What Size Solar Generator Do I Need? Free Sizing Tool for Tiny Houses

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Most solar generator advice starts with the wrong question. People ask “which one is best?” when the question that actually matters is “which one can run my stuff, in my climate, without shutting off?”

A unit that looks enormous on paper can still fail you if your well pump draws a big surge on startup, or if you park somewhere that hits 15°F in January. And a unit that looks too small can be plenty if your loads are modest and you charge daily.

The tool below does that matching for you. Enter your real appliances — the ones on your actual list, not a preset — and it screens five portable solar generators against five constraints at once: daily energy, continuous power, starting surge, operating temperature, and each unit’s verified published limits. It tells you which units clear your requirements and, just as usefully, which ones don’t and why.

Solar generator sizing tool

Add your appliances with their watts and daily run hours, tell it what needs to run and start at the same time, and set your temperature range. Nothing is guessed on your behalf — there are no wattage presets or hidden load assumptions.

What size solar generator do I need? The short answer

For most tiny houses, the honest range looks like this:

• 300–500 Wh: Phones, laptops, lights, a fan. Weekend and backup use.
• 1,000–1,500 Wh: Adds a 12V fridge, water pump, and small kitchen appliances used briefly. This is where a lot of part-time tiny house living lands.
• 2,000–3,000 Wh: Full-time off-grid with a compressor fridge running around the clock, plus an induction burner or microwave used in short bursts.
• 3,500 Wh and up: Anything with a heating element running for real stretches — space heater, electric kettle on repeat, hair dryer — or a well pump with a hard starting surge.

Treat those as a sanity check, not an answer. Two tiny houses with identical square footage can land two categories apart based on one appliance. Run your own numbers through the tool above.

The four numbers that actually decide it

1. Daily watt-hours (how much you use)

Watts × hours per day, added up across everything. A 60W fridge that cycles about 8 hours of actual compressor time uses roughly 480 Wh a day. This number tells you what capacity you need.

Add a buffer. Cloudy days happen, batteries lose capacity in cold weather, and nobody’s estimate of their own usage is generous enough. The tool defaults to 1.2×, which is a reasonable floor.

2. Continuous watts (how much runs at once)

Capacity is the size of the tank. Continuous output is the size of the hose. A unit can hold 3,000 Wh and still trip if you ask it for more watts at one instant than its inverter can deliver.

Add up everything that could plausibly be on simultaneously — the worst realistic minute of your day, not the average one. Coffee maker plus fridge compressor plus water pump is a very ordinary Tuesday morning.

3. Starting watts (the number people forget)

Anything with a motor — fridge compressor, water pump, air conditioner, power tools — pulls a brief spike when it kicks on, often two to three times its running draw. A pump that runs at 300W might demand 900W for a half second.

This is the single most common reason a correctly-sized-on-paper solar generator disappoints people. The surge trips the inverter, the unit shuts off, and it looks like a defect when it’s really a sizing miss.

4. Temperature (the number almost nobody checks)

Lithium batteries have published operating and charging temperature ranges, and the charging range is usually narrower than the discharging range. Many units will not accept a charge below freezing at all without built-in heating.

If you’re in a cold climate, this constraint can eliminate an otherwise perfect unit. The tool checks your stated temperature range against each product’s published limits, which is why it sometimes rules out a model that looks fine on capacity alone.

Three sizing mistakes worth avoiding

1. Sizing to capacity alone. Watt-hours is one of four constraints. It’s the one on the front of the box, which is why it gets all the attention.
2. Forgetting recharge. A big battery you can’t refill is a big paperweight. If you’re charging by solar, your panel wattage and available sun hours determine whether you actually get back to full each day.
3. Estimating appliance draw from memory. Check the label or the manual. Real numbers differ from remembered numbers by a lot, and always in the expensive direction.

Once you know your size

The selector screens units against your constraints — it’s a planning filter, not a purchase recommendation. When you’ve got a target range and a shortlist, these go deeper on the specific models:

• Best Solar Generators for Tiny Houses: Top 5 Picks for 2026 — hands-on comparison of the units in this tool
• Best Solar Kits for Tiny Houses — for recharging what you just sized
• How to Size a Tiny House Battery Bank — if you’d rather build a system than buy an all-in-one
• Tiny House Solar Calculator — panel and array sizing

A word on what this tool is not

This is a planning screen built on published manufacturer specifications. It is not electrical design, it is not a code compliance check, and it is not a guarantee that a given unit will work in your setup. Anything involving permanent wiring, a transfer switch, or your 240V circuits belongs with a licensed electrician.

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