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Rig power planner

Change anything on the left and the system on the right updates. Conservative on purpose: we'd rather you buy one size up than replace a melted cable. When it looks right, draw it.

Start from a typical rig

1. What you run each day

1,731 Wh/day

Watts while running and hours per day. Typical figures; check the label on your own gear.

405 Wh/day · The compressor runs about a third of the day; more in hot weather.

160 Wh/day

100 Wh/day

18 Wh/day

45 Wh/day

390 Wh/day · Charging from a 12 V USB-C charger instead skips the inverter loss.

240 Wh/day · Runs on 12 V DC through a USB-C PD adapter.

200 Wh/day · Start-up glow plug briefly draws much more.

AC means it plugs into a household outlet and runs through the inverter.

2. Where, when and how you travel

3. Your rig

Cable run lengths (feet, one way)

Your system

You use about 1,731 Wh a day.

Battery
2 x 200 Ah
12 V lithium, 5,120 Wh
Solar
400 W
40 A MPPT controller
Alternator charging
40 A DC-DC
486 Wh per day of driving
Inverter
1,000 W
Peak AC load 65 W

Solar plus driving covers 100% of a day in October in Colorado.

Priced parts: $2,136

Draw this system

Wire and fuse for each main run

Copper, 105°C insulation, sized for heat in a bundle and for under 3% voltage drop. Fuse within 7 inches of the battery or bus bar.

RunAmpsFuseWireDrop
Battery to bus bars (main cable, Class T fuse at the battery) (3 ft)120 A150 A1 AWG0.8%
Bus bars to 1,000 W inverter (4 ft)93 A125 A2 AWG1%
Solar charge controller to bus bars (6 ft)40 A50 A8 AWG2.6%
DC-DC charger to bus bars (4 ft)40 A50 A8 AWG1.7%
Starter battery to DC-DC charger (engine bay) (15 ft)48 A60 A2 AWG1.9%
Bus bars to DC fuse block (12 V loads) (5 ft)20 A25 A12 AWG2.7%
Shore charger to bus bars (4 ft)40 A50 A8 AWG1.7%

Solar panel to controller: use 10 AWG PV wire and follow the panel maker's fuse rule. DC-DC chargers tolerate more drop on the input side; your charger's manual may allow a thinner input cable. Where these numbers come from.

Parts list

Disclosure: We may earn a commission if you buy through these links. It never changes what we recommend or what the tools calculate. How we make money.

Priced parts total$2,135.90

Single-unit prices read on each maker's site on October 22, 2026. Shipping and tax not included.

Show the math
  1. 12 V loads: 1,168 Wh. AC loads: 390 Wh at the outlet, divided by 88% inverter efficiency = 443 Wh. Inverter idle: 15 W while switched on = 120 Wh. Daily total: 1,731 Wh.
  2. Battery: 1,731 Wh x 2 days, divided by 80% usable (we keep 20% of a lithium bank in reserve) = 4,328 Wh = 338 Ah at 12 V. The pick also needs a BMS rated for 110 A (inverter plus DC loads); yours: 400 A.
  3. Voltage: 12 V matches your vehicle, your fridge and almost every RV and marine accessory.
  4. Sun: a flat panel in Denver, Colorado makes 3.51 Wh per watt per day in October (PVWatts model, 14% losses). Driving: 40 A x 12.8 V x 1 h x 95% = 486 Wh.
  5. Solar to break even: (1,731 - 486) Wh / (3.51 x 95%) = 400 W, rounded to 100 W. Controller: 400 W / 12 V = 33 A, next size up.
  6. Inverter: biggest AC load plus the small AC loads that run with it = 65 W, plus 15% headroom, next standard size.
  7. Each run: fuse = 125% of the current, next standard size. Wire = the smallest size whose bundled 105°C rating (ABYC E-11 Table VI-B) is at least the fuse, and whose round-trip voltage drop is under 3%.

How the planner works

It adds up your daily energy, sizes a battery to carry you through the days with no charging, then sizes solar and alternator charging to put that energy back in the darkest month you'll be out. Sun figures come from a solar model run for a flat-mounted panel in a reference city in every state (see the table).

The battery pick is the cheapest combination of up to four identical Redodo batteries that covers your amp-hours and whose built-in management systems can deliver the current your inverter and 12 V loads can pull at once. Wire and fuse sizes follow the method on our wire gauge chart.

It does not design your AC wiring, shore power inlet or transfer switch. Those connect to 120 V and belong to a qualified electrician.

Questions people ask

How big a battery do I need for a van?

Add up your daily watt-hours, multiply by the days you want to go without sun or driving, and divide by the share of the battery you'll use. We plan on 80% of a lithium bank. A typical van with a fridge, fan, lights and phones uses 700 to 1,000 Wh a day, so two days of reserve is 1,750 to 2,500 Wh: one 200Ah lithium battery.

Why does the planner size solar for the darkest month?

Because that is when you run short. A flat roof panel makes two to five times as much in June as in December in most states, and far more in Alaska. Size for July and you'll be buying a generator in November.

Why are your wire sizes thicker than some charts?

We use the ABYC rating for wires run in a bundle or conduit (how they are actually installed in a van wall), require the fuse to protect the wire, and hold voltage drop under 3%. Charts that use free-air ratings or 10% drop give thinner wire. Thicker copper costs a little more and runs cooler.

Does the planner include inverter losses?

Yes. Everything you mark as AC is divided by 88% inverter efficiency, and we add the inverter's idle draw for the hours it is switched on. Running a laptop from a 12 V USB-C charger instead avoids both.

Sources