Most people shopping for an off-grid battery bank start by comparing brands. Renogy against Battle Born, EG4 against SOK, flooded lead-acid against whatever the guy at the hardware store recommended. That’s the wrong first question. The right one is how many batteries do I need for off-grid solar, and almost nobody answers it directly because the honest answer depends on your loads, your climate, and how many cloudy days in a row you’re willing to ride out.
This article skips the explainer on how batteries store and release energy. If you want that background, the Hub’s piece on understanding battery storage in off-grid solar systems covers the mechanics. Here, the goal is a number: a target capacity in kilowatt-hours or amp-hours that you can actually take to a supplier, sized for a Canadian winter rather than a mild October afternoon.
The question nobody answers directly
Ask five off-grid forums how many batteries a cabin needs and you’ll get five different answers, because every one of them is describing their own cabin. A hunting shack running three LED bulbs needs a fraction of what a year-round homestead with a chest freezer and a well pump needs. The number only means something once it’s attached to your specific loads, your location, and the season you’re least likely to get sun.
That last part is where most first-time buyers go wrong. They size a bank using summer numbers, because summer is when they’re standing in the cabin thinking about it, and then the bank falls short in December and January when the sun is low, the days are short, and the battery chemistry itself is working harder to deliver the same amount of power. Sizing for winter from the start avoids a second purchase a year later.
Battery types in plain terms
Four chemistries show up in off-grid systems. Here’s what each one actually is, without the electrochemistry lecture.
Flooded lead-acid (FLA) is the old standby: individual cells filled with liquid electrolyte that you top up with distilled water periodically. Cheapest per amp-hour up front, but they need ventilation, they vent hydrogen gas, and they punish you hard if you let them sit discharged.
AGM (absorbent glass mat) is sealed lead-acid with the electrolyte held in a fiberglass mat instead of free liquid. No topping up, no venting gas in normal use, and you can mount them in more positions. Still lead-acid underneath, so the same discharge limits apply.
Gel batteries are also sealed lead-acid, with the electrolyte suspended in a silica gel. They tolerate deep discharge slightly better than AGM and handle heat well, but they’re fussier about charging voltage and less common in off-grid retail now.
Lithium (LiFePO4), lithium iron phosphate, is the newer standard for a reason. It’s not just “better lead-acid.” It behaves differently: deeper usable capacity, far more charge cycles, and a built-in management system that protects the cells. It costs more per battery but less per cycle, which is the number that actually matters over a decade.
Lithium vs lead-acid: cost per cycle, weight, lifespan, and cold tolerance
The sticker price comparison is misleading on its own, so here’s how the four factors that matter actually stack up.
Depth of discharge. A lead-acid battery (flooded, AGM, or gel) should only be drawn down to about 50% of its rated capacity if you want it to last. Go past that routinely and you shorten its life dramatically. LiFePO4 can be drawn down to 80 to 90% regularly without the same damage. Practically, that means a 200Ah lithium battery delivers close to the same usable energy as a 350 to 400Ah lead-acid battery.
Cycle life. A decent flooded or AGM battery is rated for somewhere around 300 to 600 cycles at 50% depth of discharge before capacity drops off meaningfully. Quality LiFePO4 cells are commonly rated for 3,000 to 6,000 cycles at 80% depth of discharge. That’s not a marginal improvement, it’s an order of magnitude, and it’s why the cost-per-cycle math favors lithium even though the upfront price is two to three times higher.
Weight. Lead-acid is heavy. A single 6-volt golf cart battery can run 60 to 65 pounds, and a bank of eight of them for a 48-volt system is pushing 500 pounds before you’ve added a rack. LiFePO4 batteries of equivalent usable capacity typically weigh a third to half as much, which matters if you’re hauling them into a cabin by hand or up a trail.
Lifespan in years. Lead-acid batteries, even well maintained, tend to need replacement every 3 to 7 years in daily cycling use. LiFePO4 commonly runs 10 years or more under the same use pattern. Over the life of an off-grid system, that’s one lithium purchase against two or three lead-acid purchases.
Cold tolerance gets its own section below, because it’s the factor that catches the most first-time buyers off guard, and it doesn’t favor either chemistry in the way people assume.
The sizing formula: watts, amp-hours, and days of autonomy
Strip away the jargon and battery sizing is three numbers multiplied together, then adjusted for how many cloudy days you want to survive without sun.
Step one: add up your daily watt-hours. For every device, multiply its running wattage by the hours a day it runs. A fridge that draws 60 watts and cycles enough to average 8 hours of actual compressor run time a day uses 480 watt-hours. Do this for every load in the cabin and add them up. This total is your daily energy demand.
Step two: convert to amp-hours if you’re buying lead-acid, or work in kWh if you’re buying lithium. Amp-hours = watt-hours divided by system voltage. A 12-volt system needs more amp-hours to store the same energy than a 24 or 48-volt system does, which is part of why larger off-grid setups move to higher voltage.
Step three: apply days of autonomy. This is how many consecutive days with little or no sun you want the bank to cover without needing a generator. For a cabin used through a Canadian winter, two to three days of autonomy is a reasonable minimum, because a multi-day snowstorm or heavy overcast stretch isn’t rare, it’s expected.
Step four: divide by usable depth of discharge. This is where chemistry comes back in. Divide your total energy need by 0.5 for lead-acid or by 0.8 for lithium to find the actual battery capacity you need to buy, since you can’t draw a battery down to zero and expect it to survive.
Step five: apply the cold weather correction factor. This is the one most sizing guides skip, and it’s covered in detail below, but the short version is that you need to size up again once you’ve accounted for how much less capacity a battery delivers in the cold.
