A solar battery lets you store excess solar energy for use at night, during power outages, or during peak tariff periods. But how do you determine the right battery capacity? Too small, and you will run out of power during an outage. Too large, and you waste capital on unused capacity.

This guide walks through the step-by-step calculation to find the exact battery size in kWh and Ah for your home backup needs, with comparisons between tubular lead-acid and lithium LiFePO4 batteries.

                    Quick Answer: For a typical 5 kWh essential load, you need a 10.0 kWh battery with tubular (208 Ah at 48V) or a 5.6 kWh battery with lithium (117 Ah at 48V). Lithium requires roughly half the physical capacity due to deeper DoD and higher efficiency.
                

The Basic Formula

Required Battery Capacity (kWh) = Total Load (kW) × Backup Hours (h)

Required Battery Capacity (Ah) = (kWh × 1000) ÷ Battery Voltage (V)

However, this is the theoretical minimum. Real-world battery sizing must account for depth of discharge (DoD) and round-trip efficiency (RTE) losses.

Step 1: List Your Essential Home Loads

The first step is to determine which essential loads you need to power during a backup event. Below are typical loads for a medium-sized home in most global markets:

Appliance	Typical Power (kW)	4 Hours Backup	8 Hours Backup	12 Hours Backup
LED Lights (10-15 bulbs)	0.15	0.6 kWh	1.2 kWh	1.8 kWh
Refrigerator (inverter type)	0.25	1.0 kWh	2.0 kWh	3.0 kWh
TV + Streaming Device	0.15	0.6 kWh	1.2 kWh	1.8 kWh
WiFi Router + Modem	0.02	0.08 kWh	0.16 kWh	0.24 kWh
Ceiling Fans (3-4)	0.20	0.8 kWh	1.6 kWh	2.4 kWh
Laptop + Phone Charging	0.05	0.2 kWh	0.4 kWh	0.6 kWh
Water Pump (1 HP)	0.75	3.0 kWh	6.0 kWh	9.0 kWh
Air Conditioner (1.5 ton inverter)	1.50	6.0 kWh	12.0 kWh	18.0 kWh
Security System + CCTV	0.05	0.2 kWh	0.4 kWh	0.6 kWh
Electric Kettle / Microwave	1.20	0.6 kWh (30 min)	1.2 kWh (1 hr)	1.8 kWh (1.5 hr)

Load Assessment Tip: For backup sizing, only include loads that are essential during an outage. You do not need to run the AC, water pump, and microwave simultaneously. A typical essential load for a medium home during an outage is 0.5-1.5 kW, not the full 4-5 kW of the home's peak load.

Step 2: Calculate Required kWh by Backup Duration

Add the loads you want to power, then multiply by your desired backup hours. Here are common backup scenarios:

Backup Scenario	Typical Loads	Total Power	kWh Needed (4h)	kWh Needed (8h)	kWh Needed (12h)
Minimal (lights + internet + fridge)	0.15 + 0.02 + 0.25	0.42 kW	1.7 kWh	3.4 kWh	5.0 kWh
Standard (above + fans + TV + charging)	0.42 + 0.20 + 0.15 + 0.05	0.82 kW	3.3 kWh	6.6 kWh	9.8 kWh
Comfort (standard + AC + pump backup)	0.82 + 1.50 + 0.75	3.07 kW	12.3 kWh	24.6 kWh	36.8 kWh
Full Home (all loads intermittent)	Calculated average	~2.0 kW avg	8.0 kWh	16.0 kWh	24.0 kWh

Step 3: Convert kWh to Ah at Different Voltages

Batteries are typically rated in ampere-hours (Ah) at a specific voltage. Common solar battery bank voltages are 12V, 24V, and 48V. The higher the voltage, the lower the Ah for the same kWh—meaning thinner cables and lower current losses.

Required kWh	Ah at 12V	Ah at 24V	Ah at 48V	Best For
2.0 kWh	167 Ah	84 Ah	42 Ah	Minimal backup, single appliance
5.0 kWh	417 Ah	209 Ah	105 Ah	Standard essential loads, 4-8 hours
8.0 kWh	667 Ah	334 Ah	167 Ah	Full essential loads, 8-12 hours
10.0 kWh	834 Ah	417 Ah	209 Ah	Large backup with AC intermittent
15.0 kWh	1,250 Ah	625 Ah	313 Ah	Full home, extended backup
20.0 kWh	1,667 Ah	834 Ah	417 Ah	Large home, multiple days backup

Ah = (kWh × 1000) ÷ Voltage

Example: 5.0 kWh at 48V = (5,000) ÷ 48 = 104.2 Ah

Step 4: Account for Depth of Discharge and Efficiency

The theoretical Ah calculated above is the usable energy you need. However, batteries cannot be fully discharged without damage. You must install a larger nominal capacity to account for:

Depth of Discharge (DoD): Maximum safe discharge percentage
Round-Trip Efficiency (RTE): Energy lost during charging/discharging (heat, chemical losses)

Battery Chemistry	Max Recommended DoD	Round-Trip Efficiency	Cycle Life	Cost per kWh (2026)
Tubular Lead-Acid (C10 rated)	50%	80%	1,200-1,500	$120-180
Lithium LiFePO4 (LFP)	90%	95%	4,000-6,000	$250-400
AGM / Gel Lead-Acid	50%	82%	600-800	$150-220
Lithium NMC	80%	92%	3,000-4,000	$300-450

Actual Battery Capacity Needed = Theoretical kWh ÷ (DoD × RTE)

Step 5: Compare Tubular vs Lithium Sizing

Here is a direct comparison of the actual battery capacity you need for the same load, depending on whether you choose tubular lead-acid or lithium LiFePO4.

