What Size Solar Generator Do I Need? (Free Calculator)

You are standing in an aisle, or in a browser tab, staring at three numbers that all look like they mean the same thing. 1000Wh. 1800W. 2700W surge.

They do not mean the same thing. And picking wrong costs you either money you didn't need to spend, or a battery that shuts off at 3am.

Let's fix that in about two minutes.

What the three numbers actually mean

Watt-hours (Wh) is the size of the tank. It's how much energy is stored. More watt-hours, more hours.

Watts (W) is the size of the tap. It's how fast energy can come out at once. A big tank with a small tap can't run a microwave, no matter how full it is.

Surge watts is the size of the tap for one second. Motors slam a huge spike when they start. If the surge rating is too low, the unit trips instantly.

How to do the math yourself

Say you want your fridge, your internet, and your phones covered overnight.

Fridge. About 150 watts while the compressor runs. verify But it doesn't run constantly — roughly a third of the time. So over 12 hours it's 150 × 0.33 × 12, which is about 600 watt-hours.

Router and modem. About 20 watts, running the whole time. verify Over 12 hours that's 240 watt-hours.

Phones and a laptop. Call it 65 watts, half the time, over 8 hours. About 260 watt-hours.

Add them: roughly 1,100 watt-hours. Add 30 percent headroom and you want about 1,430.

So you're shopping for a 1,500Wh-class unit, or a 1,000Wh one if you're willing to be a bit careful. Not a 4,000Wh monster.

Why we add 30 percent

Because the number on the box is not the number you get.

The inverter that turns battery DC into wall-socket AC loses energy as heat. Cold weather cuts usable capacity further. And routinely draining any battery to absolute zero shortens its life.

Thirty percent covers all three without pushing you into a size you don't need.

The mistake that wastes the most money

Sizing for everything you own instead of everything you need.

Write down what actually causes damage or misery if it stops. For nearly every household that list is short: fridge, internet, phones, and medical equipment if you use it.

Your TV isn't on that list. Neither is your coffee maker, and definitely not your dryer.

The space heater row is there to make a point. Look how fast everything drains. Heating is the one thing a battery genuinely cannot do at a sensible price.

Sizes that make sense, roughly

Under 500Wh. Phones, a laptop, a CPAP for one night, lights. Not a food-saving device. Fine for camping and for people whose outages last two hours.

1,000 to 1,500Wh. The sweet spot for most US homes. Fridge overnight, internet, phones, a lamp. This is what we'd tell a friend to buy.

2,000 to 3,000Wh. Two-day outages without rationing, or an RV, or a household running medical equipment continuously.

4,000Wh and up. Well pumps, 240V circuits, genuine partial-home backup. Needs an electrician and a transfer switch to be worth it.

What size do you need for a well pump?

Different question entirely, and it's about surge, not capacity.

A half-horsepower well pump can spike to around 2,500 watts when it starts. verify Most 1kWh units surge to about 2,700 — technically enough, but with almost no margin, and margin is what you want at 3am.

Realistically a well pump means a 4,000Wh-class unit with 240V output. That's a different budget, and you should know that before you shop rather than after.

Then check one thing on the label

Go look at the back of your fridge right now. There's a plate with numbers on it.

If it gives amps instead of watts, multiply by 120. That's your real number, and it beats every estimate on the internet including ours.

A Kill A Watt meter costs about as much as a pizza and tells you exactly what each appliance uses. If you're about to spend a thousand dollars on a battery, spending twenty to know you're buying the right one is an easy call.