A worked example: lights, a fridge, a water pump, and a laptop
Here’s the math run through on a realistic cabin load list.
- LED lighting: 40 watts total, running 5 hours a day = 200 Wh
- 12V fridge: averaging 60 watts of compressor run time, 8 hours a day = 480 Wh
- Water pump: 100 watts, running 30 minutes a day = 50 Wh
- Laptop charging: 65 watts, 3 hours a day = 195 Wh
Total daily demand: 925 Wh, call it 1,000 Wh (1 kWh) once you add a small margin for odds and ends like phone charging and the occasional kitchen appliance.
For two days of autonomy, that’s 2,000 Wh of energy the bank needs to deliver without recharging.
If you’re buying lithium, divide 2,000 Wh by 0.8 usable depth of discharge: 2,500 Wh, or 2.5 kWh of rated battery capacity. On a 12-volt system that’s roughly 208 amp-hours; on a 24-volt system, about 104 amp-hours.
If you’re buying lead-acid, divide 2,000 Wh by 0.5: 4,000 Wh, or 4 kWh of rated capacity, nearly double the lithium number, and that’s before the cold weather correction factor is applied.
What freezing temperatures actually do to each battery chemistry
This is the part a lot of first-time buyers in Canada get wrong, and it’s not a minor rounding error. Cold affects lead-acid and lithium differently, and both effects point the same direction: toward a bigger bank than the summer math suggests.
Lead-acid loses usable capacity as it gets cold. At 0°C, a flooded or AGM battery typically delivers somewhere around 80% of its rated capacity. At minus 18°C, that can drop to 50% or less. The chemical reaction that releases energy simply runs slower in the cold, so the same battery that gave you 100Ah at room temperature might only give you 50Ah on a hard January night.
Lithium has a different problem: it can’t charge below freezing. Most LiFePO4 batteries, including ones with built-in battery management systems, will refuse to accept a charge below 0°C to protect the cells from damage, and charging a lithium cell below freezing can cause permanent harm to it even if the system doesn’t stop you. Discharging (drawing power out) is generally fine down to much lower temperatures, but putting power back in during a cold snap is where lithium needs help, usually in the form of an internal heating element or a heated battery box.
Practically, this means: if your lithium batteries live in an unheated space, you need a model built with internal heating, or you need to keep the bank somewhere that stays above freezing. Lead-acid doesn’t have that hard cutoff, but it pays for that flexibility with a real capacity loss on the coldest days, which is exactly when a cabin needs the most power for lighting, pumps, and anything resisting the cold.
Either way, the number from your summer sizing math needs a correction factor. A reasonable rule of thumb for a Canadian winter is to size the bank 30 to 50% larger than the room-temperature math suggests, more if your cabin sees extended stretches below minus 20°C, less if the battery compartment is insulated or heated.
Can you store batteries in an unheated cabin over winter
For a cabin that isn’t occupied all winter, the question isn’t just about charging and discharging day to day, it’s about what happens to batteries sitting unused in the cold for weeks.
Lead-acid batteries tolerate cold storage reasonably well as long as they’re kept at a high state of charge. A fully charged flooded or AGM battery has a higher freezing point resistance than a discharged one, because the electrolyte in a charged battery is denser and freezes at a much lower temperature than in a discharged one, which can actually freeze solid and crack the case. If you’re closing up a cabin for winter, top the batteries off before you leave and check them if you can get back in partway through the season.
Lithium batteries are more particular about storage temperature than lead-acid, even when they’re not being charged or discharged. Most manufacturers recommend storing LiFePO4 somewhere between minus 20°C and 45°C, with a partial state of charge (around 50%) rather than full, for long idle periods. An unheated cabin in a Canadian winter can swing colder than that range depending on your region, so if you’re going lithium and leaving the cabin dark for the season, it’s worth checking the specific battery’s storage spec and considering a small heated enclosure or bringing the bank somewhere warmer if the building won’t hold above that threshold.
Sizing mistakes that leave people short of power by February
A few patterns show up again and again in off-grid systems that run out of power mid-winter.
- Sizing from a summer load list. Longer days and more sun mean less reliance on stored capacity, so a bank that felt generous in July can feel undersized in January when the fridge runs longer to fight the cold and there’s less daylight to recharge.
- Skipping the depth of discharge correction. Buying “200 amp-hours of lead-acid” and treating it as 200 usable amp-hours instead of roughly 100 is one of the most common and expensive mistakes.
- Ignoring the cold capacity derate on lead-acid. A battery rated for its full capacity at room temperature is not the same battery at minus 15°C, and plenty of buyers size to the label number without adjusting.
- Buying lithium without a cold charging plan. A battery that refuses to charge, or that charges poorly, on the coldest days of the year isn’t doing its job on the days it matters most.
- Underestimating fridge and pump run time. These are the two loads people guess at rather than measure, and they’re usually the biggest draws in the cabin.
- Not budgeting for inverter and system losses. Every conversion from DC to AC, and every bit of wiring, loses a bit of energy as heat. A rough 10 to 15% loss factor on top of the raw math keeps the final number honest.
None of these mistakes are hard to avoid once you know to check for them. The harder part is doing the math before the money is spent rather than after a battery bank runs dry during a February cold snap.
Once this sizing math gives you a target capacity in kWh or amp-hours, the next step is matching it to the rest of the system, the panels that will recharge it, the charge controller, and the inverter that will turn it into usable power. The Hub’s guide to off-grid power systems walks through how to choose the right components to go with the battery number you’ve just worked out, before you commit to buying panels, an inverter, or the battery bank itself.
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