Most van electrical builds fail in one of two directions. Either you under-build — and spend every grey week chasing a dead battery, rationing your laptop, listening for the fridge to click off. Or you over-build — and bolt three hundred watts of panel and a slab of lithium you'll never fully cycle to a roof that's now heavier, pricier, and harder to park.
The fix is not a bigger number. It's a budget. You figure out what you actually pull in a day, in watt-hours, and then you size the battery, the panels, and the alternator feed to refill that hole on a realistic day — not a perfect one. Everything below is that process, in order.
Step 1 — Build the watt-hour budget
Energy on the road is bookkept in watt-hours (Wh): watts drawn × hours run. A device's watts come off its label or spec sheet; multiply by how many hours a day it actually runs, and sum the column. That total is your daily load, and every other decision flows from it.
Work in watt-hours, not amp-hours, while you're budgeting — it lets you add 12V loads and 120V loads in the same column without tripping over voltage. (You convert to amp-hours later, once, when you size the battery.)
A realistic worked example for a work-from-the-van rig:
| Load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| 12V compressor fridge (duty-cycled) | 45 | 8 | 360 |
| Laptop + monitor | 65 | 6 | 390 |
| Phone + tablet charging | 20 | 3 | 60 |
| LED lighting | 12 | 4 | 48 |
| Roof / Maxxair fan | 25 | 6 | 150 |
| Water pump | 50 | 0.3 | 15 |
| Starlink (if carried) | 50 | 8 | 400 |
| Daily total | | | ~1,420 Wh |
Two notes that change the number more than people expect:
- The fridge is a duty cycle, not a constant. A 45W compressor fridge does not pull 45W for 24 hours — it cycles on and off to hold temperature, so figure the equivalent run-hours (here, ~8 of every 24). In a hot van that climbs; in a cool one it drops.
- Inverter loads cost extra. Anything running off the inverter (the 120V column) pays an efficiency tax — a good inverter is ~85–90% efficient, so add roughly 10–15% to those loads before you trust the total.
Your number won't be 1,420. Build your own column. That total — call it your daily Wh — is the spine of the whole system.
Step 2 — Panel watts vs usable amp-hours: the rules of thumb
Here's where the marketing diverges hardest from reality. A "200W panel" does not make 200 watt-hours an hour, and your build will be starved if you size it as though it does.
The panel derate. Rated panel wattage is measured under lab conditions (Standard Test Conditions) that your roof never sees — perfect 25°C cells, perfect perpendicular sun, perfect clean glass. In the field, a flat-mounted rooftop array delivers roughly its rated watts × the number of peak-sun-hours for your latitude and season × a real-world loss factor of about 0.75. Heat, dust, wiring loss, a non-optimal angle, and MPPT-controller overhead all eat into the nameplate. (The 0.75 figure is the standard conservative derate used in off-grid solar sizing; see Sources.)
So the working estimate for daily solar harvest is:
> Daily Wh from solar ≈ panel watts × peak-sun-hours × 0.75
Peak-sun-hours is the catch, and it's brutally seasonal in Canada. A flat rooftop array might see 4–5 peak-sun-hours on a clear summer day in southern Canada, but 1–2 (or near zero under heavy overcast) in December. Run the math both ways:
- 400W of panel, summer (4.5 PSH): 400 × 4.5 × 0.75 ≈ 1,350 Wh/day — covers the example budget.
- 400W of panel, December (1.5 PSH): 400 × 1.5 × 0.75 ≈ 450 Wh/day — covers about a third.
That gap is why solar alone is a summer plan in this country, and why the alternator feed (Step 4) exists. Size your panels to your shoulder-season expectation, not your June best, and plan to make up winter with the engine.
Panel watts vs usable amp-hours. A second rule of thumb people get wrong: a watt of panel is not an amp-hour of usable battery. If you must think in amp-hours for a 12V battery, Wh ÷ 12.8 ≈ Ah (LiFePO4 nominal is ~12.8V, not a round 12). The 1,420 Wh daily budget is therefore roughly 111 Ah pulled from a 12V bank each day — which sets up the battery sizing.
Step 3 — Battery chemistry: LiFePO4 vs AGM
This is the decision that most shapes how the rest of the system behaves, because the two chemistries differ on the one number that matters: how much of the rated capacity you can actually use.
Usable depth of discharge. A lead-acid battery (AGM included) hates being deeply cycled. To get reasonable life out of an AGM bank you keep it above ~50% state of charge — so a "100Ah" AGM gives you about 50Ah usable. LiFePO4 is comfortable being drawn down to ~80–90% depth of discharge cycle after cycle — so a "100Ah" LiFePO4 gives you about 80–90Ah usable. On usable capacity, one 100Ah lithium ≈ two 100Ah AGM, before you've weighed anything.
Weight and space. LiFePO4 is roughly half to a third the weight of AGM for the same usable capacity — a meaningful chunk of cargo budget in a build where every kilo costs fuel, suspension, and handling (see The Just-In-Time Nomad on why dead weight is never neutral).
