Tiny Home Batteries and Inverters: Types, Sizing and Safety
Batteries store what your panels make, and the inverter turns that stored power into the outlets you are used to. Together they decide how many cloudy days you can ride through and what you can plug in. This guide covers the main battery types, how to size a bank and an inverter, and the safety details that are easy to overlook.
Battery capacity in plain terms
Battery capacity is usually printed in amp-hours (Ah) at a given voltage. Multiply the two to get watt-hours: a 12-volt, 100 Ah battery holds about 1,200 watt-hours, and a 24-volt, 100 Ah battery holds about 2,400. That watt-hour figure is what you compare with your daily use from the energy audit in our power overview.
The number on the label is not all usable. Some chemistries should not be drained completely without shortening their life, so the usable fraction matters as much as the headline capacity.
Lead-acid versus lithium iron phosphate
Two families dominate off-grid tiny homes.
- Lead-acid (flooded or sealed AGM) is the traditional choice. Purchase price per amp-hour is lower, and the technology is well understood. Batteries are heavy, and for long life they are usually discharged only about halfway. Flooded types need periodic watering and ventilation.
- Lithium iron phosphate (LiFePO4) costs more up front but typically allows a much deeper daily discharge, is lighter, holds its voltage well under load and generally lasts for more charge cycles. It includes a battery management system (BMS) that protects the cells.
There is a cold-weather catch with lithium: most lithium batteries should not be charged when the cells are below freezing. If your battery lives in an unheated space, look for a model with built-in low-temperature protection or heating, or keep the battery in an insulated, conditioned area. Always check the manufacturer's specifications for your climate.
Over the life of the system, many people find that lithium's longer life and usable capacity offset its price. Whichever you choose, never mix different battery types, ages or capacities in one bank.
Sizing the battery bank
Use this chain: daily watt-hours, times the days of backup you want, divided by the usable fraction of the battery.
Illustration with round numbers: a home using 1,500 Wh a day that wants two days without sun needs 3,000 Wh stored. With lithium that is safe to use to roughly 80 or 90 percent, you want somewhere around 3,500 Wh of rated capacity. With lead-acid used to about 50 percent, you want around 6,000 Wh. That is a big difference in weight, which matters on a trailer (see weight and balance).
Add a margin for cold weather, because most batteries deliver less capacity when it is cold, and for aging, because capacity declines over the years.
Series and parallel banks
To reach a higher system voltage, you wire batteries in series, which adds voltage. To increase capacity, you wire them in parallel, which adds amp-hours. Many banks use both. Keep cable lengths between batteries equal and use identical batteries so they share the load evenly, and follow the manufacturer's guidance on the maximum number you may connect in series or parallel.
Choosing an inverter
The inverter must handle two numbers: the continuous watts of everything running together, and the short surge some devices draw when starting, such as a refrigerator compressor, a pump or power tools. Add up your simultaneous loads, then choose an inverter whose continuous rating covers it with room to spare and whose surge rating covers the largest motor.
- Pure sine wave inverters produce clean household-style power and are the safe default for electronics, motors and medical devices.
- Modified sine wave models are cheaper but can cause buzzing, extra heat or malfunction in some appliances.
Every inverter uses some power just by being on. A very large inverter left running all night can quietly drain a modest battery bank, so match the size to your needs or choose a model with a low-power standby mode. Combination inverter-chargers also let a generator or shore power recharge the batteries, which is handy for winter backup.
Cables, fuses and placement
The currents involved are large. A 2,000-watt inverter on a 12-volt system can draw well over 150 amps, which is why higher system voltages are popular for bigger setups.
- Use cable thick enough for the current and length, following the inverter manufacturer's chart.
- Install a correctly rated, fast-acting fuse or breaker close to the battery on the positive cable.
- Keep cables short and tight to reduce voltage loss, and secure them so they cannot chafe or vibrate loose.
- Make sure terminals are clean, tight and covered.
- Mount batteries securely, especially in a home that will move, and away from heat sources.
Ventilation depends on the chemistry. Flooded lead-acid batteries release hydrogen when charging and need ventilation. Sealed batteries and lithium units have different requirements, so follow the instructions for the model you buy.
Monitoring and care
A battery monitor with a shunt measures the current in and out and calculates the true state of charge. Voltage alone can mislead, especially with lithium, whose voltage barely changes across most of its range. Check the system each week at first, then each month: tight connections, no corrosion, no swelling or heat from a battery, and sensible charging behavior.
Keep records of your actual daily use and charging. After a month or two you will know exactly how your setup performs and whether the bank is the right size.
Common mistakes
- Buying capacity by the label without allowing for the usable fraction.
- Putting lithium batteries where they may be charged below freezing.
- Choosing an oversize inverter that wastes power at idle.
- Leaving out the fuse near the battery.
- Mixing old and new batteries.
- Using too-thin cables between the batteries and the inverter.
Key takeaways
- Watt-hours equals amp-hours times volts; compare that with your daily use.
- Usable capacity differs by chemistry, so size accordingly.
- Do not charge lithium batteries below freezing unless they are built for it.
- A fuse close to the battery is essential.