How to use the 12 volt accessory load calculator
- Add a row for each accessory with its watts, the quantity and the hours it runs each day.
- Choose the system voltage: 12, 24 or 48 volts.
- Optionally enter the battery capacity in amp-hours and, under advanced options, the share of it you are willing to use.
- Read the peak current, the daily watt-hours and amp-hours, and the days between charges.
Formula
Amps = watts x quantity / system volts
Peak current = sum of the amps of all accessories running at once
Daily watt-hours = watts x quantity x hours per day
Daily amp-hours = watt-hours / system volts
Days between charges = capacity x usable share / daily amp-hours
Peak current adds every accessory as if all were on at the same time, which is the safest figure to size wires and fuses.
A weekend setup
A fridge uses 45 W for 10 hours (450 Wh), four lights use 6 W each for 4 hours (96 Wh) and two phone chargers use 15 W each for 3 hours (90 Wh). The daily total is 636 Wh.
At 12 volts that is 53.00 Ah a day, and the peak current with everything on is 8.25 A. A 100 Ah battery with half of it usable holds 50 Ah, so it lasts 0.9 days between charges.
Building a power budget
A power budget lists what you run and for how long, and turns it into a daily amount of energy. Once you have that, you can size a battery, a charger or solar panels to match. The watt-hours are the same at any voltage, while the amps depend on the system voltage, so the same load draws half the current at 24 volts as at 12.
Energy is what drains a battery and current is what heats the wire. The daily amp-hours tell you how big the battery needs to be, and the peak amps tell you how heavy the wiring must be.
Making the numbers honest
Label watts are often a maximum. A compressor fridge cycles on and off, so its hours of running time are less than the hours it is plugged in. Measure with a power meter when you can and enter the real watts and hours. Inverters, long wires and charging all lose some energy, so add a margin of your own.
The usable share matters for lead-acid batteries, which last longer if you do not discharge them deeply. A share of 50% is an example, and your battery type and its maker's guidance decide the right figure.
Assumptions and limits
- Each accessory draws constant power while it runs.
- Losses in inverters, wires and charging are not included.
- Up to 20 accessories, 24 hours a day at most for each.
- The figures are examples. Nothing is uploaded.
Frequently asked questions
Why do I need both peak amps and daily amp-hours?
Peak amps size the wires and fuses, and daily amp-hours size the battery. They answer different questions.
How do I find the watts of a device?
Read the label or use a plug-in power meter. If the label shows amps, multiply by the volts.
What if I run an inverter?
Enter the AC load as watts and add about 10% to 15% for the inverter's own use and conversion loss. The exact loss depends on the inverter.
Does it include solar charging?
No. It gives the daily need, which you can compare with what your panels make.