Cycle life. Quality LiFePO4 is rated in the thousands of cycles (commonly 3,000–5,000+ to 80% capacity); AGM is typically a few hundred to perhaps a thousand under gentle use. Over a multi-year build the lithium is usually cheaper per usable kilowatt-hour delivered, even though the sticker is higher.
Where AGM still wins. Lower upfront cost, no low-temperature charging complications (the next section), and tolerance of simpler charging gear. For a light, occasional-use, cold-climate build that mostly charges off a basic alternator, AGM is still a defensible choice.
For a full-time, work-from-the-road rig with a real daily budget, LiFePO4 is the standard answer — provided you respect its one genuine weakness, cold.
| | AGM | LiFePO4 |
|---|---|---|
| Usable capacity (of 100Ah rated) | ~50Ah | ~80–90Ah |
| Weight vs usable capacity | heavy | ~⅓–½ |
| Cycle life | hundreds–~1,000 | thousands |
| Upfront cost | lower | higher |
| Cold-charge risk | none | must be managed |
Step 4 — Alternator DC-DC charging: the winter workhorse
When solar collapses to 1–2 peak-sun-hours, the engine becomes your generator. But you cannot simply wire the house battery to the alternator and call it done — a modern "smart" ECU-controlled alternator varies its voltage and shuts down when it decides the start battery is full, and a depleted lithium bank can pull more current than the alternator or wiring can safely deliver. The correct part is a DC-DC charger (a B2B / battery-to-battery charger) sitting between the start battery and the house bank.
A DC-DC charger does three jobs: it limits the current drawn to a fixed, safe ceiling; it runs a proper multi-stage charge profile suited to your house chemistry (lithium, AGM, gel); and it uses engine-running detection so it only draws while the alternator is actually producing, never flattening your start battery. A common modern unit, the Victron Orion XS 12/12-50A, is a 12V-to-12V charger with a settable output up to 50A, built-in engine-shutdown detection so it only charges while the engine runs, a configurable input-lockout that protects the start battery from deep discharge, and selectable lead-acid and lithium charge profiles — exactly the three jobs above. (Victron — Orion XS 12/12-50A manual)
The math is what makes this the winter workhorse. A 50A DC-DC charger into a 12.8V lithium bank delivers roughly 50 × 12.8 ≈ 640 Wh per hour of driving. Two hours of driving between camps is ~1,280 Wh — nearly the whole example budget, refilled by the engine you were running anyway. In December, when 400W of panel scrapes together 450 Wh on a good day, the alternator isn't a supplement — it's the main supply, and the panels are the supplement.
Size the DC-DC charger to your alternator's spare capacity, not just your battery's appetite. A 30A B2B is gentle on a small or older alternator; a 50A unit refills faster but demands an alternator (and wiring gauge) that can give 50A continuously on top of the vehicle's own loads without cooking itself. When in doubt, the smaller charger that runs cool for the whole drive beats the bigger one that overheats and throttles.
Step 5 — Inverter sizing
The inverter turns your 12V bank into 120V AC for anything without a DC plug. Size it by two separate numbers, not one:
- Continuous watts must exceed the sum of everything you'd run at once off AC, with headroom. Add up your realistic simultaneous AC loads, then size up — a 2000W inverter for a ~1500W simultaneous draw, not a 1500W inverter run at its ceiling.
- Surge watts must cover the startup spike of motor- and compressor-driven gear. An induction cooktop, a microwave, a power tool, or an air compressor can momentarily pull 2–3× its running watts at the instant it kicks on. An inverter that's fine on continuous watts will still trip on surge if you ignore this.
Two more decisions:
- Pure sine vs modified sine. Buy pure sine wave. Modified-sine is cheaper but makes some electronics buzz, run hot, or refuse to work — laptop bricks, CPAP machines, variable-speed motors, and anything with a sensitive power supply. The savings aren't worth the failure modes.
- Right-size it; don't max it. An oversized inverter has higher idle (no-load) draw — it burns watt-hours just being switched on. Match the inverter to your real AC loads, switch it off when nothing's plugged in, and run DC-native wherever you can (12V fridge, 12V fans, USB charging) to keep it off entirely.
The honest move in a small rig: minimize the AC side. Every load you can run natively on 12V is a load that skips the inverter's efficiency tax and idle draw both.
Step 6 — The Canadian-winter LiFePO4 cold-charging problem
This is the section that protects your most expensive component, so read it twice. Lithium iron phosphate batteries must not be charged below freezing. Charging a cold LiFePO4 cell causes lithium plating — metallic lithium deposits on the anode that permanently and irreversibly reduce capacity, and in the worst case create an internal short. (Battle Born — temperature limits)
Critically, discharging cold is fine — it's only charging cold that does the damage. You can run your lights, fridge, and laptop off a lithium bank in deep cold; you just can't safely push current back into it until it warms.
