The Physics of Power: Why Your "Guestimate" Will Fail
In tiny house construction, there is no room for "close enough." This is especially true for electrical systems. If you undersize your solar array, you’ll be sitting in the dark by 8 PM in mid-winter. If you oversize it without a plan, you are throwing thousands of dollars at equipment that will degrade before you ever utilize its full capacity. Precision is the only path to energy independence.
As an engineer, I look at a tiny house as a closed-loop system. To achieve 100% off-grid reliability in 2026, we must move past "kit-based" thinking and move into Load Calculation. You need to know your peak demand, your base load, and your "autonomy days" (the number of days you can survive without sun). This guide provides the raw formulas and data you need to build a system that actually works.
1. Step One: The Daily Load Requirement (Wh)
You cannot buy a battery until you know your numbers. We calculate energy in Watt-hours (Wh). The formula is simple: Watts (Power) × Hours (Time) = Watt-hours (Energy).
List every appliance you intend to run. Look at the UL sticker on the back of the device to find the wattage. If only Amps are listed, multiply Amps by your voltage (usually 120V in the US) to get Watts.
| Appliance | Watts | Daily Use (Hrs) | Total Wh |
|---|---|---|---|
| LED Lighting (Total) | 60W | 5 | 300 Wh |
| Laptop / Workstation | 85W | 8 | 680 Wh |
| 12V High-Efficiency Fridge | 50W | 8 (Duty Cycle) | 400 Wh |
| MaxxFan / Ventilation | 30W | 12 | 360 Wh |
| TOTAL ESTIMATED LOAD | 1,740 Wh | ||
Don't Guess on Your Wiring
Calculating the load is only half the battle. You need to know the gauge of wire required to handle that current without melting. Our 2026 Master Plan Book includes full electrical schematics for 30A and 50A systems.
Get The Master Plan Book ($19)2. Sizing the Battery Bank: LiFePO4 Standards
In 2026, Lithium Iron Phosphate (LiFePO4) is the only logical choice for tiny homes. Unlike lead-acid (AGM), which can only be discharged to 50%, LiFePO4 can be safely discharged to 90% or 100%.
The Autonomy Rule: You should always size your battery bank to cover 2 to 3 days of use without a charge. If your daily load is 1,740 Wh, you need a minimum of 5,220 Wh of storage for a 3-day buffer.
To convert Watt-hours to Amp-hours (which is how batteries are sold), use the formula: $$Ah = \frac{Wh}{V}$$
For a 12V system: $$5,220 / 12 = 435Ah$$
For a 24V system (Recommended for Tiny Houses): $$5,220 / 24 = 217.5Ah$$
3. Calculating Solar Input: The "Sun Hour" Variable
A 400W solar panel does not produce 400W all day. It produces its rated wattage only during "Peak Sun Hours." Depending on your location (e.g., Arizona vs. Washington State), you may get 6 hours of peak sun or 2.5 hours.
To recharge your daily use of 1,740 Wh in a location with 4 peak sun hours, you need:
$$1,740 Wh / 4 Hours = 435 Watts$$
Engineer’s Warning: System inefficiency (Inverter loss, wire resistance, heat) typically consumes 20% of your power. Multiply your required wattage by 1.25. To be safe, you would need roughly 550 Watts of solar panels to maintain that load.
4. Safety Critical: Inverters and Peak Surges
Your Inverter converts DC battery power to AC wall power. You must size your inverter based on your Simultaneous Peak Load. If you run your 1000W microwave and your 800W coffee maker at the same time, a 1500W inverter will trip its breaker or fail.
Always buy a "Pure Sine Wave" inverter. "Modified Sine Wave" inverters are cheaper but will destroy sensitive electronics like laptop power bricks and high-efficiency refrigerators over time. In a tiny house, your inverter is your heart. Do not buy a budget model off an unverified marketplace.
5. The Off-Grid Hierarchy of Needs
If your calculations show you can't afford the battery bank needed for your current lifestyle, you have two choices: Increase your budget or decrease your load. In the engineering world, we prioritize loads like this:
- Tier 1 (Critical): Lighting, Water Pump, Ventilation, Communication.
- Tier 2 (Essential): Refrigeration, Cooking (Ignition), Minimal Workstation.
- Tier 3 (Luxury): AC, Space Heating (Electric), Microwave, Hair Dryer.
If you are serious about off-grid living, Tier 3 items should be shifted to Propane or Diesel. Using electricity to generate heat is the fastest way to kill a battery bank.
Final Checklist: Before You Buy
- Check your Weights: Batteries and solar panels add significant weight. Ensure your trailer GVWR can handle the bank.
- Cold Weather: LiFePO4 batteries cannot charge below 32°F (0°C). Ensure your batteries are in a conditioned space or have built-in heating blankets.
- Over-Current Protection: Every circuit must have a fuse or breaker sized to the wire’s capacity. No exceptions.
Building a solar system isn't magic; it's math. Run the numbers twice, buy the equipment once.
Stay safe on the job site.
— Martin
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