Plenty of people travel with equipment that has to be charged or run from the rig: a CPAP machine, a power wheelchair or scooter, a wheelchair lift, an oxygen concentrator. The electrical question is always the same. How many amp hours does it take, and what has to be in the system so the circuit is safe?
This guide is about power sizing only. It is not medical advice and we are not qualified to give any. Confirm what your device actually needs with its maker or the supplier who provided it, not with a blog and not with a forum thread. If your equipment is life sustaining, plan your backup power with your equipment provider and your clinician. We know cable, fuses and batteries, not devices.
Start with the label, not a guess
Every device has its rating printed on the unit, on the power brick, or in the manual. Volts times amps gives you watts. Find it before you buy anything.
- Check whether the rating is AC or DC. "1.0 A at 120 V" is 120 watts from the wall. The same 120 watts from a 12 volt battery is about 10 amps, ten times the current, which is why the DC cable is so much fatter.
- A label is a maximum, not an average. The only way to know your average is to measure it.
- Count the chargers too. They are loads like any other, and that total sizes your bank.
Makers also tell you which power supply to use, and they mean it. ResMed's compatibility document says it in one line: "Only use compatible ResMed power supplies with ResMed devices."
CPAP: 12 volt DC or through an inverter
Direct 12 volt DC. Several CPAP makers sell their own DC cord or DC/DC converter that runs the machine from a vehicle outlet. That skips the inverter, so you avoid the conversion loss and the idle draw. Buy the maker's accessory for your model. Do not build one.
Through an inverter. You plug the machine's normal AC supply into an inverter on the house battery. Simple, and it uses the supply the machine came with.
The humidifier changes the arithmetic. The blower is a small fan load. A heated humidifier and a heated hose are resistive heaters, and heating is where the watts go. Check your own machine's accessory list for a DC option, and use only the supply the maker names. Find out what it draws with humidification on and off, and size for how you use it.
Idle draw is not free. A Victron Phoenix VE.Direct inverter uses about 4.2 watts at zero load on the 12/250 and 6 watts on the 12/500, which is 2.6 to 3.8 amp hours over eight hours before the machine draws anything. Do not use ECO mode for equipment you rely on: it cuts idle to about 1 watt by sleeping until a load appears, and a low draw device may not wake it.
Why pure sine wave
Modified sine wave inverters put out a stepped, blocky waveform. Transformers, motors and switch mode chargers run hotter on it, and many equipment makers specify pure sine. Every inverter we stock is pure sine: the Phoenix VE.Direct range with the NEMA 5-15R outlet, the Phoenix 12/3000 and 24/3000, the MultiPlus-II and the budget 1000 watt unit. If you own a modified sine inverter, replace it first.
Charging a 24 volt chair or scooter from a 12 volt bank
Most power chairs and many scooters run 24 volts. Most house systems run 12. Two ways to bridge it.
Route one: an inverter and the chair's own charger, exactly as you would at home. The maker's charge profile and its own socket stay in the loop, which is usually what the maker wants, so this is the safer default. It costs amp hours, because you pay the inverter's losses on top of the charger's. Work yours out from the charger's label.
Route two: a 12 to 24 volt DC-DC unit, which never goes through AC. We stock two:
- Orion-Tr Smart 12/24-15A non-isolated, a charger rather than just a converter. Victron rates it 15 amps and 360 watts at 40 C, input 8 to 17 volts, output adjustable 18 to 30 volts, with a three stage charge algorithm, Bluetooth and engine running detection. At 88 percent efficiency it pulls about 33 amps from the 12 volt side.
- Orion IP67 12/24-50, 24 volts at 50 amps, 1200 watts, 96 percent efficient and waterproof. It is a converter, not a charger, so there is no charge profile. At full output it draws about 100 amps from the 12 volt side, so size the feed and fuse for that.
Before wiring either one to a chair, ask its maker or dealer. Many chairs only accept a charge through their own socket with their own charger, and some lock out driving while charging.
The wheelchair lift gets its own fused feed
A lift is a motor, and motors pull hard for a few seconds at a time. Harmar's manual for the AL6000 and AL6000HD says the motor "can draw up to 20 Amps at some points, requiring all of the available wire to flow proper current", specifies a 20 amp self resetting breaker about six inches from the battery, and runs its harness from the vehicle battery to the cargo area. Poor connections, it adds, are the top cause of a slow, warm or intermittent motor.
Numbers differ by model, so read your own manual. The principles do not. Give the lift its own feed from the battery or busbar, protected at the source, sized for the full round trip, with clean tight terminations. If it runs off the chassis battery, think about what a few cycles with the engine off leave you. That is why people add a house battery and a DC-DC charger.
Sizing the bank: worked example
Replace every number with yours.
- Nightly energy. Say the equipment averages 120 watts for 8 hours. That is 960 watt hours.
- Inverter loss. At 90 percent, the battery supplies about 1,067 watt hours.
- Amp hours. At 12.8 volts, about 83 amp hours.
- Add the rest of the rig. Fridge, lights, fans, pump. Say 30 amp hours. Total 113.
- Leave headroom. Plan on using about 80 percent of a lithium bank, so 113 / 0.8 is about 141 amp hours of nameplate capacity as a minimum.
- Plan the recharge. Those 113 amp hours go back in daily, from solar, the alternator or shore power.
One Eco-Worthy 12 V 280 Ah LiFePO4 covers that example with margin; the Victron Lithium SuperPack range covers smaller builds, including a 25.6 volt version for 24 volt systems. On shore power the Blue Smart IP22 recharges it.
Fuse every circuit
Every conductor leaving the battery needs protection at the battery end, sized to protect the cable: the inverter feed, the DC-DC unit's input and output, the lift feed, each 12 volt socket.
- MEGA fuses in a holder, 40 A to 500 A, for big feeds.
- MIDI fuses in a holder for smaller branches.
- A busbar so each circuit leaves from its own fused stud instead of stacking rings on a battery post.
Manufacturers say the same. The Inogen One G5 manual tells owners the vehicle socket must be fused for the concentrator, minimum 10 amp and preferably 15, and warns against splitters and extension cords on its DC cable. It also specifies a DC input of 13.5 to 15.0 volts, and a 12 volt lithium battery at rest sits below 13.5 volts, so ask the maker whether that cord suits a house battery. Our cable, fuse and busbar chart has the ratings.
Know your state of charge, do not guess it
Lithium voltage is nearly flat across most of its range, so a voltmeter tells you little until it is too late. A shunt counts every amp in and out and reports real state of charge. Fit a Victron SmartShunt and you can watch each device's draw in the app, which is how you replace the numbers above with your own.
What we stock, and what we do not
We sell the power system: pure sine inverters, inverter/chargers, 12 to 24 volt DC-DC units, lithium batteries, shore chargers, fuses, holders, busbars, cable and monitors. We do not sell lifts, chairs, scooters, CPAP machines, oxygen equipment or any medical cord or adapter. Buy those from your provider. Nothing we sell is a medical device, and no inverter or battery makes equipment safe to depend on.
Related guides
Send us your device labels, your house voltage and how many nights you go without hookups, and we will size the bank, the inverter and the fuses. We ship from Upland, California.