kWh is a unit of energy measurement. This is how much energy is stored in the battery. kW rating of the battery is how quickly it can get that energy out. To size a battery we need to know how much energy your house uses and how long you want to back it up. To me - I wouldn’t pay the premium for a battery because power outages where I am are infrequent and rarely last more than a few hours. To backup just the freezers - you need to look at the specs of them and calculate how much energy they draw. Before investing in batteries, it’s probably worth paying $200-$500 for an energy monitoring system. More on reddit.com

Old rule of thumb for offgrid: battery capacity 3x daily use, and enough panels to charge fully in 5 hours More on reddit.com

Size the battery to your household usage and goals, not to total solar production. Solar production just sets the ceiling of what’s available to charge it. A battery doesn’t exist to “catch” all your solar, it exists to shift energy in time or provide backup. A few ways to think about it: Grid-tied, good net metering / export rate If you have 1:1 net metering or a decent export tariff, batteries are usually about: • Evening + overnight usage • Peak shaving (TOU arbitrage) • Backup for essentials In that case, look at average kWh used from sunset → sunrise and size the battery to cover that. Example: • Home uses ~20 kWh/day • ~8–10 kWh happens after the sun goes down → A ~10–15 kWh battery makes sense Even if your array produces 40 kWh/day, oversizing the battery doesn’t add value. 2. Poor export rates / TOU pain If exports are paid poorly but imports are expensive (very common now), then the battery’s job is: • Maximize self-consumption • Avoid buying peak power Still, you size to load, not array size. You just bias toward: • Core household loads • Things you actually want to run after solar hours Example: • Midday solar surplus: 25 kWh • Evening + morning load you want to offset: 12–18 kWh → Battery in that range Anything bigger just sits partially unused most days. 3. Backup-driven systems For backup, the question becomes: • What do I need to keep alive? • For how long? You don’t size to total production — you size to: • Critical load kW • Desired runtime (hours or days) Example: • Critical loads average 1.2 kW • Want ~24 hours of autonomy → ~30 kWh usable storage Solar just helps recharge it when the sun’s out. The mistake people make People see a big array and think: “I need a big battery to match it” In reality: • Solar is about energy generation • Batteries are about energy timing and resilience Unless you’re off-grid, batteries bigger than your actual usable demand usually don’t pencil out. Rule of thumb • Start with household load profile • Overlay utility rules (net metering / TOU / export rates) • Then check that your solar can reasonably refill the battery on a typical day If the battery reliably charges by early afternoon and empties overnight, you nailed it. That’s the balance. More on reddit.com

Battery needs lots of solar and/or good off peak tariff. So battery size ideally sized to cover 90-100% of winter day usage. More on reddit.com