At the end of April, Elon Musk announced that his new company, Tesla Energy, would manufacture industrial-scale lithium-ion batteries for homes (the PowerWall) and businesses (the PowerPack). Across the country, the thought took hold that renewables would suddenly become cost-effective and sufficient for all our energy needs.

But fantastic battery storage doesn’t actually lower the cost of renewable energy. It doesn’t decrease the amount of energy that needs to be produced. Nor does it increase renewable’s low capacity factor or its low efficiency.

The Tesla PowerWall will sell for $3,500 or so. This price does not include installation or the inverters needed to turn the batteries' direct current into the alternating current that US households run on. This price is much lower than expected and a fraction of the cost of similar systems from its major competitors, Sungevity and Sonnenbatterie. The PowerWall will store a kilowatt hour (kWh) of electricity for only 2 cents.

The PowerWall does not save any energy, since we don't throw any of that energy away. We can deal with the intermittency. It’s annoying having to ramp natural gas, coal, or hydroelectric plants up and down to accommodate, or load-follow, wind and solar, but we do it all the time.

The PowerWall is really good if you want to get off the grid. And you only want to do that if you’re geographically challenged, need emergency response power, or have enough disposable income that you can just play.

Batteries have always been an expensive way to store energy, costing anywhere from 9 cents to 50 cents to store a kWh. So the Tesla PowerWall is quite a breakthrough at 2 cents.

The PowerWall was designed to work in concert with rooftop photovoltaic solar arrays. It’s the dream of tech-savvy, device-heavy, gadget-loving people. It is basically an uninterruptible power supply large enough to keep refrigerators, routers, lights, and other devices powered up when extreme weather and other unforeseen incidents take down the grid. Or when the sun goes away at night.

It’s perfect if you don’t want to depend on the grid at all.

Musk said, “We’re going to unveil the Tesla home battery … [a] consumer battery that will be for use in people’s houses or businesses, fairly soon… I’m really excited about it.”

I am, too. I just put a 4 kW rooftop solar array on my house, so right away I calculated what a PowerWall would do for me, my electricity bill, and my carbon footprint.

I wasn’t really surprised to see it would have no effect at all, except to cost me about 7 cents more per kWh of electricity used.

Right now solar energy generated during the day is used during the day. The way our grid works, none is thrown away. Renewables are always taken first, as dictated by state mandates and other laws, so if it’s produced when not really needed, other sources like gas, nuclear or hydro, ramp down to accept it. When renewables stop producing, something else has to fire up to keep the power coming. This process is called load following and is a common gripe of regional transmission organizations and other grid operators that spend lots of time and money dealing with this intermittency of renewal energy sources.

Most of the time, my house uses all my solar generation as it’s produced. However, at peak solar incidence—from 2 p.m. to 5 p.m. for my house—I generate more electricity than I use, which goes back into the grid to be used by others, for which I get paid. At night, I use electricity from other sources, like nuclear and hydro.

The economic competitiveness of rooftop solar, the amount of solar generation needed to replace fossil fuels, the low efficiency of solar, and the huge amount of space needed for solar, are not affected by having good battery storage. None of the long-term calculations about achieving a sustainable energy future ever include the cost of load following, battery storage, or the intermittency of renewables. They all assume that all the renewable energy produced will be used.

The PowerWall simply makes that assumption more real. And would make a difference if we could achieve the fantasy of 100 percent renewables for this country. There wouldn’t be anything else to load-follow, so batteries would be essential.

But in the case of my rooftop solar array, I would lose money with a PowerWall. I live in Washington State with an amazing renewable program that gives me 54 cents for every kWh of excess solar electricity production that I give back to the grid. But I only pay 8 cents per kWh for energy I get from the grid. If it costs me 2 cents per kWh to store the surplus energy my solar system generates via the Powerwall, then it would cost me a lot more to store and use my own excess energy than to sell that energy to the grid when it's generated during the day and simply buy additional energy I might need from the grid at night for much less cost.

When these mandates, subsidies, and net-metering arrangements go away after 2021, then having a PowerWall will be more useful for rooftop solar.

Musk pointed out that the PowerWall and PowerPack could cleverly charge themselves by drawing power at specific times of the day when prices are low, and then using it when prices are high, something that could save businesses lots of money, but still doesn’t change the energy equation itself or the amount of energy needing to be generated.

A full home battery system would provide one undeniable benefit: It would allow a homeowner to go completely off the grid.

With a PowerWall, I could place an additional 8 kW solar array on my roof and store more energy at peak sunlight, so it would be used when the array is not producing enough electricity for my house. So I wouldn’t need to buy any electricity from the grid.

The system I put on my roof in February is the latest and greatest, and over its lifespan, it will produce about 150,000 kWhs of electricity for about 13 cents per kWh. If I sell a third of that back to the grid at present net metering rates of 56 cents a kWh, I’ll make $6,000 in profit after subtracting the original capital costs of the array and assuming no operating, maintance, or decommissioning costs.

If, instead, I emplace enough rooftop solar to cover my total power needs, and install three PowerWalls, then I could generate all my electricity for the next two decades at 15 cents per kWh. Which is OK if you care about the environment and don’t mind spending more to decrease carbon emissions, which I don’t mind at all. But the electricity I buy now from the grid is already carbon-free; it comes from hydroelectric and nuclear sources. So, in my case, it doesn’t really help the environment to go all solar.

Plus I don’t have enough roof space for that many panels.

But this concept—rooftop solar plus batteries, to cover an entire home's electric needs—really could help reduce carbon footprints in areas dominated by fossil fuel. Enough of these megabattery systems could remove a lot of the need for back-up power, or load following, of renewables by traditional, carbon dioxide-spewing sources.

And this is not trivial. Right now, wind and solar need a 1-to-1 backup by other sources of electrical generation, usually natural gas or coal. So the savings in carbon emissions using these battery systems could be wonderful.

But the cost to generate the renewable energy itself is unaffected.

The development of the Tesla PowerWall and PowerPack battery systems will allow the United States—and eventually other countries—to install a really low-carbon energy mix, including hydro, new nuclear, and new solar power, without needing to balance with fossil fuel. This new energy portfolio would be completely dispatchable, take up a small amount of room, involve the lowest carbon emissions imaginable, provide the power to charge a future fleet of fully electric vehicles, and provide all the energy the country needs for a very long time.

But let’s not confuse a great battery with solving the real problems of our energy future.