Solar battery size calculator
Storage is sized from the energy you want to back up, not from your panel count. Pick the kWh a day you need to carry, the days you want it to last, and your chemistry; the tally returns the nominal capacity to shop for, in kWh, in whole modules, and in amp-hours at 48 volts. Rated capacity always runs bigger than the energy stored, and the two reasons why are below.
How the sizing works
Two multipliers turn the energy you want into the pack you buy. First, depth of discharge: you never run a battery to empty. Draining lead-acid past half, or lithium past about 90%, cuts its cycle life short, so the usable energy is only a fraction of the nameplate. Second, round-trip efficiency: roughly a tenth of the energy is lost charging the pack and inverting the DC back to household AC. So the rated capacity is your usable energy divided by the depth of discharge and then by the efficiency.
Worked example, the defaults: to back up 10 kWh a day for 1 day on LiFePO4 at 90% depth of discharge and 90% efficiency, that is 10 ÷ 0.9 ÷ 0.9 = 12.3 kWh of nominal capacity, which rounds up to 3 of the common 5 kWh wall batteries or about 257 amp-hours at 48 volts. Every figure on this page runs that same arithmetic; the methodology page lists the depth-of-discharge and efficiency constants.
Nominal capacity by daily backup
LiFePO4 at 90% depth of discharge, 90% round-trip efficiency, one day of autonomy. Amp-hours are at a 48-volt nominal bus (a 51.2V lithium rack); halve the voltage and the amp-hours double for the same kWh.
| Daily backup | Nominal capacity | 5 kWh modules | 10 kWh modules | Ah at 48V |
|---|---|---|---|---|
| 2 kWh/day | 2.5 kWh | 1 | 1 | 51 Ah |
| 5 kWh/day | 6.2 kWh | 2 | 1 | 129 Ah |
| 8 kWh/day | 9.9 kWh | 2 | 1 | 206 Ah |
| 10 kWh/day | 12.3 kWh | 3 | 2 | 257 Ah |
| 15 kWh/day | 18.5 kWh | 4 | 2 | 386 Ah |
| 20 kWh/day | 24.7 kWh | 5 | 3 | 514 Ah |
| 30 kWh/day | 37 kWh | 8 | 4 | 772 Ah |
What one day of essentials looks like
Backup is not your whole bill. Size it from the loads you truly need through an outage and leave the big ones (central heat, AC, electric range) off unless you plan for them on purpose. A common critical set (fridge, freezer, lights, network, phones, furnace fan) adds up to about 5.5 kWh a day; add water pumps on top if your well or sump depends on power. Our figures; the fridge, freezer, and well-pump numbers match the appliance pages.
| Load | kWh/day | Note |
|---|---|---|
| Refrigerator | 1.5 | A modern full-size unit runs 1.4 to 1.8 kWh a day. |
| Chest freezer | 1 | Full and shut, it also holds temperature for a day on its own. |
| Lights (LED, evenings) | 0.5 | Whole-house LED lighting run in the evening, not all day. |
| WiFi router and modem | 0.3 | Small, constant draw; runs day and night. |
| Phone and laptop charging | 0.2 | A couple of phones and a laptop topped up daily. |
| Gas furnace blower | 2 | The fan only; a winter-heating figure, zero in summer. |
| Well pump | 1.5 | Household water use, not the surge, which sizes the inverter. |
| Sump pump (wet season) | 0.5 | Intermittent; a storm week runs it far more. |
Add your own list, drop the total into the calculator, and read the pack. If your must-run list includes a heat pump or central AC, size those from the single-load pages first; a compressor can double this whole table on its own.
Chemistry and autonomy move the number most
For the same 10 kWh of usable energy, the chemistry you pick sets the rated capacity more than any other input. Lead-acid needs close to double a lithium pack for the same job, and weighs far more:
| Chemistry | Usable depth | Nominal capacity | At 48V |
|---|---|---|---|
| LiFePO4 (lithium), 90% DoD | 90% | 12.3 kWh | 257 Ah at 48V |
| LiFePO4, conservative 80% DoD | 80% | 13.9 kWh | 289 Ah at 48V |
| Lead-acid / AGM, 50% DoD | 50% | 22.2 kWh | 463 Ah at 48V |
Days of autonomy scale the pack straight up: two days is twice the capacity, three is triple. Grid-tied backup rarely needs more than a day because the grid recharges the pack; off-grid sites in cloudy country plan for the run of dark days, not the average one.
| Autonomy | Usable energy | Nominal (LiFePO4) | Modules |
|---|---|---|---|
| 1 day | 10 kWh | 12.3 kWh | 3 × 5 kWh |
| 2 days | 20 kWh | 24.7 kWh | 5 × 5 kWh |
| 3 days | 30 kWh | 37 kWh | 8 × 5 kWh |
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Questions people ask
How big a battery do I need to back up 10 kWh a day?
About 12.3 kWh of nominal LiFePO4 capacity for one day of autonomy, at 90% depth of discharge and 90% round-trip efficiency. That is 10 kWh of usable energy divided by 0.9 and 0.9 again, which is why the rated number is bigger than the load. In common hardware it is 3 of the 5 kWh wall batteries, or about 257 amp-hours at 48 volts.
Why is the rated capacity bigger than the energy I want to store?
Two reasons, both hardware. You do not drain a pack to empty (draining lead-acid past 50% or lithium past 90% shortens its life), and you lose roughly 10% charging the pack and inverting the DC back to AC. So the nameplate has to be your usable energy divided by the depth of discharge and again by the round-trip efficiency. At LiFePO4 defaults that is usable divided by about 0.81.
How many kWh per day should I plan to back up?
Add up only the loads you actually need through an outage, not your whole bill. A common critical set (fridge, freezer, lights, network, phones, furnace fan) lands near 5.5 kWh a day. Heat, central AC, and electric cooking are the big ones to leave off backup unless you size for them on purpose; each can rival that whole critical set by itself.
LiFePO4 or lead-acid: how much does chemistry change the size?
A lot. For 10 kWh of usable energy, LiFePO4 at 90% DoD needs about 12.3 kWh nominal; lead-acid at 50% needs about 22.2 kWh, close to double the rated capacity and far more weight for the same job. That is why nearly every new storage pack is lithium.
How many days of autonomy should I size for?
One day is the default here: enough to ride a normal night or a short outage before the panels or the grid recharge. Grid-tied backup rarely needs more, since the grid refills the pack. Off-grid or storm-prone sites plan two to three days so a cloudy stretch does not run the pack flat, which multiplies the capacity straight up.
Can I wire this battery bank myself?
Sizing is planning; wiring is not a job to freelance. Battery banks, inverters, and any tie to your home or grid involve high DC currents, code-required disconnects and grounding, and permits in most places. Take the kWh and amp-hour figures from this page to a licensed installer for the actual bus, fusing, and hookup.