Step 2. How To Choose A Solar Battery For Your Off-Grid Home

The battery is often the most expensive part of your off-grid solar setup. Get this part right and the rest gets easier.

In Step 1 you worked out how much energy your home uses per day. Now you turn that figure into a box on the wall. Here's how to choose a solar battery in five checks, and how to wire several of them together once you have them.

What Are Solar Batteries?

A solar battery stores the extra electricity your solar panels make during the day, so your appliances keep running at night or on a gray day.

Think of it like a water tank on a roof. The sun fills it up while it shines, and you open the tap whenever you need power. With no power line to fall back on, this energy storage is the only thing keeping the lights on after sunset.

There's no special "solar" technology inside. It's the same kinds of batteries you find elsewhere, picked for one job: soaking up the power your panels generated all day, then giving it back after dark, for years. If you want to see what goes on inside one, this guide to home batteries explains it.

Three Solar Battery Types

Most homeowners pick from these.

LiFePO4. Lithium iron phosphate, a member of the lithium ion family. It's the favorite on the market today. It's much less fire-prone than the battery in your phone, it lasts a long time, and it's light for the energy it holds.

Lead-acid. The old reliable, like the battery under a car hood. Lead-acid batteries cost less up front, but they're about three times heavier for the same energy and wear out much sooner.

Saltwater. Salt water inside, so it's the hardest to set on fire. However, it holds less energy per kilo and the main makers went out of business, so saltwater batteries are hard to find today.

For most homes the decision comes down to LiFePO4 or lead-acid. The second is cheaper on day one, while LiFePO4 is generally cheaper per year of use.

Five Things To Check Before You Buy

1. Voltage. Your battery and your inverter have to match. 12V, 24V, or 48V are the common ones. Bigger systems usually run at 48V because thinner wires can carry the same power.

2. Battery size (capacity). How much energy the battery holds, in amp-hours (Ah) or kilowatt-hours (kWh). Multiply amp-hours by volts to convert: a 100Ah battery at 12V stores 1,200 watt-hours, or 1.2 kWh.

3. Lifespan (cycle life). How many times you can charge and drain it before it drops to about 80% of its storage. This is where the two main types split up.

4. Temperature range. LiFePO4 doesn't like freezing. A normal lithium ion battery must not be charged below 0°C (32°F), because a thin metal layer builds up inside and damages it for good. If your battery lives in a cold shed, look for one with a built-in heater, or keep the space warm.

5. Warranty. How many years, and how much capacity the maker promises is left at the end. Tesla, for example, promises its Powerwall still holds 70% of its energy after ten years. A clear warranty tells you more about real life than anything else on the box.

How Much Battery Size Do You Need?

Short answer: your daily kWh, times the days you want to last without sun, divided by how deep you can safely drain the battery.

Take the sample home from Step 1. It uses about 3.1 kWh a day, and wants 2 cloudy days of backup, so it needs 6.2 kWh of storage.

• LiFePO4 is often planned at 80% depth of discharge. 6.2 ÷ 0.8 = about 7.75 kWh. At 48V that's about 160Ah.
• Lead-acid wears out fast if you drain it past half, so plan at 50%. 6.2 ÷ 0.5 = 12.4 kWh, a much bigger and heavier bank for the same home.

In winter your solar panels make less, so consider an extra day of backup if your gray spells run long.

The battery calculator does this math for you at 12, 24 or 48 volts, so you can determine the right size before you buy.

Why Lifespan Matters More Than Price

Batteries wear out one cycle at a time, and how deep you drain them matters a lot.

Victron's LiFePO4 battery, for example, is rated for 2,500 cycles if you use 80% each time, 3,000 at 70%, and 5,000 only at 50%. A deep-cycle lead-acid one manages about 400 to 500 cycles at 50%, and only 150 to 200 if you drain it all the way.

Heat shortens both: a sealed lead-acid battery loses half its lifespan for every 8°C above 25°C. Batteries also age just from time passing, so 5,000 cycles does not simply mean 13 years.

That's why a battery that costs twice as much can still be the cheaper solution. Compare the price per kWh you'll get out of it over its whole lifespan, not the price tag.

Wiring Multiple Batteries Together

One battery is rarely enough to power a real home, so you wire several into one battery storage system. There are two ways.

In series. Plus of one to minus of the next. This adds the voltages together. Two 12V batteries in series make a 24V system, and capacity stays the same.

In parallel. Plus to plus, minus to minus. This adds the capacities together. Two 12V 100Ah batteries in parallel give you 12V 200Ah, and voltage stays the same.

You can also mix the two. Put four 12V units in series to make one 48V string, then add more strings of four in parallel. That's how you build a 48V battery bank from smaller 12V units.

Rules When Combining Batteries

Same age, same brand, same size. Mixing an old battery with a new one drags the new one down to the old one's level. Don't do it.

Read the battery's manual. Each type has its own rules. Some batteries with a built-in BMS can't be put in series at all, others only up to a set number. Victron's lithium batteries, for example, may only go in series with the same capacity and part number, and no more than 3 years apart in age. Never mix chemistries, like LiFePO4 with lead-acid.

Match the charger. Whatever charges the battery (the charge controller, the inverter) has to be set to the right voltage and chemistry.

Fuse the connections. Fuse each battery (or each series string) on its plus side, plus the main cable to the bank. A short circuit between two house-sized batteries can melt cable in a heartbeat.

Once your solar battery is picked, the next steps are Step 3, the controller, and Step 4, wiring the system.

→ Read more about fuses

→ Calculate your battery bank size

→ How to build a home battery

Sources

• Battery University, BU-201: How does the Lead Acid Battery Work?
• Battery University, BU-214: Summary Table of Lead-based Batteries
• Battery University, BU-216: Summary Table of Lithium-based Batteries
• Battery University, BU-410: Charging at High and Low Temperatures
• Victron Energy, Lithium iron phosphate batteries Smart datasheet
• Victron Energy, Lithium Battery Smart manual
• Tesla, Powerwall limited warranty (US)
• Sandia National Laboratories, Energy storage overview (NECPUC 2021)
• Frontiers in Chemistry, thermal runaway of lithium-ion battery chemistries (2024)
• pv magazine, Aquion files for Chapter 11 bankruptcy
• NSW Government, About home solar batteries