Theoretical Load (kWh)	Tubular: Nominal kWh	Tubular: Ah at 48V	Lithium: Nominal kWh	Lithium: Ah at 48V
1.0 kWh	2.5 kWh	52 Ah	1.2 kWh	25 Ah
2.0 kWh	5.0 kWh	104 Ah	2.4 kWh	50 Ah
3.0 kWh	7.5 kWh	156 Ah	3.5 kWh	73 Ah
5.0 kWh	10.0 kWh	208 Ah	5.6 kWh	117 Ah
8.0 kWh	15.0 kWh	313 Ah	8.8 kWh	183 Ah
10.0 kWh	20.0 kWh	417 Ah	11.1 kWh	231 Ah
15.0 kWh	30.0 kWh	625 Ah	16.7 kWh	348 Ah
20.0 kWh	40.0 kWh	834 Ah	22.2 kWh	463 Ah

                    Tubular Calculation (5 kWh load): 5.0 kWh ÷ (0.50 DoD × 0.80 RTE) = 5.0 ÷ 0.40 = 12.5 kWh nominal. Nearest standard tubular battery: 10.0 kWh (208 Ah at 48V) or two in parallel for 12.0 kWh.

                    Lithium Calculation (5 kWh load): 5.0 kWh ÷ (0.90 DoD × 0.95 RTE) = 5.0 ÷ 0.855 = 5.85 kWh nominal. Nearest standard lithium battery: 5.6 kWh (117 Ah at 48V).

Tubular Derating Warning: Tubular lead-acid batteries are typically rated at C10 (10-hour discharge rate). If you discharge them faster (e.g., a heavy AC load draining the battery in 3 hours), the effective capacity drops by 15-25%. Always apply an additional safety factor of 1.20-1.30 for tubular if you have high-power loads like AC or water pumps.

Worked Example: Complete Battery Sizing

Scenario: A home in South Asia needs 8 hours of backup for: 15 LED lights (0.15 kW), 1 inverter fridge (0.25 kW), 3 ceiling fans (0.20 kW), 1 TV (0.15 kW), WiFi (0.02 kW), and a 1 HP water pump running 2 hours during the backup (0.75 kW × 2h = 1.5 kWh). No AC during outage.

Step 1: Total continuous load = 0.15 + 0.25 + 0.20 + 0.15 + 0.02 = 0.77 kW.

Step 2: Energy for 8 hours continuous = 0.77 × 8 = 6.16 kWh. Plus pump: 1.5 kWh. Total = 7.66 kWh.

Step 3: At 48V, Ah needed before DoD/RTE = (7.66 × 1000) ÷ 48 = 160 Ah.

Step 4a (Tubular): 7.66 ÷ (0.50 × 0.80) = 7.66 ÷ 0.40 = 19.15 kWh nominal. At 48V: 19,150 ÷ 48 = 399 Ah. Use 2 × 200 Ah tubular batteries in series/parallel for a 48V bank.

Step 4b (Lithium): 7.66 ÷ (0.90 × 0.95) = 7.66 ÷ 0.855 = 8.96 kWh nominal. At 48V: 8,960 ÷ 48 = 187 Ah. Use 1 × 200 Ah LiFePO4 battery (9.6 kWh).

Cost Comparison for This Example: Tubular solution (2 × 200Ah @ 48V) costs approximately $1,200-1,800 and lasts 5-7 years with proper maintenance. Lithium solution (1 × 200Ah @ 48V) costs approximately $2,000-3,200 and lasts 10-15 years. Over 15 years, lithium often has a lower total cost of ownership due to fewer replacements and higher efficiency.

Related Guides

Size your battery bank instantly

Open the Battery Sizing Tool →

How Solar Metrix Pro Helps: The Battery Backup Sizing Calculator in the sidebar above handles the math. Enter your day and night loads, backup hours, and system voltage. It instantly compares tubular lead-acid vs lithium capacities, energy waste, and recommends the best chemistry for your needs.
                

Open All Solar Tools in Pro Suite →

Data sources: Battery manufacturer datasheets (Luminous, Exide, Trojan, Amaron for tubular; BYD, Pylontech, Powerwall, SolarEdge for lithium). DoD and RTE values based on C10 and C5 test standards. Pricing is indicative for Q2 2026 and varies by market. Always consult a certified solar installer for final system design and local electrical codes.

Last updated: July 2026 | Browse all guides

Solar Battery Size Calculator: Find Ah & kWh for Backup