How AMP'D 12V calculates your system
Most 12V calculators either tell you the biggest battery to buy or assume perfect conditions. We do neither. This page explains exactly how the calculator works — the formulas, the assumptions, and why our numbers are deliberately conservative — so you can decide whether to trust the recommendation rather than taking it on faith.
Our approach
The goal isn't to sell you the biggest system — it's to size one that runs your gear reliably, matches how you actually camp, and doesn't cost more than it needs to. Every recommendation is built from transparent formulas and conservative, disclosed assumptions, so the tool errs slightly toward safety rather than toward optimistic best-case numbers.
Step 1 — Your daily power use
Everything starts here. As you add gear, the calculator estimates the daily amp-hours (Ah) each item draws. Fridges, lighting, water pumps, diesel heaters, Starlink, CPAP machines, and 240V appliances run through an inverter are all modelled. Where possible the figures come from manufacturer specifications; where real-world use varies a lot between brands, we use researched averages chosen to reflect Australian touring rather than lab conditions.
Fridge draw changes with the weather
A compressor fridge is usually the biggest single load, and its consumption rises sharply in heat. The calculator scales fridge draw with the season you select — higher in summer and outback conditions, lower on cool overcast days — because a flat figure would badly under-count a fridge working hard in 35-degree heat.
Step 2 — Battery capacity
From your daily use, we size the battery using your chosen chemistry's safe depth of discharge (about 85% usable for LiFePO4, 50% for AGM), your desired days of autonomy, and charging efficiency, then round up to a real, purchasable size. We also check the battery against the peak current your inverter loads demand — not just the energy — and flag when a typical 100A BMS would be marginal. Sizing on energy alone is a common mistake this avoids.
Step 3 — Solar
Solar is sized to replace the energy you use, accounting for seasonal peak-sun hours, panel size, MPPT or PWM controller efficiency, shade, and real-world charging losses. A panel almost never makes its rated output all day, so the recommendation reflects a realistic whole-system yield, not the sticker figure. This is why your solar recommendation changes between summer and winter.
Step 4 — DC-DC charging
For vehicle charging, the calculator sizes a DC-DC charger to replace the energy used across your typical driving, considering driving time, battery chemistry and sensible charge rates. On higher-output chargers it also reminds you to check your alternator can supply the current and to size the cable run properly — a long run to a rear canopy needs heavy gauge to avoid voltage drop.
Step 5 — Inverter
Unlike batteries, an inverter is sized for peak power, not daily energy. Running a 1500W appliance briefly needs little battery but does need an inverter that can supply that peak safely. We size the inverter from your largest load (or the combined load if you'd run several at once), plus a safety margin — and account for the idle draw of an inverter left switched on all day, which quietly adds up.
Why our numbers may differ from other calculators
You may notice we recommend more (or a different mix) than a simpler tool. That's usually because we include depth of discharge, controller efficiency, seasonal solar, chemistry differences, charging limits and conservative margins — factors many calculators skip. We'd rather recommend a system that keeps working when conditions aren't perfect than one that only works on paper.
We recommend specifications, not brands
The calculator is brand-neutral. It recommends specifications — say, 200Ah lithium, 400W solar, a 40A DC-DC charger — and there are often many suitable products from different manufacturers. Where affiliate links appear, they're chosen because the product matches the recommended spec, never because a brand pays more. You're always free to buy an equivalent from anyone.
What no calculator can predict
Every setup is different. Real power use depends on weather, ambient temperature, driving habits, battery age, appliance settings, installation quality, cable sizing and panel orientation. Our recommendations are planning guidance and a starting point — not a substitute for professional electrical advice where it's required. Treat the numbers as a well-reasoned estimate to check against your own situation.
Why we built it
Too many people spend thousands on batteries, solar and chargers without really knowing what they need. AMP'D 12V exists to help Australian tourers size reliable 12V systems from transparent calculations rather than marketing claims or generic rules of thumb. As the tool evolves, we keep refining the formulas using manufacturer data, industry guidance and feedback from real users.