Manufacturers handle this three ways, and you need to know which one your battery does:
- Hard BMS cutoff. Battle Born's BMS simply stops accepting charge below ~25°F (−4°C) and won't resume until the battery climbs back above freezing. Discharge stays available down to about −4°F (−20°C). The protection is automatic — but a battery that won't accept charge in winter is a battery you can't refill, which is its own problem. (Battle Born — temperature limits)
- Current derate. A standard RELiON battery doesn't hard-cut at freezing; it throttles the allowed charge current as it gets colder — no more than 0.1C between 32°F and 14°F (0°C to −10°C), and no more than 0.05C between 14°F and −4°F (−10°C to −20°C). Charge is permitted, but slowly, and you must respect the limit. (RELiON — cold-temperature charging)
- Self-heating. Purpose-built cold-weather batteries (RELiON's LT series, Battle Born's heated models) carry an internal heating element. When a charge cycle starts below freezing, the BMS diverts the incoming charge current to the heater — RELiON's BMS sources roughly 5–15A depending on model and how cold it is — until the cells reach a safe charging temperature, then charges normally. This is the cleanest answer for genuine winter use: the battery thaws itself off the same charge source, no heated blanket required. (RELiON — heating-element draw)
Set your charge sources to honour this. On Victron gear, the low-temperature cut-off default for the LiFePO4 profile is 5°C (41°F) — deliberately above freezing, with a margin — and charging only resumes once the battery has warmed ~0.5°C past the cut-off (a small hysteresis). Note that Victron's own Smart Lithium batteries manage this through their own BMS, but a third-party lithium bank on a Victron MPPT or DC-DC does need the cut-off configured and a battery temperature reading fed in (e.g. via a temperature sensor on a VE.Smart network) — the controller's internal sensor alone is not on the battery. (Victron Community — stop charging below 5°C)
The practical winter build comes down to three honest options:
- Heated battery. Buy a self-heating LiFePO4 (LT / heated model). Simplest, most robust, most expensive.
- Keep the bank warm. Mount the battery inside the heated, insulated living space — not in an exterior bay or under the chassis — so your cabin heat keeps it above freezing and it charges normally. Cheapest if your layout allows it.
- Run AGM for the cold months. If your build genuinely lives in deep Canadian winter and can't keep a lithium bank warm, AGM sidesteps the cold-charge problem entirely — at the cost of weight and usable capacity.
What you must not do is bolt a plain (non-heated) LiFePO4 bank in an unheated exterior compartment and feed it an alternator or solar charger with no temperature sensor and no cut-off. That's the configuration that plates lithium onto the anode and quietly kills a battery you paid a premium for. For the rest of the cold-weather build — heat, condensation, frozen water lines, and the CO detector that isn't optional — see [Winter & cold-weather van living in Canada](winter-vanlife.md).
Putting it together
The whole sizing process, in order:
- Budget your daily watt-hours from a real device-by-device column (Step 1).
- Size the battery so its usable capacity covers 1–2 days of that budget — Wh ÷ 12.8 ≈ Ah for a 12V bank — choosing chemistry on usable depth, weight, and cold (Steps 2–3).
- Size the panels to refill the budget in your shoulder-season sun, not your June best, using panel watts × peak-sun-hours × 0.75 (Step 2).
- Size the DC-DC charger to your alternator's spare capacity so the engine covers the winter gap solar can't (Step 4).
- Size the inverter for simultaneous AC load plus surge, pure-sine, and switch it off when idle (Step 5).
- Solve the cold before winter, not during it — heated battery, warm install, or AGM (Step 6).
Build the budget first. Everything downstream is just arithmetic against that one honest number.
Sources
- Battle Born Batteries — Understanding Temperature Limits of LiFePO4 Batteries (charge blocked below ~25°F, discharge to ~−4°F, lithium-plating risk). https://battlebornbatteries.com/blogs/academy/understanding-temperature-limits-of-lifepo4-batteries-the-battle-born-educational-series
- RELiON — How do LiFePO4 batteries perform in cold temperatures? (0.1C charge-current derate 0°C to −10°C; 0.05C derate −10°C to −20°C). https://www.relionbattery.com/knowledge/how-do-lifepo4-batteries-perform-in-cold-temperatures
- RELiON — How many amps does the heating element draw? (LT self-heating BMS diverts ~5–15A to the heater before charging the cells). https://www.relionbattery.com/knowledge/how-many-amps-does-the-heating-element-draw
- Victron Energy — Orion XS 12/12-50A DC-DC battery charger manual (12V→12V, up to 50A output, engine-shutdown detection, input lockout, lead-acid/lithium profiles). https://www.victronenergy.com/upload/documents/Orion_XS_12-12-50A_DC-DC_battery_charger/124067-Orion_XS_DC-DC_battery_charger-pdf-en.pdf
- Victron Community — Stop charging below 5°C (LiFePO4 low-temperature cut-off default of 5°C, ~0.5°C resume hysteresis, external battery-temperature input required). https://community.victronenergy.com/t/victron-multiplus-ii-mppt-smartsolar-stop-charging-below-5-degrees/18092
- Solar derate and peak-sun-hours figures (0.75 system loss factor; ~4–5 PSH summer in southern Canada, dropping to ~1–2 in December) are standard off-grid solar-sizing conventions; Natural Resources Canada publishes Canadian photovoltaic potential and insolation data at https://natural-resources.canada.ca/energy-sources/renewable-energy/solar-photovoltaic-energy