Off-Grid Solar System Sizing for a Tiny Home: A Step-by-Step Calculator Guide

Sizing an off-grid solar system is the single most important decision in the entire build — and the most commonly botched. Undersize it and you're rationing power by the first cloudy week. Oversize it and you've spent thousands of dollars on panels and battery capacity your daily routine will never use.

The good news is that proper sizing isn't guesswork. It comes down to three formulas, run in order: your daily load, your battery bank, and your solar array. Get the first number right, and the other two follow directly from it.

If you're still deciding whether off-grid living is the right fit at all, start with our Living Off-Grid for Beginners guide. This post picks up from there and turns the general planning into an actual system spec.

Step 1: Calculate Your Daily Load

List every electrical device you plan to run. For each one, note its wattage and how many hours a day you'll realistically use it, then multiply watts by hours to get watt-hours (Wh). Add every device together to get your total daily load.

A few details matter a lot here:

• Use running wattage, not peak wattage, for cycling appliances. Fridges, pumps, and other compressor-based devices don't draw their peak rating continuously — check the nameplate or use a plug-in power meter to find real running watts.
• Account for inverter efficiency. Some of the power drawn from your battery is lost in the DC-to-AC conversion, so the battery needs to supply slightly more than your appliances actually use.
• Be honest about resistive heating loads. If you plan to heat water or space-heat using electricity from the battery bank, your system size can balloon dramatically. Propane, wood, or solar thermal keep the electrical side of the budget realistic.

Our Off-Grid Solar Inverter & Battery Sizing Calculator automates this step — enter your appliances and hours of use, and it totals your daily watt-hours for you.

Step 2: Size Your Battery Bank

The formula: (daily Wh × days of autonomy) ÷ (system voltage × usable depth of discharge × inverter efficiency) = battery bank size in amp-hours.

Worked example: a tiny home using 4,000Wh per day, planning for 2 days of autonomy, on a 48V LiFePO4 system with roughly 95% usable capacity and 95% inverter efficiency:

(4,000 × 2) ÷ (48 × 0.95 × 0.95) ≈ 184 amp-hours at 48V

That translates to roughly a single stackable LiFePO4 module in the 180–200Ah range — with the option to add a second module later if your usage grows, rather than needing to guess the final size up front.

Two to three days of autonomy covers most weather in sunnier climates. Cloudier regions, or setups without a generator for backup, often plan for four to five days instead.

Step 3: Size Your Solar Array

The formula: daily Wh ÷ (peak sun hours × system efficiency) = array size needed, in watts.

Peak sun hours vary significantly by location and season — and sizing for a sunny summer day is a common mistake. Your system has to work in December, not just June. You can look up average peak sun hours for your specific location using the PVWatts Calculator, a free tool that models solar production by ZIP code and season.

Continuing the example above: a 4,000Wh daily load with 4 peak sun hours and 85% system efficiency:

4,000 ÷ (4 × 0.85) ≈ 1,176W of solar panel capacity

In practice, that rounds up to a 1,200W array — comfortably covered by four 300W panels from RichSolar or Renogy, or a pre-matched kit from our Off-Grid Solar Kits collection.

Common Sizing Mistakes

• Choosing a modified sine wave inverter to save money. Modified sine wave can damage compressor motors, some electronics, and anything with a transformer. Pure sine wave is worth the extra cost for a permanent system.
• Undersizing the charge controller. A cheap, undersized controller throttles your panel output no matter how much solar you've installed. Buy one size above the calculated minimum.
• Skipping generator integration entirely. Even in sunny climates, a handful of low-production days per year is normal. A small backup generator prevents a multi-day outage from becoming a real problem, and most modern inverter-chargers make the integration simple.
• Sizing for a sunny day instead of a typical winter day. This is the single most common cause of an off-grid system that "should work" but doesn't, especially in northern or cloudy climates.

Once you've run these three calculations, you have an actual system spec — not a guess. If you want a sense of what these numbers look like in practice for different lifestyles, our companion piece on typical tiny home energy budgets walks through real-world examples ranging from minimalist to full-time, high-demand setups.