Most tiny houses sip electricity rather than gulp it - LED lights, a 12-volt fridge, phone and laptop charging, a water pump, and a fan add up to far less than a conventional home uses. That is why a modest solar array of 400 to 1,000 watts can run an entire off-grid tiny house. The trick is sizing each piece of the system to your actual loads instead of guessing, because an undersized battery bank is the single most common reason off-grid setups disappoint.

This guide walks through sizing a tiny-house solar system step by step: auditing your loads, sizing batteries and panels with simple formulas, and choosing the charge controller and inverter that tie it all together. No electrical engineering degree required - just a calculator and an honest list of what you plug in.

Everything starts with knowing what you use. List every electrical device in the house with its wattage and the hours it runs per day, then multiply to get watt-hours. A typical audit: LED lighting at 50 watts for 5 hours is 250 watt-hours; an efficient fridge cycling 8 hours at 60 watts is about 480; a laptop at 50 watts for 6 hours is 300; phones, a water pump, a fan, and a router add a few hundred more.

Most one-to-two-person tiny houses land between 1,500 and 3,000 watt-hours per day. Add a 20 to 30 percent buffer for inverter losses and wiring resistance. That buffered number is your daily target - every other calculation flows from it.

Batteries are the heart of an off-grid system and the component beginners most often undersize. Decide how many days of autonomy you want - how long the system should run with no sun. Two to three days is the standard choice for full-time off-grid living.

The formula is simple: battery capacity in watt-hours equals daily use times days of autonomy, divided by the usable depth of discharge. Lithium iron phosphate (LiFePO4) batteries can safely use about 80 to 90 percent of their capacity, while old lead-acid batteries should only be discharged to 50 percent. For a 2,000 watt-hour daily load with two days of autonomy on lithium at 85 percent usable, you need roughly 4,700 watt-hours of storage - about 400 amp-hours at 12 volts.

This is why lithium has taken over tiny-house builds despite the higher upfront price. A LiFePO4 bank lasts ten or more years with zero maintenance, weighs far less, and delivers nearly its full rated capacity - while lead-acid needs double the rated size for the same usable energy and dies in two to three years of daily cycling.

The formula: required panel watts equals daily watt-hours divided by the product of peak sun hours and system efficiency (use 0.8 to account for losses). For our 2,000 watt-hour example at 4 peak sun hours: 2,000 divided by 3.2 gives 625 watts of panels. Round up to what is available - three 250-watt panels or two 400-watt panels both land in the right zone.

The charge controller sits between the panels and the batteries, regulating voltage so the batteries charge safely. For any system above a couple hundred watts, choose MPPT over PWM. MPPT controllers convert excess panel voltage into extra charging current and run about 30 percent more efficiently, especially in partial shade and cold weather.

Size the controller by current: divide your array wattage by your battery voltage. A 625-watt array on a 12-volt bank needs about 52 amps, so a 60-amp MPPT controller fits with headroom. If you are building at 24 volts instead - worth considering for systems over 1,000 watts, since it halves current and lets you use thinner wiring - the same array needs only about 26 amps.

The inverter converts battery DC power into the AC power your household outlets and appliances expect. Size its continuous rating at least 20 percent above your maximum simultaneous load, and check that its surge rating covers motor startups - fridges and well pumps can draw two to three times their running watts for a few seconds.

Always choose a pure sine wave inverter. Modified sine wave units are cheaper but can damage sensitive electronics, run motors hot, and make some chargers buzz or fail. For a typical tiny house, a 1,500 to 2,000 watt pure sine wave inverter covers lights, fridge, laptop, and small kitchen appliances comfortably.

Wire the system in this order: mount panels, connect the charge controller to the batteries first, then connect the panels to the controller, and finally the inverter to the batteries. Fuse every positive wire near the battery, ground the system, and use wire gauges rated for your current - undersized wiring is a fire risk.

If any of this feels beyond your comfort level, there is no shame in hiring an electrician for the final hookup after doing the sizing math yourself. You will still save substantially over a turnkey quote, and you will understand your system well enough to maintain it for the decade it serves you.