Here's everything we know about Tesla's big April 30 announcement

Last month, CEO Elon Musk tweeted that Tesla would unveil a new product - not a car - on April 30, at the company's facility in Hawthorne, California.

Now that the big event is near, we can pretty accurately predict what to expect when Musk takes to the stage: a battery.

Actually, two batteries. Tesla investor relations head Jeffrey Evanson revealed on Wednesday that the company would reveal a home battery and a "utility scale" battery - a battery for normal folks and a big battery for businesses.

We could still be surprised, by a heretofore unheard of or unrumored Tesla product. But that's unlikely (although there was a bit of speculation that Tesla would roll out ... a motorcycle!)

So here's everything we know:

Tesla home batteries are already out there

Elon Musk is also chairman of SolarCity, a provider of home solar panels. SolarCity has in the past offered home energy storage, using Tesla batteries. According to SolarCity's website, the company will have installed about 450 of these systems through 2015. The idea is that a customer always has backup power.

A Tesla home battery and utility scale battery would take on the economics of energy consumption

When you draw power from the grid, you pay whatever the price is at the time you use the power. A Tesla battery would enable you to draw power during off-peak periods and "bank" that power, using it during peak periods. The big Tesla battery would permit businesses and institutions to do likewise.

Tesla's battery could prevent brownouts and blackouts

As Bill Howard and Extreme Tech noted, "The [energy] draw is highest in late afternoon and evening, especially in air-conditioning months. So the Tesla battery, or competitors, could take a load off the grid at peak periods." The US electricity infrastructure has endured some rough episodes in recent years, typically when the heat rises and millions of customers are making demands on the grid to stay cool. If some of those customers were using off-grid battery power, the burden would be eased.

The batteries are already being tested

Various news outlets have reported that hundreds of home batteries are already in use, as Tesla beta tests the technology. Wal-Mart stores are also using the tech.

The home batteries don't look like much

It's a white box with a Tesla logo and a bunch of sealed lithium-ion batteries inside. Minimalist chic.

The home battery should appeal to fans of alternative energy

Obviously, if you have a solar setup, you want to be able to bank as much power as possible when the Sun is shining. A high-capacity Tesla battery would allow you to do that.

Governments will make it worth your while to install Tesla's batteries

Again, here' s Extreme Tech's Howard: "[The battery] would work financially through energy company rebates for conservation devices. There could also be tax credits. That lowers the effective cost of the battery packs, just as credits do for high-efficiency furnaces or solar panels."

The Tesla batteries make it very, very important for the company to finish building its $5-billion Gigafactory in Nevada

The facility is supposed to produce enough lithium-ion cells to build battery packs for, ultimately, 500,000 Tesla vehicles. Each Tesla car has about 8,000 li-ion cells in its battery pack. The home battery will contain a similar number. There aren't enough cells being produced globally to fulfill Tesla demands on the vehicle side. So it's imperative that the Gigafactory come online to serve the home battery market.

This expands a line of business for Tesla

Currently, most people have to spend around $100,000 to obtain a Tesla battery - by buying a Model S sedan. The battery is the most expensive part of a Tesla, but compared to buying an entire car, the pack alone is a relative bargain.

Tesla is simply awesome at managing power systems

Before Tesla came along with its offbeat design, nobody could create a battery pack that was relatively compact and could keep an electric car going for 200 miles. Tesla uses sophisticated battery and powertrain management systems to optimize the electricity that one of its cars draws from a battery pack. The company's home and utility scale batteries should be similarly well managed - and probably will have software that would be frequently updated over-the-air.