Does adding a battery to plug-in solar pay for itself?

It depends on whether your utility charges different rates at different times of day. Plug-in solar without a battery offsets whatever your home is drawing while the sun is up, and anything beyond that goes to the grid uncompensated. A battery stores the surplus so you can use it in the evening, which is worth more if evening electricity costs more than daytime electricity where you live. On a flat rate, the battery allows you to install more plug-in solar because you can save the solar energy for when it's needed. On a steep time-of-use rate, the benefit is more substantial.

The other half of the equation is system cost. A plug-in battery system runs several times the price of the panels, and prices are moving quickly as these products reach the US market. We currently do not sell batteries, so please treat this as a description of the trade-off rather than a recommendation.

How do I work out whether it makes sense for me?

Find your peak and off-peak rates on your bill, work out the gap, and multiply it by the battery capacity. For example, if the difference between your daytime and evening rates is 20 cents per kWh, a 2 kWh battery can save you 40 cents per day, multiplied by the battery's roundtrip efficiency, which can be anywhere between 70 and 90%.

Can I use a portable power station or solar generator?

A portable power station cannot feed energy into an outlet, but some members use it as their emergency setup. When the grid goes out, they disconnect their solar panels from the microinverter and plug it into the power station instead, then plug their essential loads directly into the power station.

Can I add a battery to my existing kit?

Yes, but not as a bolt-on. Plug-in battery systems with integrated inverters replace the microinverter rather than sit alongside it. Some are already selling in Europe and are beginning to appear in the US.

Would a battery give me backup power in an outage?

A plug-in solar kit will not provide whole-home backup because the microinverter shuts down without the grid. Some battery systems provide localized backup with outlets directly on the battery that you can use to charge your devices or even run an extension cord to the fridge.

This question produced a spirited discussion thread in the Bright Saver Community, with members running competing numbers.