How to Calculate Solar Inverter and Battery Bank Capacity for a Solar Panel Array

The Golden Rules of Solar System Sizing

A good off-grid or battery-backed system is not decided by how many panels fit on the roof. Three parts have to be balanced: the panels that make energy, the inverter that delivers it, and the batteries that hold it. Get the balance wrong and the symptoms are predictable. A big inverter on a small battery bank sags in voltage whenever a motor starts, and trips its low-voltage cutoff. A big battery bank on a small array never fully recharges, and a bank that lives at partial charge ages faster (lead-acid sulfates, lithium simply wastes capacity).

It helps to size each part against the quantity it actually has to handle:

Step 1: Calculating Daily Energy Consumption (kWh/day)

Everything downstream depends on this number, so start here. A monthly utility bill gives a rough average, but it hides busy days and seasonal swings. For an off-grid system, write down every load, its wattage, and how many hours it really runs in a day.

Daily energy is the sum over all loads:

Divide by 1,000 to get kWh. Nameplate watts overstate loads that cycle, like a fridge compressor, so use a plug-in energy meter where you can.

Handling Phantom Loads and Standby Drain

Small loads that never switch off, such as routers, set-top boxes, cameras, chargers and clocks, are easy to forget. A router drawing 15 W around the clock uses:

15 W × 24 h = 360 Wh/day (0.36 kWh/day)

Ten such devices at that level would already add up to about 3.6 kWh a day, and even a modest handful can reach 1 to 2 kWh. On top of the audited total, add 10% to 15% for forgotten loads and for the inverter's own idle consumption.

Step 2: Determining Solar Panel Array Requirements (kWp)

With daily energy (E_daily) known, the next question is how many watts of panels will produce it on an ordinary day at your location.

Understanding Peak Sun Hours (PSH)

Panels only reach their rated output at full midday sun. Light starts near zero at dawn, peaks around solar noon and fades by evening. Peak Sun Hours (PSH) squash that curve into one easy number.

One Peak Sun Hour is 1 kWh of sunlight falling on each square meter, the same as an hour at the standard test intensity of 1,000 W/m². If a roof receives 4,800 Wh/m² spread across 11 hours of daylight, that day counts as 4.8 Peak Sun Hours. Look up your own figure for the worst month you must cover, not the annual average.

Accounting for System Derating Factor (eta_sys)

Panel ratings come from a lab test at 25°C cell temperature, 1 kW/m² and AM 1.5 light. A real roof does worse, and the losses stack up:

• Heat: Cells in direct sun often run at 50°C to 65°C. Crystalline silicon loses roughly 0.35% to 0.40% of its power for each degree above 25°C, which is a 10% to 15% loss on a hot day.
• Dirt: Dust, pollen and bird droppings commonly cost 3% to 6%, more in dusty regions.
• Wiring: Voltage drop in string cables and connectors takes about 1.5% to 3%.
• Conversion: A good MPPT charge controller is 96% to 98% efficient.

Multiply the factors together (the heat loss is not at its worst all year, so use an average) and a typical overall efficiency, eta_sys, lands between 0.75 and 0.80. Battery losses come in later, in step 4.

Step 3: Sizing the Solar Inverter (Continuous & Surge Margins)

Panels and batteries are sized by energy over a day. The inverter is different: it has to cope with whatever is running right now, plus the moment a motor kicks in.

1. Continuous Rated Wattage

Add up the AC appliances that could realistically run together. Say a well pump (1,200 W), refrigerator (180 W), air conditioner (1,500 W) and office computers (350 W) are all on: that is 3,230 W.

Then add a 20% to 25% margin so the inverter is not running flat out all day and does not derate or shut down for heat on a hot afternoon. For the example, 3,230 W × 1.25 ≈ 4,040 W:

Inverter Continuous Rating = Continuous Load × 1.25

2. Inrush Surge Rating for Inductive Motor Loads

Motors in pumps, compressors, fridges and power tools draw a burst of current when they start from standstill (the locked-rotor current). It can be 2.5 to 5 times the running wattage, and sometimes more for a hard-starting compressor, lasting from tens of milliseconds to a second or so.

Check the inverter's surge rating against your biggest motor. Many off-grid inverters, especially low-frequency transformer models, can supply about 200% of rated power for several seconds, but figures vary by model, so read the datasheet. The estimator below assumes a surge of 2.5 times the continuous load. If one pump or compressor is the problem, a soft starter can cut its start-up current.

