Most solar system sizing mistakes come from estimating rather than calculating. An undersized array leaves you without power on cloudy winter days. An oversized one costs thousands of dollars in equipment you'll never fully use. Neither outcome is acceptable when power is the only thing standing between you and a working kitchen and a charged laptop.
This guide works through the actual calculations — daily load, battery sizing, solar array sizing, and inverter selection — so you arrive at the right numbers before you spend anything.
Step 1: Calculate Your Daily Load in Watt-Hours
You cannot size a battery bank until you know how much energy you consume in 24 hours. The formula is straightforward: Watts × Hours = Watt-hours. List every appliance you plan to run, find the wattage on the device label (if only amps are listed, multiply amps by 120V to get watts), estimate your daily use time, and multiply.
| 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 Daily Load | 1,740 Wh | ||
This is the number everything else is built around. Run it against your actual appliance list — the profile above is illustrative. If you add a mini split or an induction cooktop, the number changes significantly.
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 overheating. Our 2026 Master Plan Book includes full electrical schematics for 30A and 50A systems.
Get The Master Plan Book ($19)Step 2: Size the Battery Bank
LiFePO4 (Lithium Iron Phosphate) is the right chemistry for a full-time tiny home in 2026. Unlike lead-acid batteries, which need to stay above 50% state of charge to avoid damage, LiFePO4 can be safely discharged to 90–100% of rated capacity. That means the capacity you pay for is the capacity you can actually use.
The standard rule is to size for 2 to 3 days of autonomy — the number of overcast days the system can sustain your full load without any solar input. Using the 1,740 Wh daily load example with a 3-day buffer:
Required storage: 1,740 Wh × 3 days = 5,220 Wh
Batteries are sold by Amp-hours (Ah), not Watt-hours. To convert: divide Wh by the system voltage.
12V system: 5,220 ÷ 12 = 435 Ah
24V system (recommended for tiny homes): 5,220 ÷ 24 = 217.5 Ah
24V systems are recommended because they operate at lower current for the same power output, which means thinner, cheaper cable and less resistive heat loss in the wiring.
Step 3: Size the Solar Array
A 400W panel doesn't produce 400 watts all day. It produces its rated output only during peak sun hours — a measure of daily solar energy that varies significantly by location. Arizona averages around 6 peak sun hours per day. Washington State averages closer to 2.5. Look up your location's average peak sun hours before sizing the array.
Using the 1,740 Wh daily load example at 4 peak sun hours:
Required panel output: 1,740 Wh ÷ 4 hours = 435 W
That's the theoretical minimum. In practice, system inefficiencies — inverter losses, wire resistance, heat — consume approximately 20% of available power. Multiply the required wattage by 1.25 to account for this:
Real-world requirement: 435 W × 1.25 = approximately 545 W of panel capacity
In this example, two 300W panels would cover the load with a reasonable safety margin.
Step 4: Size the Inverter Correctly
The inverter converts DC battery power to AC power for standard appliances. The critical number is your simultaneous peak load — the maximum wattage you might draw at any single moment. A 1,000W microwave and an 800W coffee maker running at the same time requires at least 1,800W of inverter capacity. Size below this and the inverter trips or fails under load.
Always specify a pure sine wave inverter. Modified sine wave units are cheaper but produce a power waveform that degrades sensitive electronics over time — laptop power supplies, high-efficiency refrigerators, and variable-speed motors are all affected. In a small home where these appliances are in continuous use, the damage accumulates faster than in an occasional-use setup. The cost difference doesn't justify the risk.
Step 5: Prioritise Your Loads
If the battery bank the calculation calls for is beyond budget, the answer is either increase the budget or reduce the load. The practical approach is to tier your loads by criticality:
- Tier 1 — Critical: Lighting, water pump, ventilation, communication. These run regardless.
- Tier 2 — Essential: Refrigeration, cooking ignition, minimal workstation. These run when power is available.
- Tier 3 — High-draw: Air conditioning, electric space heating, microwave, hair dryer. These consume disproportionate power relative to their use time.
Tier 3 items that generate heat or cold are the most efficient candidates to shift to propane or diesel alternatives. Electric resistance heating and cooling are among the highest-draw loads in any small home. Moving them off the battery bank dramatically reduces the required system size and cost.
Final Checks Before You Buy
- Weight: Batteries and panels add significant mass to a trailer. Check total system weight against the trailer's GVWR before ordering.
- Cold weather: LiFePO4 batteries cannot accept a charge below 0°C / 32°F. In cold climates, the battery compartment needs to be within the thermal envelope or the batteries need integrated heating elements.
- Overcurrent protection: Every circuit needs a fuse or breaker sized to the wire capacity. This is not optional and has no exceptions.
Solar system sizing is arithmetic, not guesswork. Run the numbers before you buy anything and the system will work exactly as expected.
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