I have learned a lot about energy storage by investing in companies that are making batteries better and more affordable. There is some fantastic research going on and some fantastic companies being built, but we need even more innovation.

Why? Because although solar and wind power are great sources of low-carbon energy, they also have their downsides. One is that they’re not constant sources. With solar, it’s not just that the sun goes away at night; cloudy days also make it hard for some places to use solar year-round. According to this list from NOAA, my hometown of Seattle gets less sun than all but 9 cities in the United States.

When you hear about this problem with wind and solar, it is tempting to ask: Can’t we generate extra energy on days when the sun and wind are strong, and store it for those days when they’re not?

Here’s the problem: Storing energy turns out to be surprisingly hard and expensive.

As I wrote in this year’s Annual Letter: “If you wanted to store enough electricity to run everything in your house for a week, you would need a huge battery—and it would triple your electric bill.” Let’s break that sentence down.

“If you wanted to store enough electricity to run everything in your house for a week, you would need a huge battery …”

According to this U.S. Energy Information Administration fact sheet, in 2014 the typical U.S. household used 911 kilowatt-hours a month, which works out to roughly 210 kilowatt-hours per week (911 per month / 30 days per month x 7 days per week). The best lithium-ion batteries store less than 0.2 kilowatt-hours per kilogram.

So a lithium-ion battery large enough to store 210 kilowatt-hours would weigh at least 210 / 0.2, or 1050 kg. 1050 kg is about 2314 pounds, or more than one ton.

“…and it would triple your electric bill.”

This figure is based on the capital cost of a lithium-ion battery amortized over the useful life of the battery. For example, a battery that costs $150 per kilowatt-hour of capacity with a life cycle of 500 charges would, over its lifetime, cost $150 / 500, or $0.30 per kilowatt-hour.

So if a consumer tried to store enough electricity in this lithium-ion battery to run her house, she would be paying at least $0.30 per kilowatt-hour for the battery.

According to the EIA, the average price of electricity for consumers in the United States is around $0.10 per kilowatt-hour. The European Union, where prices average 20 cents per kilowatt-hour, and India, where they range from 2 to 15 cents, would see similarly dramatic increases.

This is one of the reasons why we need new inventions that improve our ability to store energy cheaply and efficiently. Getting them will make it easier for solar and wind to be a big part of our zero-carbon future.