Off-grid tiny house systems, with the math

The short answer

Off-grid is a budget problem, measured in kilowatt-hours. Figure out how many you use per day, design for the worst month, and size solar and batteries to match. Everything else is details.

In Tampa Bay, one appliance decides the whole design: air conditioning. Without it, a tiny house can run on a small solar array. With it, the system roughly quadruples.

Step 1: Count your kilowatt-hours

Use spec sheets, not guesses. Here's a real example with published numbers:

The usage hours are our example assumptions; swap in your own. The mini-split figure is rough: 12,000 Btu divided by its 23.5 SEER2 rating is about 510 watts on a seasonal average, times 12 hours. Real draw depends on heat, insulation and shading.

Add your own lights, fans, laptop, phone and water pump from their labels (watts times hours).

For scale, the EIA's 2020 survey found the average Florida household used 50.6 million Btu of site energy a year, about 40 kWh a day in electric terms. Homes under 1,000 square feet nationally averaged 42.1 million Btu, about 34 kWh a day. A tiny house that runs on 2 to 8 kWh is already doing most of the work by being small.

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Step 2: Size the solar

How much sun Tampa gets

Profile Solar, using NASA POWER data, estimates Tampa solar production at:

Design to the worst season, winter. Real systems lose output to wiring, inverter, heat and dirt; NREL's PVWatts model uses 14% total system losses as its default. Applying that: 4.32 x 0.86 = about 3.7 kWh per day per kW in winter.

The math

• No AC, 2.0 kWh/day: 2.0 / 3.7 = about 0.54 kW of panels minimum
• With AC, 8.1 kWh/day in summer: 8.1 / (6.36 x 0.86) = about 1.5 kW minimum

Then add margin for cloudy stretches, panel aging and loads you forgot. Roof space is the real limit on a tiny house; a 24 by 8.5 foot roof doesn't hold many panels, and ground mounts or a carport roof are common workarounds.

Florida note: if you install solar under the owner-builder solar provision, the statute requires a licensed electrical contractor to wire the panels (Fla. Stat. 489.103(7)).

Step 3: Size the batteries

Batteries carry you through nights and cloudy days.

Formula: daily kWh x days of autonomy = storage needed.

Example battery: Battle Born 100Ah 12V LiFePO4, listed at $925 (SolarTown, October 2026), 31 lb, rated for 3,000 to 5,000 cycles, with a 10-year defect warranty per the Explorist listing. At a 12.8 V nominal lithium voltage, 100 Ah is about 1.28 kWh, and SolarTown describes it as 100Ah of usable power.

Our math from the listed specs. Note the weight: 13 batteries is 400 pounds on your trailer. See the tiny house trailer guide.

The battery bank also has to survive hurricanes: plan where it goes during a tow or evacuation (tiny homes and hurricanes).

Step 4: Use propane for heat jobs

Heat is where batteries struggle. One gallon of propane holds 91,452 Btu, which equals about 26.8 kWh of heat (EIA conversion factors). The EIA's U.S. average residential propane price was $2.638 per gallon for the week of October 12, 2026.

Example: a 12,000 Btu gas burner at full flame for an hour burns about 0.13 gallons, roughly 35 cents at that price. Covering the same hour with electric resistance from batteries would take 3.5 kWh.

That's why many off-grid tiny houses cook and heat water with propane even when everything else is solar. Compare cooktops in small kitchen appliances. Propane appliances need proper venting and a CO alarm, and indoor combustion in a small space is a safety issue to design for, not an afterthought.

Step 5: Water

How much you use

EPA WaterSense says each American uses an average of 82 gallons a day at home. Standard showerheads use 2.5 gallons per minute; WaterSense-labeled models use no more than 2.0. Off-grid tiny house living usually means cutting that use drastically.

Rainwater in Tampa Bay

UF/IFAS says one inch of rain on 1,000 square feet of roof yields about 600 gallons. Tampa's normal annual rainfall is 49.48 inches (NWS, 1991 to 2020), but it's lopsided: August averages 9.03 inches and November 1.40.

For a 240-square-foot tiny house roof (our example):

Our math, before first-flush diversion and overflow losses. Compare that to 82 gallons a day: rainwater can supplement, but on a tiny roof it won't fully supply a typical household. Storage matters too: UF/IFAS notes a 6 by 6 by 6 foot cistern holds 1,600 gallons, and water weighs 8.3 pounds per gallon.

Is it legal in Florida?

Yes for non-potable use. UF/IFAS says stored rainwater is not public water, so you don't have to follow watering restrictions. It also says:

• Don't drink stored rainwater
• Don't connect rain barrels or cisterns to in-home plumbing
• Check with your building department about backflow protection before tying pumps or irrigation to anything connected to potable water

Drinking water from rain needs filtration, disinfection and testing. Talk to your county health department first.

Step 6: Toilets and graywater

Florida's rule

Florida Administrative Code 62-6.009 allows waterless, incinerating and composting toilets that meet NSF/ANSI 41 or NSF Protocol P157. But:

• Graywater and other wastewater must still be collected and disposed of under the onsite sewage rules
• Liquids from the toilet must be plumbed into the onsite system
• Solids are mixed with lime, containerized and disposed of as solid waste
• For residences using these toilets for all toilet waste, the drainfield can be 25% smaller, not eliminated

So a composting toilet shrinks your septic system; it doesn't replace it. Full bathroom planning is in tiny house bathroom.

Is it legal to live off-grid in Florida?

The equipment is legal. Whether you can live off-grid on a particular lot is a zoning and sewage question, answered by your city or county.

• Solar: Florida law bars local ordinances and deed restrictions, HOA covenants included, from prohibiting solar collectors. An HOA can choose where on the roof they go, but only facing south or within 45 degrees east or west of due south, and only if that spot doesn't impair the system's performance (Fla. Stat. 163.04, checked October 2026).
• Sewage: a composting toilet shrinks the drainfield but doesn't remove the need for one, because graywater still has to go to an approved system (see Step 6). If public sewer later becomes available, the owner of a working septic system must connect within 365 days of written notice from the utility (Fla. Stat. 381.00655, checked October 2026).
• Zoning: where a tiny house can be lived in is set locally. Start with where to park a tiny house and are tiny homes legal in Florida; for a backyard unit in Tampa or St. Petersburg, see Tampa Bay ADU rules.

Off-grid checklist

• Daily kWh from spec sheets, with and without AC
• Solar sized to winter, with losses
• Battery bank sized to days of autonomy, and its weight added to the trailer budget
• Licensed electrician for solar wiring (Florida)
• Propane for cooking and water heating, with venting and CO alarm
• Water source, storage and a non-potable/potable plan
• Toilet that meets NSF/ANSI 41 or P157, plus a legal graywater path
• Zoning confirms you can live there (where to park a tiny house)

Price the system alongside everything else in tiny house cost.