System voltage
12 V 24 V
Circuit type
Critical — 3% max drop
Standard — 5% max drop
Non-critical — 10% max drop
Current draw—
Voltage drop—
Cable rated for—
How this is calculated
Current — I = W ÷ V at nominal system voltage.Cable — smallest standard size meeting both a rating of 1.25 × load
and your drop limit, over 2 × run length for the return path.Voltage drop — V = 2 × ρ × L × I ÷ A, copper at ρ = 0.0175 Ω·mm²/m (~50 °C).Fuse — protects the cable , not the appliance. At least 1.25 × load,
never above the cable's rating. If no standard fuse fits, the cable is stepped up.
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Daily energy use
—
watt-hours per day
At 12 V that's
—
amp-hours per day
DC loads—
AC loads (via inverter)—
Biggest single draw—
How this is calculated
Per appliance — Wh = watts × hours × quantity.AC appliances add ~13% for inverter conversion loss (87% efficiency), because
they draw from the battery through the inverter.Amp-hours — Ah = Wh ÷ 12.8 V (nominal LiFePO4 bank voltage).Fridge and heater hours are compressor run time , not hours switched on — a fridge
cycles roughly half the time.
Daily use (Wh)
Battery type
LiFePO4 (lithium) AGM / lead-acid
System voltage
12 V 24 V
Depth of discharge—
Days without charging—
Approx. weight—
How this is calculated
Ah = daily Wh × 1.2 ÷ (depth of discharge × bank voltage), rounded up to a size you can buy.1.2 is a 20% headroom buffer — never plan to run a bank to empty every day.Depth of discharge — LiFePO4 80%, AGM 50%. This is why a 100 Ah lithium
battery replaces roughly 160 Ah of AGM.Bank voltage is 12.8 V nominal for LiFePO4, 12.0 V for AGM (doubled on a 24 V system).
Roof space
—
approx. panel area
What that array actually generates
How this is calculated
Watts = daily Wh ÷ (peak sun hours × system efficiency), rounded up to a standard panel size.System efficiency 0.72 — MPPT losses, wiring, plus a heat and soiling penalty.
Van panels lie flat on a hot roof and rarely get cleaned or tilted.Peak sun hours are for a horizontal surface, because a van roof can't tilt toward the sun.
Current draw—
Depth of discharge—
Nameplate capacity—
How this is calculated
Usable Wh = Ah × bank voltage × depth of discharge.Runtime = usable Wh ÷ load watts.Depth of discharge — LiFePO4 80%, AGM 50%. Running below this shortens
battery life sharply, so nameplate capacity is never what you actually get.AC loads through an inverter draw ~13% more from the battery than their rated watts.
Guidance only — always check component manuals, which take precedence, and consult a qualified
auto electrician for anything you're unsure of. Mains AC work is not covered here.