV modules	3	3
Installed array STC power	990 W	990 W

VeraSol Examples 18–20 · PWM current method

Source: VeraSol pp.53–56, Examples 18–20. The p.56 statement '22.3' conflicts with p.53's 23.3 A and the numerator of p.56's own division. Use the correct 23.3 A. Published Imp=8.73 A is rounded; rated P/Vmp gives 8.73016 A. No separate cable loss is included in that example.

Inputs: Lighting: total running power = 150 W; Lighting: equivalent full-power hours/day = 10 h; Refrigerator: total running power = 0 W; Electronics: total running power = 0 W; Pump / other AC: total running power = 0 W; Direct DC loads: total running power = 28 W; DC loads: equivalent hours/day = 4 h; Additional inverter idle draw = 0 W; Inverter energy efficiency = 0.9; Battery bank nominal voltage = 24 V; Individual battery nominal voltage = 12 V; Minimum operating battery voltage = 21 V; Worst design month's peak sun hours/day = 4.6 h; Array oversizing factor = 1.3; Charge controller type = PWM · size from ampere-hours and current; PV modules in each series string = 1; PV-side wiring efficiency = 1; Cold design temperature below 25 °C = 5 K; the others as above.

Result	Source	This calc
PWM required array operating current	23.3 ± 0.1 A	23.272 A
Derated PWM module operating current	8.3 ± 0.02 A	8.294 A
Selected PV strings in parallel	3	3
Installed array STC power	990 W	990 W

VeraSol Example 28 · cold string voltage

Source: VeraSol p.64, Example 28: three modules, 46.2 V Voc, −0.29%/°C, 20 °C cells (5 K below STC). Published 140.7 V rounds per-module voltage to 46.9 V first; unrounded physics gives 140.6097 V.

Inputs: PV modules in each series string = 3; Cold design temperature below 25 °C = 5 K; the others as above.

Result	Source	This calc
Cold open-circuit voltage per module	46.9 V	46.87 V
Cold string open-circuit voltage	140.7 V	140.6 V

Victron · 100 W MPPT voltage conversion

Source: Victron, Which solar charge controller: PWM or MPPT? (20 Jan 2020), p.3. Published ideal current is 7.7 A from 100 W at 13 V. This checks ideal power conversion; the selected controller current rating separately uses minimum battery voltage and allowance.

Inputs: Lighting: total running power = 1 W; Lighting: equivalent full-power hours/day = 1 h; Refrigerator: total running power = 0 W; Electronics: total running power = 0 W; Pump / other AC: total running power = 0 W; Direct DC loads: total running power = 0 W; Additional inverter idle draw = 0 W; Additional simultaneous starting demand = 0 VA; Module rated STC power = 100 W; Module STC maximum-power voltage = 18 V; Module STC open-circuit voltage = 21.6 V; Module STC short-circuit current = 6.12 A; PV modules in each series string = 1; Battery bank nominal voltage = 12 V; Individual battery nominal voltage = 12 V; Minimum operating battery voltage = 11 V; Maximum battery charging voltage = 13 V; MPPT voltage headroom above charging voltage = 0 V; Retained Vmp at hottest design cells = 0.9; the others as above.