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Batteries, UPS & solar

Home Inverter and Battery Backup Sizing (Lights, Fans, Router, TV)

Where the grid drops out for hours at a time, many homes run a few essential circuits from an inverter and a battery bank: lights, fans, the internet router, a television and phone chargers. The two questions are how big the inverter must be and how long the batteries will last.

The calculator below is preset for a 600 W essential load at a power factor of 0.8, two 12 V 200 Ah lead-acid batteries in series (24 V), 50 % usable depth of discharge, 90 % inverter efficiency and a 4-hour backup target. Replace the load with your own total; motors such as fridges and pumps need extra headroom for starting current.

Last reviewed

In a double-conversion UPS the rectifier feeds the DC bus and charges the battery; the inverter rebuilds AC for the load. When the mains fails, the battery carries the DC bus through the inverter's efficiency.

For estimation and education. Not a substitute for a licensed engineer, the applicable codes (IEC 60364, NEC/NFPA 70, BS 7671, local regulations) or manufacturer data. Read the disclaimer.

Inputs

Load

Load entered as
W

Sum of the equipment on the UPS output

From the equipment data; 0.8 is only an example

%

25 % is a common practice, not a standard

Battery bank

V
Ah

Series adds voltage

Parallel adds Ah. Enter 1 and 1 to type the bank voltage and Ah directly.

Fraction, e.g. 0.8 = 80 %

From the UPS datasheet at your load; 0.9 is an example

Runtime target

min

Gives the battery Ah needed for this backup time

Result

UPS and inverter sizing — Engineerly. Estimate for education only; verify against applicable codes and manufacturer data.

Required UPS rating (with margin)
937.5VA
Backup runtime
216min

The battery bank falls short of your 240 min target; about 222.2 Ah at 24 V is needed.

Load real power
600W
Load apparent power
750VA
Required UPS rating (with margin)
750W
Next common catalogue size
1,000 VA
Battery bank
24 V, 200 Ah (4.8 kWh)
Battery current at the DC bus
27.78A
C-rate
0.139C
Backup runtime
3.6h
Capacity needed for 240 min
222.22 Ah@ 24 V
Parallel strings of 200 Ah needed
2

Working, with your numbers

  1. 1. Load apparent power
    S = P / PF
    S = 600 W / 0.8
    = 750 VA
  2. 2. Required UPS rating with margin (VA)
    S_UPS = S × (1 + m)
    S_UPS = 750 VA × (1 + 25 / 100)
    = 937.5 VA
  3. 3. Required UPS rating with margin (W)
    P_UPS = P × (1 + m)
    P_UPS = 600 W × (1 + 25 / 100)
    = 750 W
  4. 4. Bank voltage (series adds volts)
    V = V_unit × N_s
    V = 12 V × 2
    = 24 V
  5. 5. Bank capacity (parallel adds Ah)
    C = Ah_unit × N_p
    C = 200 Ah × 1
    = 200 Ah
  6. 6. Battery current
    I = P_load / (V × η)
    I = 600 W / (24 V × 0.9)
    = 27.778 A
  7. 7. Runtime (ideal capacity)
    t = C × V × DoD × η / P
    t = 200 Ah × 24 V × 0.5 × 0.9 / 600 W
    = 3.6 h
  8. 8. Required bank capacity for the target runtime
    C = P × t / (V × DoD × η)
    C = 600 W × 4 h / (24 V × 0.5 × 0.9)
    = 222.22 Ah
  9. 9. Parallel strings needed
    N_p = ⌈C / Ah_unit⌉
    N_p = ⌈222.222 / 200⌉
    = 2

Assumptions

  • Design margin 25 % on the load, a common practice for growth and ageing, not a requirement of a standard.
  • Constant power load for the whole discharge; battery voltage taken as the nominal bank voltage.
  • Usable depth of discharge 50 % of rated capacity.
  • Inverter efficiency 90 %, constant over the load range.
  • All batteries in the bank are identical, healthy and fully charged at the start of the outage.
Load power: 600 W
Load power factor: 0.8
Design margin: 25 %
Battery bank: 2 × 12 V 200 Ah, 1 strings
Depth of discharge: 0.5
Inverter efficiency: 0.9
Target runtime: 240 min

Check before you rely on this

  • The UPS maker's runtime chart or table (runtime against load for the actual battery model) overrides this estimate. Runtime also falls as the batteries age and at low temperature.
  • A UPS has both a VA and a W limit. Check that the load stays within both ratings on the datasheet.
  • Laser printers, heaters, motors and compressors draw high starting or surge currents; many UPS makers advise against connecting them.
  • Lead-acid capacity falls at high discharge rates, which is normal in UPS service. Turn on the Peukert option for a more realistic figure, or use the battery maker's constant-power discharge table.
  • Estimation and education only. Not a substitute for a licensed engineer, the applicable code (IEC 60364, NEC/NFPA 70, BS 7671 or local regulations) or manufacturer data.

Notes for this case

  • Runtime here is energy-based. Lead-acid capacity falls at high discharge rates; enable the Peukert option for a more conservative figure.
  • Recharge matters as much as discharge: the charger or grid must refill the bank between cuts, otherwise each day starts lower.
  • Check the inverter's surge rating against the largest motor it must start.

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How these formulas were checked: methodology and testing.