Propane vs Electric in Tiny Houses: Sizing Tanks and Calculating Energy Demand

Propane or electric? Answer wrong and you either haul a tank you cannot refill in February or watch your battery bank die trying to run a resistance heater. Every tiny house has to pick its fuel, and the choice ripples into tank size, electrical service, solar array, and monthly cost. This guide runs the full comparison in plain numbers — BTU per gallon, BTU per kilowatt-hour, and your actual appliances.

1. Propane vs Electric in Tiny Houses at a Glance

Propane vs electric in tiny houses is not a taste question; it is an energy-accounting question. Propane stores a large amount of heat in a portable tank — 91,500 BTU in every gallon — while electricity delivers 3,412 BTU per kilowatt-hour and has to be generated, bought from the grid, or pulled from a battery the moment it is used. The right answer depends on three things: how much heat your appliances actually demand each day, how each fuel’s efficiency changes what you pay per delivered BTU, and whether your house sits on the grid or off it. The table gathers the whole comparison in one place; the sections after it take the energy math, the demand audit, tank sizing, safety, service sizing, the hybrid standard, and the ten-year costs apart.

91,500 BTU in a gallon against 3,412 in a kilowatt-hour — the fuel decision on one card.

Read the table as a funnel. The energy content sets the exchange rate between the fuels; the demand audit sets how much of either fuel your house burns; and the supply question — grid, solar, or a delivery truck — decides which of the two columns your house can actually live on. Most tiny houses end up answering with both fuels at once, each assigned to the loads it serves cheapest. That hybrid is not a compromise; by the original guide’s own verdict, it is the engineering optimum for off-grid tiny living.

2. Energy Content: The Common Currency

Compare fuels in BTU and the marketing falls away. Propane carries 91,500 BTU per gallon. Electricity carries 3,412 BTU per kilowatt-hour. Divide one by the other and you get the exchange rate the whole decision runs on: one gallon of propane ≈ 26.8 kWh of electricity, before efficiency. A single gallon in a tank therefore holds the heat energy of a respectable battery bank — which is why propane keeps winning the storage argument in houses that have to carry their energy with them.

Efficiency then adjusts the rate. A propane furnace at 90% AFUE delivers about 82,000 BTU per gallon — the rest leaves with the exhaust. Resistance electric heat is 100% efficient at the point of use, though the power plant that generated the electricity was not. And a mini-split heat pump at COP 3.0 delivers roughly 10,200 BTU per kWh — three times resistance heat from the same kilowatt-hour — because it moves heat rather than making it. That multiplier is why the heat pump anchors so many of the heating line-ups compared in the guide to how to heat a tiny house, and why it changes the cost result in the next comparison.

At the original guide’s reference prices — $3 per gallon of propane against $0.15 per kWh of electricity — the delivered cost of heat lands like this: propane heat costs about $36.60 per million BTU, resistance electric about $44 per million BTU, and the heat pump about $14.70 per million BTU. The heat pump wins on operating cost wherever electricity is available. Propane’s case rests on the other half of the ledger: it stores densely, refills anywhere, and asks nothing of a battery bank.

3. The Demand Audit: What Burns What

Before sizing anything, audit the loads. Space heating is the big one: 5,000–15,000 BTU/hr in winter, which compounds to 120,000–360,000 BTU/day in cold climates. Water heating takes 30,000–40,000 BTU/day for two people — showers plus dishes. Cooking adds 10,000–20,000 BTU/day. Clothes drying, if the dryer is gas, takes 20,000 BTU per load. Add the propane loads together — heat, hot water and cooking — and a tiny house burns 0.5–2 gallons/day in heating season and just 0.2–0.5 gallons/day in summer.

The electric equivalents frame the same demand in the other currency. Heating via mini-split runs 2–5 kWh/day in mild weather and 10–20 kWh/day in the cold. Water heating takes 3–5 kWh/day; cooking 1–2 kWh/day. Two lessons fall out of the audit. First, heat dominates everything: space and water heating together dwarf lights, electronics and cooking combined, so the fuel decision is really a heating decision. Second, the seasonal swing is enormous — a house can need four times the fuel in January that it needs in July, which is exactly why tank sizing in the next section is done against winter demand, not the annual average. The wider context for these numbers — what a complete tiny house draws across all its systems — is tallied in the breakdown of how much energy a tiny house uses.

