Batteries
Array size answers "how much do I make". Battery size answers a different question: how long can I run when I make nothing?
bank kWh = daily load × days of autonomy ÷ depth of discharge ÷ round-trip efficiency
Those last two terms are why nameplate capacity is not comparable across chemistries. A 22 kWh lithium bank and a 40 kWh lead-acid bank do the same job, because lead-acid can only give up half its capacity without shortening its life dramatically.
Depth of discharge is a lifespan setting, not a limit
| Chemistry | Usable DoD | Cycles at that DoD |
|---|---|---|
| Flooded lead-acid | 50% | 1,000–1,500 |
| AGM | 50% | 500–1,000 |
| LiFePO₄ | 80–90% | 3,000–6,000 |
Compare banks by cost per delivered kWh — purchase price ÷ (usable kWh × rated cycles). It is the only comparison that survives the chemistry differences, and it usually reverses the answer that price-per-nameplate-kWh gives.
Days of autonomy
Two days is the usual off-grid figure with a generator as backstop. Past about three, a generator is cheaper than the equivalent battery and only burns fuel on the days it runs.
Size the bank for the worst month, not the annual average. Off-grid systems fail in January.
Charge controllers
MPPT converts surplus panel voltage into extra current; PWM discards it. The gap is 20 to 30%, and it is widest exactly when you need the energy most — cold clear days, when panel voltage is highest and the battery is lowest.
Size the controller's maximum input voltage against the array's cold open-circuit voltage, not its rated Voc. Voc rises as temperature falls, and this is the most common way controllers are destroyed.
Temperature
Lead-acid loses about 20% of capacity at 0 °C. Lithium must not be charged below freezing without a heater or a low-temperature cutoff. An unheated shed in a northern climate is not a battery location without allowing for both.
4 articles
How Many Batteries for a 10 kW Solar System? The Array Size Does Not Decide ItA 10 kW array does not imply any particular battery size. What decides it is which loads you want to carry, and for how long, after the sun goes down.
LiFePO4 or Lead-Acid: The Sticker Price Compares the Wrong ThingLead-acid costs less per kilowatt-hour on the shelf and more per kilowatt-hour delivered. Two numbers explain the whole reversal.
Sizing an Off-Grid Battery Bank: Days of Autonomy, Not Panel CountThe number that sizes a battery bank is not how much you generate. It is how long you need to run when you generate nothing.
MPPT or PWM: What the Extra Cost Actually BuysOne of these converts surplus voltage into extra current. The other throws it away. That single difference decides which one belongs in your system.