Solar & Battery Storage Guide & FAQs
Verified engineering answers on sizing, batteries, inverters, and subsidies.
Formula:
System Size (kW) = Daily kWh / (Peak Sun Hours × 0.85). For 15 kWh/day with 5.5 sun hours: 15 / (5.5 × 0.85) ≈ 3.2 kW DC.
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| Solar & Battery Calculators |
Solar & Battery Calculators: Size Your Solar System and Battery Backup Accurately Buying solar panels, an inverter, or batteries without proper calculation usually ends in one of two ways: a system that is too small to cover your load, or one that is larger and costlier than you need. These free solar and battery calculators help you get the numbers right before you spend money.
Table of Contents
- What Solar & Battery Calculators Do
- Calculators You Can Use
- How to Use These Calculators
- Input Details Explained
- Formulas Behind the Results
- Worked Examples
- Battery Type Comparison
- Common Mistakes to Avoid
- Frequently Asked Questions
What Solar & Battery Calculators Do
A solar system has three connected parts: panels that generate power, a battery that stores it, and an inverter that converts it for your appliances. If one part is sized badly, the whole system suffers.
These calculators take your appliance load, daily usage, and location-based sunlight data, then give you a recommended panel capacity, battery size, and inverter rating.
Calculators You Can Use
| Calculator | What It Tells You | Best For |
|---|---|---|
| Solar Panel Size Calculator | Required system capacity (kW) and number of panels | Homes, shops, small offices |
| Inverter Size Calculator | Inverter rating in VA or kVA | Choosing the right inverter |
| Battery Backup Time Calculator | How many hours a battery will run your load | Power cut planning |
| Battery Capacity Calculator | Battery size in Ah or kWh for a target backup | Buying a new battery |
| Solar Battery Bank Calculator | Number of batteries and series/parallel layout | Off-grid and hybrid setups |
| Solar Savings and Payback Calculator | Monthly savings and years to recover cost | Deciding if solar is worth it |
How to Use These Calculators
- List your appliances. Note the wattage of each device you want to run, such as fans, lights, refrigerator, TV, and pump.
- Estimate daily running hours. Enter how many hours each appliance runs in a day.
- Enter your location or sunlight hours. Most parts of India receive roughly 4 to 6 peak sun hours per day.
- Select battery type and system voltage. Choose lead-acid or lithium, and 12V, 24V, or 48V.
- Click Calculate. The result shows the recommended panel capacity, battery size, or backup time.
- Add a safety margin. Round the result up to the next standard size available in the market.
Input Details Explained
| Input | Meaning | Typical Value |
|---|---|---|
| Load (W) | Total wattage of appliances running together | 300 W to 3,000 W for a home |
| Daily Usage (kWh) | Units consumed per day | 5 to 15 kWh for an average home |
| Peak Sun Hours | Hours of full-strength sunlight per day | 4 to 6 hours |
| System Losses | Loss from wiring, dust, heat, and conversion | 20% to 25% |
| Battery Voltage (V) | Voltage of the battery or bank | 12V, 24V, 48V |
| Battery Capacity (Ah) | Charge the battery can store | 100 Ah to 200 Ah per battery |
| Depth of Discharge (DoD) | Share of battery capacity you can safely use | 50% lead-acid, 80% to 90% lithium |
| Inverter Efficiency | Power left after DC to AC conversion | 85% to 95% |
Formulas Behind the Results
| Calculation | Formula |
|---|---|
| Solar system size (kW) | Daily usage (kWh) ÷ (Peak sun hours × 0.75) |
| Number of panels | System size (W) ÷ Panel wattage |
| Inverter size (VA) | Total load (W) ÷ 0.8, plus 25% margin |
| Battery energy (Wh) | Voltage × Ah |
| Usable battery energy (Wh) | Voltage × Ah × DoD × Inverter efficiency |
| Backup time (hours) | Usable battery energy (Wh) ÷ Load (W) |
The 0.75 factor in the solar formula accounts for typical system losses of 25%.
Worked Examples
Example 1: Solar Panel Sizing
A home uses 10 kWh per day and gets 5 peak sun hours.
- System size = 10 ÷ (5 × 0.75) = 2.67 kW
- Using 550 W panels: 2,670 ÷ 550 = 4.85, so you need 5 panels
A 3 kW system with 5 or 6 panels is a practical choice.
Example 2: Battery Backup Time
A 12V 150Ah lead-acid battery runs a 300 W load. DoD is 50% and inverter efficiency is 85%.
- Usable energy = 12 × 150 × 0.5 × 0.85 = 765 Wh
- Backup time = 765 ÷ 300 = 2.55 hours, about 2 hours 33 minutes
Example 3: Inverter Sizing
Total running load is 1,500 W.
- 1,500 ÷ 0.8 = 1,875 VA
- Adding a 25% margin: 1,875 × 1.25 = 2,344 VA
A 2.5 kVA inverter is the right choice here.
Battery Type Comparison
| Feature | Tubular Lead-Acid | AGM / Gel | Lithium (LiFePO4) |
|---|---|---|---|
| Safe DoD | About 50% | 50% to 60% | 80% to 90% |
| Round-trip Efficiency | 80% to 85% | 80% to 90% | 95% or more |
| Maintenance | Needs water top-up | Maintenance free | Maintenance free |
| Weight | Heavy | Heavy | Light |
| Upfront Cost | Low | Medium | High |
| Cycle Life | Lower | Medium | Much higher |
Lithium costs more at purchase, but the higher usable capacity and longer life often make it cheaper over the years.
Common Mistakes to Avoid
- Ignoring depth of discharge. A 150Ah lead-acid battery does not give you 150Ah of usable power.
- Sizing for average load only. Motors, pumps, and ACs draw extra power at start-up. Include this surge.
- Skipping system losses. Dust, heat, and wiring reduce real output below the panel rating.
- Mixing old and new batteries. Different ages and capacities in one bank shorten the life of all of them.
- Choosing the exact calculated size. Always go to the next standard size for headroom.
- Placing panels in partial shade. Even a small shadow can cut output sharply.
Frequently Asked Questions
1. How many solar panels do I need for my home?
It depends on daily electricity use and sunlight hours. A home using 10 units per day typically needs a 3 kW system, which is about 5 to 6 panels of 550 W. Use the solar panel calculator with your own units for a precise figure.
2. How do I calculate battery backup time?
Multiply battery voltage by Ah, then by depth of discharge and inverter efficiency. Divide the result by your load in watts. The battery backup calculator does this for you in one click.
3. What size inverter do I need for solar?
Add up the wattage of all appliances that will run together, divide by 0.8 to get VA, and add a 25% margin. Round up to the nearest standard inverter size, such as 1 kVA, 2.5 kVA, or 5 kVA.
4. Are the results from these calculators exact?
The results are reliable estimates for planning and shopping. Real output varies with weather, roof angle, shading, and equipment quality. For installation, confirm the final design with a certified solar installer.
