Home Solar PV System Sizing: Load, Panels, Battery and Inverter Worked Out

Sizing a solar system is not about buying the biggest panel the roof can hold. It starts with how much energy the house uses each day, then works backwards to the panel capacity, battery bank, inverter and charge controller. Undersize and the batteries never charge fully; oversize and money sits idle on the roof. This article sizes a complete off-grid or hybrid system for a typical Indian home, step by step, with every assumption stated so you can replace it with your own numbers.

Step 1: Estimate Daily Energy Use

List every load you want on solar, its power and how many hours it runs per day. Use measured or label figures where possible.

Load	Qty	Watts	Hours/day	Wh/day
LED bulbs	5	9	6	270
Ceiling fans	3	75	8	1800
LED TV	1	100	4	400
Refrigerator (from label, averaged)	1	-	24	1200
Laptop	1	60	4	240
Phone chargers	2	10	2	40
Total				3950

The daily energy requirement is about 3.95 kWh. Replacing the three 75 W fans with BLDC fans rated around 30 W would save over 1 kWh a day, and that is usually cheaper than adding solar capacity to supply it. Always reduce load first.

Step 2: Size the PV Array

Peak sun hours (PSH) is the daily solar energy per square metre expressed as hours of full 1000 W/m² sunlight. Much of India receives roughly 4.5 to 5.5 kWh/m² per day on average, but it varies by location and season, so use data for your city, for example from NASA POWER or the NREL solar resource tools, and design for the weaker months if the system is off-grid.

A derating factor covers real-world losses: module temperature (panels lose output when hot), dust, wiring, mismatch, charge controller and battery charging losses. A combined factor around 0.7 to 0.75 is commonly used for battery systems.

Array size (Wp) = Daily energy (Wh) / (PSH × derating factor)

= 3950 / (5 × 0.75)
= 3950 / 3.75
= 1053 Wp

Choose 3 × 400 Wp modules = 1200 Wp

The extra margin from 1053 to 1200 Wp helps on hazy days and as panels degrade slowly over the years. Mount the panels facing south, tilted roughly at the site's latitude for best annual yield, and keep them free of shade from water tanks and parapets, since even partial shading on one cell can sharply cut a string's output.

Step 3: Size the Battery Bank

Battery Ah = Daily Wh × Days of autonomy / (System V × DoD × Battery efficiency)

LiFePO4, 48 V system, 1 day autonomy, 80% DoD, 95% efficiency:
Ah = 3950 × 1 / (48 × 0.80 × 0.95) = 3950 / 36.5 = 108 Ah
=> 48 V (51.2 V nominal) 120 Ah LFP bank

Lead-acid, 48 V, 1 day, 50% DoD, 85% efficiency:
Ah = 3950 / (48 × 0.50 × 0.85) = 3950 / 20.4 = 194 Ah
=> 4 × 12 V 200 Ah tubular batteries in series

Note that in a hybrid system much of the daytime load is supplied directly by the panels, so the battery only needs to cover the evening and night share. If two-thirds of the energy is used after sunset, the battery can be sized for about 2.6 kWh instead of 3.95 kWh. For a strictly off-grid site with frequent cloudy spells, add one or two days of autonomy instead.

System voltage	When to choose	Current for 1 kW
12 V	Very small systems, below about 1 kW	83 A
24 V	About 1 to 2 kW	42 A
48 V	Above about 2 kW, or any LFP home system	21 A

Step 4: Inverter and Charge Controller

The inverter must supply the largest set of loads running at the same time, plus starting surges. Here, all lights, fans, TV, laptop and the refrigerator running together take about 45 + 225 + 100 + 60 + 150 = 580 W, assuming the refrigerator compressor draws around 150 W while running. Compressor starting can draw several times that briefly. A pure sine wave inverter of about 1.5 to 2 kVA, matched to the 48 V battery, gives comfortable margin.

An MPPT charge controller tracks the maximum power point of the array and converts the higher panel voltage down to the battery voltage. Check two limits: its maximum PV input voltage and its maximum charging current.

Assume module Voc = 37 V, Vmp = 31 V (check your datasheet).
Three modules in series:
  String Voc = 3 × 37 = 111 V (plus a margin for cold mornings)
  String Vmp = 3 × 31 = 93 V
Controller must accept at least about 130 V PV input.

Charge current = Array W / Battery V = 1200 / 51.2 ≈ 23 A
=> choose a 30 A or larger MPPT controller (or hybrid inverter)

Many homes now use a single hybrid inverter that combines the MPPT controller, battery charger and inverter. For grid-connected rooftop systems with net metering, the application goes through your DISCOM, and central schemes such as PM Surya Ghar provide subsidies for eligible residential systems; check the current rules on the official portal.

Summary

Add up daily Wh for all solar loads, after cutting waste.
Array Wp = daily Wh / (PSH × derating).
Battery Ah = daily Wh × autonomy / (V × DoD × efficiency).
Inverter kVA from simultaneous load plus surge.
Controller from string voltage and array Wp / battery V.

For this 3.95 kWh/day home: 1.2 kWp of panels, a 48 V 120 Ah LFP battery, a 30 A MPPT controller and a 1.5 to 2 kVA pure sine inverter.