3. Power Factor (kVA vs. kW)

Motors, pumps and older ballast lighting draw current that is out of step with the voltage, so the inverter supplies more volt-amps (VA) than the watts the load actually uses:

If a household's power factor averages 0.85 to 0.90, a 4.0 kW load needs about 4.4 to 4.7 kVA before any safety margin. Check whether an inverter's rating is in VA or in watts, because the two are not interchangeable.

Step 4: Sizing the Battery Bank (Ah, DoD, and Autonomy)

The battery bank is usually the biggest cost in an off-grid system. Too small and it cycles deeply every day and wears out early. Too large and you have paid for storage the array can never refill.

1. Days of Autonomy

Autonomy is how many days in a row the batteries can run your loads with little or no help from the panels or a generator.

• 1 to 1.5 days: Fine for a hybrid system that can fall back on a reliable grid.
• 2 to 3 days: A common choice for off-grid homes in temperate climates.
• 4 to 5 days: Remote equipment such as telecom repeaters, or sites with long rainy seasons.

2. Depth of Discharge (DoD): Lithium vs. Lead-Acid

Draining a battery to empty shortens its life, so only part of the nameplate capacity counts. The share you plan to use is the depth of discharge (DoD). Figures below are typical; the datasheet of the battery you buy wins.

3. System DC Voltage Selection: 12V, 24V, or 48V?

Since P = V × I, the same load draws less current at a higher battery voltage, and current sets cable size, fuse size and heat:

• 12 V: Campervans and systems under about 1,000 W. A 2,400 W load pulls 200 A at 12 V, which needs very thick cable.
• 24 V: Small cabins and tiny homes, roughly 2,000 to 3,000 W.
• 48 V: The usual choice above about 3,000 W. Currents stay manageable, copper losses are lower, and most rack-mount lithium batteries are 48 V.

System Architecture & Complete Sizing Flowchart

The diagram shows how one number, daily energy use, feeds the sizing of the array, the inverter and the battery bank.

Interactive Quick Sizing Estimator

Enter your own daily use, sun hours and load below to get a first-pass size. It runs in your browser and uses the same factors as this guide: 0.78 system efficiency, a 1.25 inverter margin, 0.85 DoD for lithium and 0.50 for lead-acid. Treat the result as a starting point, not a design.

Practical Calculation Walkthrough: 14.5 kWh/day Home

Here is the whole process for an off-grid house using 14.5 kWh per day at a site with 4.8 Peak Sun Hours:

Compare the two chemistries. For the same 2 days of autonomy, lead-acid needs almost double the nominal capacity (70.7 kWh vs 35.9 kWh) and is much heavier. Lithium costs more per kWh up front, but with far more cycles it often works out cheaper over its life. Prices vary, so compare quotes for your area. A requirement of 748 Ah means eight 100 Ah units, since seven would fall short.

Five Costly Solar Sizing Mistakes to Avoid

1. Counting all of the battery as usable. Routinely discharging lead-acid far beyond 50% can shorten its life dramatically. Always include the DoD divisor when calculating amp-hours.
2. Forgetting cold-weather open-circuit voltage (V_oc). Panel voltage rises as temperature drops, typically by about 0.25% to 0.32% per °C. A string that looks fine on a warm day can exceed the controller's maximum input voltage on a freezing morning and damage it. Work out cold V_oc from the panel datasheet before you fix string lengths.
3. Ignoring the battery's current limit. Lithium batteries have a maximum continuous discharge rate, often 0.5C to 1C. A 5 kW inverter draws about 104 A at 48 V. On a single 50 Ah battery rated 0.5C (25 A), the battery management system would cut out under load. Add up the batteries' combined rating and compare it with the inverter's maximum current.
4. Thin DC cable. Low-voltage DC means high current. Undersized cable between batteries and inverter drops enough voltage that the inverter shuts down on low-voltage under heavy load while the batteries are still half full. Size it with the cable calculator linked above, and fuse every battery connection.
5. Mixing up sun hours and daylight hours. Designing as if the panels produce full power for 10 or 12 hours leaves the array less than half the size it needs. Use Peak Sun Hours from a local solar data source.

Frequently Asked Questions