4. Tank Sizing: Capacity Matched to the Refill Interval

Propane tanks come in four working sizes for tiny houses. The 20-lb BBQ cylinder holds 4.6 gallons — the portable option, swappable at any gas station. The 30/40-lb RV cylinders hold 7–9.5 gallons and form the tiny house standard pair. The 100-lb cylinder holds 23.6 gallons and stretches the interval between refills. And the 120-gallon ASME tank is the permanent, homestead option.

The sizing math runs off winter demand. At 1.5 gallons/day for 30 days, a house needs 45 gallons a month. Against that figure: two 40-lb cylinders (19 gallons together) last about 12 days; a 100-lb cylinder (23.6 gallons) lasts about 16 days; and a 120-gallon tank — 96 usable gallons, because tanks are filled to 80% — lasts about 64 days. Hence the rule: size for the refill interval you tolerate. Monthly refills mean a 100-lb cylinder or bigger; seasonal refills mean the 120-gallon ASME.

Placement is part of sizing, because a tank you cannot place safely is not capacity at all. Tanks live outside, in a ventilated compartment or on an open rack. Propane sinks, so it must never sit in a sealed basement where a leak can pool, and placement stays 10+ feet from ignition sources per code. The pair of 40-lb cylinders earns its standard status here: small enough to mount on the trailer tongue or an exterior rack, swappable by hand, and big enough together to measure the refill interval in weeks rather than days.

5. Propane Safety: The Non-Negotiables

Propane’s safety list is short and absolute. First, the detector: a propane detector mounted LOW — because propane sinks — at $30, hardwired with battery backup. The original guide calls it the cheapest life insurance in the house, and at that price there is no argument against it. A detector mounted at ceiling height, where a smoke alarm would go, watches the wrong layer of air entirely.

The plumbing has its own rules. Lines are black iron or CSST — corrugated stainless steel, flexible and DIY-friendly with the proper fittings — and every installation is pressure-tested at install, with the pro’s habit adopted as your own: soap-bubble every joint. The regulator is two-stage, bringing tank pressure down to 11″ water column, the standard the appliances are built for, and it is protected from the weather. Shutoffs come in two layers: an accessible main shutoff plus a shutoff per appliance, each one labeled.

The nevers are just as specific. Never store spare cylinders inside — outside only. Never use unvented heaters as primary heat, for the moisture and carbon-monoxide risk. And never ignore the rotten-egg smell: leave, shut the gas off outside if it is safe to do so, and call for help. None of this is exotic — the same service discipline that governs every utility in the house, set out system by system in the tiny house utilities guide — but propane is the one system where skipping a step can fill a low corner with fuel.

6. Electric Service: Sizing for the Peak, Not the Average

An all-electric tiny house is sized by its worst moment, not its typical day. The running loads: mini-split heating at 1,000–2,000W, a water heater at 1,500–4,500W, a cooktop at 1,800–3,600W, a dryer at 2,500–5,000W — plus a baseline 500W that never sleeps. Run the plausible combination — heater, water heater recovery, one burner, baseline — and peak concurrent demand lands easily at 8,000–12,000W. That is why all-electric houses need 50-amp service, which provides 12,000W of capacity: the service is sized to the peak, with the mini-split’s draw kept in check by the discipline in the mini-split installation guide.

Off-grid, the same audit turns severe. An all-electric house drawing 15–25 kWh/day needs a 4–6 kW solar array and 30–45 kWh of batteries — $20,000–35,000 for the power system alone. The problem is not the daytime loads; it is that electric resistance and water-heating loads arrive in winter, when the array produces least, and every one of those kilowatt-hours has to be stored or generated on the spot. Sizing an array and battery bank to a stated daily demand is its own exercise, worked through in the guide to solar panel kits and battery bank sizing — and run honestly, it is the calculation that pushes most off-grid houses toward the hybrid.

7. The Hybrid: The Tiny House Standard

Most successful off-grid tiny houses run both fuels, each where it is strong. Propane takes the thermal loads: water heating — on-demand, the big win — cooking, which is instant and powerful with no electrical load, and backup space heat from a small propane furnace or wall heater for the coldest nights. Solar electric takes everything else: the mini-split as primary heating and cooling, the efficiency king; lights; electronics; the fridge; and the small appliances.

