How to use it
- Describe your battery. Voltage, type and amp-hours.
- List your plug-in gear. An example is loaded. Change the watts and hours, untick anything you never run at the same time as the rest, and add more from the list.
- Set up the inverter. Its size, how long it stays on each day, and the cable run to the battery.
- Read the results. Run time, how hard you're pulling on the battery, the inverter size and surge you need, and the cable and fuse.
How the numbers are worked out
Energy
Each item uses watts × hours × quantity watt-hours a day at the outlet. The inverter loses some of that as heat, so the battery supplies outlet watt-hours ÷ efficiency, plus the inverter's idle draw for every hour it is on.
The default 85% efficiency is a planning figure for typical loads. Good inverters peak higher, around 91% for a Victron Phoenix 12/1200, and lose more at very light loads. The default 7 W idle draw is that Victron model's zero-load figure.
Run time
Lead-acid batteries deliver fewer amp-hours under heavy draw, so the battery's rating is adjusted with Peukert's law (k = 1.25 flooded, 1.15 AGM, 1.05 lithium) at your average current while the inverter is on, and never credited above the label. "Everything on at once" uses the full current of every item ticked "At once".
Inverter size and surge
Continuous watts are everything ticked "At once" running together. Surge is that total plus the biggest extra start-up kick from any one motor or compressor, the same method generator makers use. Running and starting watts come from Champion Power Equipment's wattage chart and Virginia Cooperative Extension's appliance list.
Cable and fuse
Battery current at full load is inverter watts ÷ (battery volts × efficiency). The fuse is sized at 125% of that, the usual rule for continuous loads, rounded up to a standard size. The cable must carry the fuse rating (105°C marine wire, 33 CFR 183.425) and keep voltage drop to 3% using the ABYC formula.
Planning estimates. If your inverter's manual lists a different fuse or cable size, follow the manual. Mount the fuse close to the battery on the positive cable. Never run fuel-burning or open-element heaters inside a tent.
Sources
- Victron Phoenix 12/1200: 91% maximum efficiency, 7 W zero-load power
- Champion Power Equipment wattage chart (running and starting watts)
- Virginia Cooperative Extension: appliance wattages
- CPAP power use (CPAP.com)
- Starlink Mini power consumption (Anker, citing Starlink)
- 33 CFR 183.425: allowable amperage of conductors
- Ancor: 3% voltage drop formula