Solar Battery Sizing for Daily kWh and Days of Autonomy

A solar battery is sized from energy, not from a round kWh figure on a datasheet. Start with how much energy the battery must deliver (a number of backup hours at your typical load, or a number of full days of autonomy at your daily kWh), divide by the usable depth of discharge and the round-trip efficiency to get nominal capacity, and then check that the battery can also deliver the peak power and that the array can recharge it. This guide walks through each of those steps.

It builds on how to size a solar PV system from your load, which covers the load list, daily kWh, and typical versus peak load. The formulas below are general engineering relationships, and the worked figures are illustrative only. Your loads, climate, and battery datasheet will give different numbers.

Step 1: Decide whether you need storage at all

Storage is optional. A home rooftop scheme that runs at zero export and can import from the grid when the array is dark may need no battery. A clinic, workshop, or remote site that must run after sunset or through grid outages usually does. In the load2pv online tool, storage is added only if you tick it and enter backup hours; if you leave it off, no battery is added quietly, and PV and the inverter or MPPT still follow your load.

Step 2: Choose a sizing basis: backup hours or days of autonomy

There are two common ways to express how much energy the battery must hold. Use the one that matches how the site actually fails.

Backup hours suit sites where the grid or a generator normally carries the load and the battery bridges a gap. This is the basis load2pv uses: usable battery energy follows backup hours × typical load.

Usable energy (kWh) = typical load (kW) × backup hours

Days of autonomy suit off-grid sites that must keep running through days of poor sun without any other source.

Usable energy (kWh) = energy drawn from the battery per day (kWh) × days of autonomy

For a fully off-grid site on a cloudy day, the energy drawn from the battery can approach the whole daily load. On a sunny day it is mainly the evening and night share. Be explicit about which case you are designing for.

Step 3: Convert usable energy to nominal capacity

A battery cannot deliver all of its nameplate energy over a useful life, and some energy is lost in charging and discharging. The usual correction is:

Nominal capacity (kWh) = usable energy ÷ (depth of discharge × round-trip efficiency)

Depth of discharge (DoD) is the share of nominal capacity you plan to use in a normal cycle. The right figure comes from the battery manufacturer's datasheet and warranty terms. Lithium iron phosphate (LFP) batteries are generally specified for a deeper usable window than lead-acid types, but the exact value depends on the product.

Step 4: Worked examples (illustrative)

Using the illustrative load from the PV sizing guide: 24 kWh per day, a typical load of 2 kW, and an assumed DoD of 0.8 with 0.95 efficiency.

The gap between those rows is the main reason a battery budget changes so much: each extra day of autonomy adds a full day of energy. That is why the autonomy target should come from the real consequence of an outage, not from habit.

Step 5: Convert kWh to amp-hours at your system voltage

Battery banks are often quoted in amp-hours (Ah). The conversion uses the nominal DC bus voltage:

Capacity (Ah) = capacity (kWh) × 1000 ÷ nominal system voltage (V)

For the 10.5 kWh case on a 48 V bus, that is 10.5 × 1000 ÷ 48 ≈ 219 Ah. For 63.2 kWh, it is about 1,317 Ah at 48 V. Higher bus voltages reduce current for the same energy, which usually keeps cables and fuses smaller. Match the voltage to the hybrid inverter or the MPPT controllers you plan to use.

Step 6: Check power, not only energy

A battery can hold enough kWh and still fail to run the loads. Check these points against the datasheet:

1. Continuous discharge power must be at least the peak load, including loads that start together. load2pv applies inverter and MPPT limits when it sizes storage.
2. Surge capability matters for motors, pumps, and compressors, whose starting current is higher than their running current.
3. Charge power must accept what the array and charger can deliver on a good day, or the extra energy is wasted.
4. Temperature. In hot MENA sites, battery rooms and cabinets can run warm, which affects life and available capacity. Check the manufacturer's operating range and plan ventilation or shading.

Step 7: Make sure the array can refill the battery

Autonomy is only useful if the battery is full when the bad weather starts. After a cloudy spell, the array has to supply the daily load and recharge the battery at the same time. A system sized only for the daily load may take many days to recover. If autonomy matters, check that the array has enough margin above daily energy, or plan a generator to support recovery. The array itself is sized from daily energy, local sun, and losses, as described in the PV sizing guide; the About page explains how load2pv applies climate and voltage-window checks.

Quick checklist

• Is storage actually needed, or does the grid cover the night?
• Is the basis backup hours or days of autonomy, and why?
• Usable energy calculated from typical load or daily kWh
• Nominal capacity corrected for datasheet DoD and efficiency
• Ah calculated at the real bus voltage
• Discharge and surge power cover the peak load
• Array and charger can recharge the battery after autonomy days
• Temperature and ventilation checked for the site

Next step

If you know your typical and peak load and how many backup hours you need, run the free online design with storage switched on and compare the preview with your own estimate. For a checked scheme and official files, the Contact page lists the six facts to send, including whether you need storage and for how many hours.

Worked examples use illustrative round numbers. Online output is an indicative scheme, not a construction-permit set.