The typical setup is compact: two 40-lb propane cylinders, 2–3 kW of solar, 10–15 kWh of batteries, and a 9k BTU mini-split — with monthly propane running $30–80. The logic is storage physics. Heating water and cooking need enormous amounts of energy in short bursts, and propane stores that energy densely in a cheap steel cylinder; solar is perfect for the moderate, spread-out loads. Each fuel is asked to do only the work it does cheaply, and the battery bank shrinks to a size a tiny house can actually carry — compare the 10–15 kWh of the hybrid with the 30–45 kWh the all-electric off-grid house demanded in the previous section.

8. Heat Pumps, Resistance Heat and Radiant Floors

The electric side deserves one more distinction, because “electric heat” names three very different machines. Resistance heat converts electricity to heat one-for-one — 3,412 BTU per kWh, no more. The mini-split heat pump at COP 3.0 delivers about 10,200 BTU from the same kWh, which is why it, and not a resistance heater, is the hybrid’s primary heat source. And radiant floors change the delivery rather than the source: a hydronic loop or an electric mat warms the floor mass itself, the comparison drawn in the underfloor heating comparison, where the same running-cost math decides between hot water and cable.

The practical ordering that falls out of §2’s cost table — heat pump at ~$14.70 per million BTU, propane at ~$36.60, resistance at ~$44 — explains the hybrid’s shape. Where grid electricity is available, the heat pump is the cheapest heat in the guide and should carry the load. Where it is not, the question becomes which fuel can be stored: propane in a cylinder, or electricity in a battery at far greater cost per BTU held. Resistance heat is the fallback of last resort — simple and silent, but the most expensive way to turn either fuel supply into warmth.

9. Refills, Deliveries and the February Problem

Propane’s hidden cost is logistics, and the original guide’s hook names it: the tank you cannot refill in February. A cylinder system refills when you haul it — the 20-lb size swaps at any gas station, and a pair of 40-lb cylinders rides in a truck bed — so the chore is measured in trips, not appointments. The 120-gallon ASME tank inverts the problem: 96 usable gallons and roughly 64 days of winter autonomy, but refilling means a delivery truck reaching your site, which is a homestead answer, not a traveller’s.

Electricity has the mirror-image logistics. Grid power has no refill problem at all — it arrives by wire, in any weather, at $0.15/kWh in the guide’s reference math. Solar’s version of the February problem is production: the array that covers a house in June underproduces in the short, grey weeks when heat demand peaks, which loops back to why the off-grid all-electric house needed such an oversized array and battery bank. The hybrid is, among its other virtues, a logistics answer: the fuel that winter stresses hardest is the one that arrives in a tank and waits patiently to be used.

10. Costs: The Ten-Year Comparison

Total the three strategies over a year of operation. All-propane — heat, hot water and cooking — burns roughly 400–600 gallons a year, at $3 a gallon some $1,200–1,800/year. All-electric on the grid uses about 6,000–9,000 kWh a year at $0.15, which is $900–1,350/year with a heat pump — or $2,000–3,000 a year on resistance heat. The hybrid off-grid burns 150–250 gallons of propane ($450–750) plus the solar system amortized at roughly $800–1,200/year over 20 years, for $1,250–1,950/year with zero utility dependence. Those annual energy figures sit inside the wider ownership ledger drawn in the breakdown of the true cost of tiny houses, where the power system is one line among many.

The verdict follows the supply. Grid-tied heat-pump electric is the cheapest option where the grid exists — the lowest annual figure and no tanks, detectors or deliveries. Off-grid, the hybrid minimizes both cost and complexity: near the all-propane annual cost, without asking propane to do the jobs electricity does better, and without asking a battery bank to store a winter’s heat. Pure resistance electric off-grid is the most expensive option by far — the one configuration the math never rescues.

Final Considerations

Do the demand audit first: 5,000–15,000 BTU/hr of space heat and 30,000–40,000 BTU/day of water heating decide everything downstream. Size the propane for your refill tolerance — 19 gallons for roughly 12 winter days, 96 usable gallons for roughly 64 — and size the electric for the peak concurrent load, 8,000–12,000W against a 50-amp service. Default to the hybrid, propane for the thermal loads and solar electric for the rest, and never skip the $30 detector mounted low. Fuel choice is the invisible architecture of the tiny house — get the math right and comfort follows for decades.