How to size a solar generator

Specifications verified 2026-07-29 · How we work

Almost everyone buys on capacity and then discovers the inverter rating was the number that mattered. Here is how to work it out properly, in four steps, with the arithmetic shown.

How we get our numbers: Every specification on this page comes from the manufacturer's published documentation or an authorized retailer's spec table — each one linked and dated at the foot of the page. Runtimes are calculated from those specs using the formula shown, so you can check them against your own equipment. Full method.

Step 1 — list what you genuinely need, not what you own

The instinct is to total up the house. Resist it. During an outage almost nobody needs their dishwasher; they need food kept cold, phones charged, the internet up, a few lights, and whatever medical equipment is in use. That list is usually far smaller than people expect — and it is the difference between a 1 kWh purchase and a 5 kWh one.

Write down each item with its wattage. The nameplate is on the appliance itself, or in the manual. If it gives amps instead, multiply by 120.

Step 2 — apply duty cycle

This is the step that is nearly always skipped, and it changes the answer by a factor of three for the biggest load in most households.

Anything thermostat-controlled — refrigerator, freezer, air conditioner, electric blanket — does not draw its rated wattage continuously. It runs until it hits temperature, then stops. A fridge labelled 150 W averages closer to 50 W across a day.

average draw = rated watts × duty cycle fridge 150 W × 0.33 = 50 W router 20 W × 1.00 = 20 W two lamps 20 W × 1.00 = 20 W phones ~ 5 W ------ 95 W average

That is a realistic essentials load for a typical household: roughly 95 W averaged, or about 2.3 kWh across a full day.

Step 3 — decide how long you need to last

Outage length	Energy needed at ~95 W	What that means
Overnight (12 h)	~1.1 kWh	A 1,000–1,200 Wh unit covers it
One full day	~2.3 kWh	1 kWh unit plus solar, or a 2 kWh unit
Three days	~6.8 kWh	Not a battery-only problem — you need solar input or fuel

The important realisation: beyond about a day, panels matter more than capacity. A 1 kWh battery with 400 W of solar attached outlasts a 3 kWh battery with none, because the first one refills every morning and the second never does.

Step 4 — check the inverter, then the surge

Capacity tells you how long. The inverter rating tells you what will run at all, and it is a hard ceiling — plug in something above it and nothing happens regardless of how full the battery is.

Continuous rating must exceed the total wattage of everything running simultaneously. Note "simultaneously": your fridge and your microwave rarely overlap, but plan as if they might.
Surge rating must cover motor startup. Anything with a compressor or motor — fridge, freezer, sump pump, power tools — draws several times its running wattage for a second or two. Allow roughly 6× the running figure for a fridge.

This is why a 300 W unit cannot run a 150 W refrigerator: the arithmetic on capacity works, and the startup surge kills it anyway.

Putting it together

For most households the answer is a 1,000–1,200 Wh unit with an inverter of at least 1,500 W, plus 200–400 W of panels if outages where you live routinely last beyond a day. That covers the fridge, the router, the lights and the phones overnight with margin, and the panels convert it from a one-night solution into an indefinite one.

All three units we cover sit in exactly that class — see the comparison, or the runtime reference for 20 loads calculated against each one.

When to size up instead

Sump pump in a wet basement. High surge, and the consequence of failure is a flood rather than an inconvenience.
Well water. A well pump typically exceeds every unit on this site outright.
Medical equipment beyond a CPAP. Oxygen concentrators in particular draw far more than people expect, often 300–600 W continuously.
Heat. If your plan involves keeping warm electrically, a battery is the wrong tool — see solar versus gas.

Questions people actually ask

What size solar generator do I need for my house?

For essentials only — refrigerator, router, lights, phones — a 1,000–1,200 Wh unit with a 1,500 W or larger inverter covers a typical household overnight. That works out to about 95 W averaged across the day once you account for the fridge compressor cycling. Whole-house backup, including heating or air conditioning, is a different category of equipment entirely.

Is it better to buy more capacity or add solar panels?

Beyond roughly one day of outage, panels win. A 1 kWh battery with 400 W of solar attached will outlast a 3 kWh battery with none, because it refills every morning while the larger one only ever depletes. Below a day, capacity is what matters.

Why does my 300W power station not run my 150W fridge?

Starting surge. A fridge compressor draws several times its running wattage for the first second or two — a 150 W fridge can spike to 900–1,200 W. If the inverter cannot deliver that instantaneous peak it faults out and shuts down, no matter how much battery capacity sits behind it.