Which off-grid battery do you need, and how long does it last?

For an off-grid system in Belgium, choose a lithium iron phosphate (LFP) battery with a battery management system. Size it on what you use at night, plus a short reserve for grey days. A generator that starts automatically covers the dark winter weeks, not an ever larger battery. In our fully off-grid projects the battery ranges from 15.4 kWh for a hobby room to 46.2 kWh for a farm. We always size it as one part of the off-grid installation, together with the panels, the inverters and the generator.

You are planning a building, a farm shed or a cottage without a grid connection, and the battery quotes are all over the place. One offers a lead-acid bank at a fraction of the price. Another offers a lithium rack of 30 kWh. On forums one person swears by lead, the next one lost a battery in the first winter.

That choice weighs more off-grid than anywhere else. Without a grid, the battery works every single night and wears a little with every cycle. Being your own power company also means the replacement bill is yours, and it rarely comes at a good moment. A battery of the wrong type or size does not fail on day one. It simply gives up years earlier than planned.

A stable and a cottage in Kasterlee lie very far from the public grid. With 16 kWh of storage and 5 kVA of inverter power, the owner has enough self-generated electricity for a full year. On a farm in St-Georges-sur-Meuse, 46.2 kWh of batteries turned a generator that ran around the clock into a back-up.

What does the battery do in an off-grid system?

In an off-grid system the battery is not an extra. It is the heart of the installation. The inverter builds the electricity for your site from it, and every kWh you use after sunset comes out of it.

Energy that is not used right away is stored in the battery. In the evening, or on days with little sun or wind, the battery supplies what you need. The battery decides how long your site keeps running without the generator. That is why the choice of chemistry, capacity and placement matters so much.

A home battery next to a grid connection has an easier life. When it is empty, the grid takes over. Off-grid, there is no grid to fall back on. Some home batteries can also run off-grid, but for a building without a connection the battery is designed together with the whole system.

LFP or lead-acid: which battery is best for off-grid?

For a professional off-grid system the answer is almost always lithium iron phosphate (LFP or LiFePO4). The reason is not fashion. It is how often an off-grid battery is cycled, and how it behaves when it is rarely full.

Per kWh of capacity, an LFP battery delivers about six times more energy over its life than an AGM battery. It also loses less along the way.

Lead-acid still has a place in a small system that is used now and then. Think of a back-up that stays charged all year. For a building that runs on its battery every night, LFP is the safer bet.

How much battery capacity do you need to go off-grid?

Less than most people think, as long as the rest of the system is designed well. The battery does not have to bridge a dark week in January. That is the generator's job.

We start from what you actually use. YouPower measures the actual and peak consumption over a period. Everything is sized on that, from the generator to the battery, the inverters and the cables. For a new building without a history, we work from the planned loads.

In winter the sun delivers only a fraction of the summer yield. How that works out for your panels and battery is explained step by step in our article on off-grid solar panels with a battery. The short version: a battery that has to cover a dark week alone becomes very large and very expensive. The rest of the year it is barely used.

Why kWh alone does not decide your off-grid system

Two numbers describe an off-grid system, and they are often confused. The battery capacity in kWh says how much energy is stored. The inverter power in kVA says how much power you can draw at the same moment.

Turnhout and Kasterlee store about the same energy, but Kasterlee can deliver more power at once. The farm needs ten times the power of the hobby room for its machines, yet only three times the storage. Your largest machines set the inverter power; your nights set the battery. Size them separately.

How long does an off-grid battery last?

An LFP battery lasts a number of cycles, and that number depends on how deep you discharge it. Victron states 2,500 cycles at 80% discharge, 3,000 at 70% and 5,000 at 50%. After those cycles the battery still holds at least 80% of its original capacity.

Turned into years: if the battery went through one 80% cycle every single day, 2,500 cycles would last almost seven years. At 50% per day, the datasheet gives 5,000 cycles, or more than thirteen years of daily cycling. In a real off-grid system not every day is a deep cycle, and on summer days the panels do much of the work directly.

Three things shorten the life of an off-grid battery: deep discharges every night, charging in the cold, and a missing battery management system. That system must also talk to the inverter. The battery management system watches voltage, current and temperature, and intervenes before damage occurs.

In our worked off-grid case the batteries came with a 10-year warranty on 80% of their capacity. Ask for that figure in every quote. It tells you what the manufacturer itself expects.

What does an off-grid battery cost?

The price per kWh on a quote says little on its own. What counts is the cost per kWh the battery delivers over its life. Divide the price by the capacity, the number of cycles, the depth of each cycle and the efficiency.

That is where the chart above matters. An LFP bank delivers about six times more energy per kWh of capacity than an AGM bank. So even at a clearly higher purchase price, it is often the cheaper battery per kWh it actually delivers.

What else drives the price of the battery part:

We do not publish standard prices, because the battery is always sized together with the rest of the system. After the pre-study you get one price for the whole installation.

Can you expand an off-grid battery later?

Yes, if the system is designed for it from the start. In Beernem, a research and education site, YouPower set the battery capacity in the pre-study and kept it expandable for a later phase. No oversizing on day one, and room to grow when the needs grow.

Plan for it early. The inverters, cables and protection must be large enough for the future battery, not only for today's. Extending works best with the same type of battery modules, managed by one battery management system. Mixing different types or ages of batteries without one system to manage them makes them work against each other.

Where should an off-grid battery be installed?

Placement is often forgotten in quotes, yet it decides how long the battery lasts. Our battery installations follow these conditions:

• A watertight room with a stable, level floor.
• A room temperature between 0 °C and 50 °C, as constant as possible, with little dust and constant humidity.
• Ventilation, and at least 2 m from a heat source.
• No explosive or flammable materials nearby, and the battery never covered.
• Out of reach of children and pets.

Outside those temperatures the battery switches off automatically. An LFP battery also charges only from +5 °C (Victron LiFePO4 Smart). A cold shed in a Belgian winter is therefore a poor place for the battery of an off-grid system. More about the battery installation is on our page with a worked off-grid case.

When is a lithium off-grid battery not the right choice?

What happens to an off-grid battery at the end of its life?

Even an LFP battery reaches the end of its life one day. YouPower has its own Battery Disassembly Center for lithium batteries. Each battery is screened first. If it is still suitable, it can be reused in a second-life application. If not, it is identified as end of life and disassembled.

So you do not have to look for a recycler yourself: YouPower takes in lithium batteries at its own centre.

What happens when you contact YouPower?

1. Step 1: You fill in the quotation form. It asks for your yearly consumption in kWh, and you tell us what keeps running at night.
2. Step 2: We measure your actual and peak consumption over a period, or work from the planned loads for a new building.
3. Step 3: Our engineering office calculates the type and capacity of the battery, together with the panels, the inverters and the generator.
4. Step 4: We install the whole system, test it and put it into service.
5. Step 5: You follow production, storage and consumption in real time in the app, with alerts on your phone, and we remain reachable 24/7.

How the battery fits into the whole system is explained on our page about the off-grid installation. For a business site, also read when off-grid pays off for a business.