Why TOU rates change everything about solar
For two decades, most US homeowners were on flat rates — every kilowatt-hour cost the same whether you used it at 2 am or 7 pm. Solar economics under flat rates are simple: produce kWh, offset kWh, multiply by one rate. But TOU pricing has fundamentally changed this equation for tens of millions of Americans, and most solar calculators haven't caught up.
The mismatch problem
Solar panels generate most of their energy between 9 am and 3 pm. TOU peak periods typically run 4–9 pm, after the sun has started going down. Without a battery, you're selling cheap midday solar at a low export rate, then buying expensive evening power at the peak rate. Depending on your utility, this can quietly subtract $200–$600/yr from what a simpler flat-rate analysis would predict.
How a battery flips the equation
A battery stores your midday solar surplus and discharges it into your home during the 4–9 pm peak window. Instead of exporting at $0.06/kWh and re-importing at $0.40/kWh, you're using solar you stored at near-zero cost to replace the most expensive electricity of the day. On utilities with extreme TOU spreads like California PG&E or Arizona APS, this can be worth $500–$1,200/yr — changing the battery payback from 12 years to 7.
No battery? Still plenty of optimisation
The cheapest TOU optimization is behavioural. Moving EV charging to midnight, setting the dishwasher to run at 10 pm, and pre-cooling your home to 72°F at 2 pm (using free solar) before the peak window can save $150–$400/yr with zero hardware investment. This calculator quantifies both paths so you can decide which fits your situation.