Battery Bank Sizing Recommendations

These calculations are intended for off-grid systems, not for grid-connected systems with battery backup.

PV
system
size Ave. daily PV
generation (kWh)¹ Recommended battery
capacity (kWh)² Battery
bank
voltage⁶ Recommended battery
capacity (Ah)² Estimated cost
of batteries⁷

Min. Max. Ave. Min.³ Max.⁴ Ave.⁵ Min.³ Max.⁴ Ave.⁵ Min. Max. Ave.

1 kW 1.54 4.06 2.87 3.08 8.12 5.74 48 64 169 120
Due to recent market volatility in the cost of lead, it is next to impossible to give a good cost estimate for a lead-acid battery bank at this time.
As of mid-August, 2007, costs were around $400 per kWh of battery capacity.

24 128 338 239

12 257 677 478

2 kW 3.08 8.12 5.74 6.16 16.2 11.5 48 128 338 239

24 257 677 478

12 513 1,353 957

3 kW 4.62 12.2 8.61 9.24 24.4 17.2 48 193 508 359

24 385 1,015 718

12 770 2,030 1,435

4 kW 6.16 16.2 11.5 12.3 32.5 23.0 48 257 677 478

24 513 1,353 957

12 1,027 2,707 1,913

5 kW 7.70 20.3 14.4 15.4 40.6 28.7 48 321 846 598

24 642 1,692 1,196

12 1,283 3,383 2,392

6 kW 9.24 24.4 17.2 18.5 48.7 34.4 48 385 1,015 718

24 770 2,030 1,435

12 1,540 4,060 2,870

7 kW 10.8 28.4 20.1 21.6 56.8 40.2 48 449 1,184 837

24 898 2,368 1,674

12 1,797 4,737 3,348

8 kW 12.3 32.5 23.0 24.6 65.0 45.9 48 513 1,353 957

24 1,027 2,707 1,913

12 2,053 5,413 3,827

9 kW 13.9 36.5 25.8 27.7 73.1 51.7 48 578 1,523 1,076

24 1,155 3,045 2,153

12 2,310 6,090 4,305

10 kW 15.4 40.6 28.7 30.8 81.2 57.4 48 642 1,692 1,196

24 1,283 3,383 2,392

12 2,567 6,767 4,783

¹ Ave. daily PV generation (kWh) assumes a minimum insolation for Binghamton NY of 2.2 hr/day of "full sun" (1000W/m²) in December, a maximum of 5.8 hr/day in June, and an average of 4.1 hr/day annually, a year-to-year uncertainty of +/-11% (NREL data), and an overall PV system efficiency factor of 0.70.
² Recommended battery capacity (kWh or Ah) is set equal to twice the average daily PV generation. For longest life, sealed maintenance-free batteries (aka VRLA, gelled electrolyte, AGM, etc.) should not be discharged to greater than 50% of their total capacity rating (at the 24hr discharge rate). As a rule-of-thumb, recommended battery capacity (in kWh) turns out to be approximately 3x the PV system rating (in kW).
³ Min. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average mid-winter day. Min. recommended battery capacity (kWh or Ah) assumes no other means of battery charging other than the PV system. A battery bank of this size or smaller would be fully charged (on average) by the PV system every day of the year without other charging sources. This is usually the best choice unless expected daily energy usage is more than 1/2 of the min. battery capacity.
⁴ Max. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average mid-summer day. In general, other charging sources (such as an auxiliary generator or utility grid) will be needed at other times of the year to bring the battery bank to a full state of charge at regular intervals (at least once per week is recommended). Without other charging sources, in order for this battery bank to remain fully charged at times of the year other than the summer months, energy usage must be equal to or less than that provided by the PV system on any given day. This can be difficult to achieve, especially during winter months.
⁵ Ave. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average day. In general, other charging sources (such as an auxiliary generator or utility grid) will be needed during the winter months to bring the battery bank to a full state of charge at regular intervals (at least once per week is recommended). Without other charging sources, in order for this battery bank to remain fully charged in winter months, energy usage must be equal to or less than that provided by the PV system on any given day.
⁶ Battery bank voltage is the nominal value, not the fully-charged voltage. 48V values are highlighted due to better economics and efficiency of higher voltage battery charging systems utilizing 48V DC-to-AC inverters.
⁷ Estimated cost of batteries includes batteries, cabling, conduit and enclosure(s) - all of which are required for code compliant systems.

PV system size	Ave. daily PV generation (kWh)¹			Recommended battery capacity (kWh)²			Battery bank voltage⁶	Recommended battery capacity (Ah)²			Estimated cost of batteries⁷
	Min.	Max.	Ave.	Min.³	Max.⁴	Ave.⁵		Min.³	Max.⁴	Ave.⁵	Min.	Max.	Ave.
1 kW	1.54	4.06	2.87	3.08	8.12	5.74	48	64	169	120	Due to recent market volatility in the cost of lead, it is next to impossible to give a good cost estimate for a lead-acid battery bank at this time. As of mid-August, 2007, costs were around $400 per kWh of battery capacity.
							24	128	338	239
							12	257	677	478
2 kW	3.08	8.12	5.74	6.16	16.2	11.5	48	128	338	239
							24	257	677	478
							12	513	1,353	957
3 kW	4.62	12.2	8.61	9.24	24.4	17.2	48	193	508	359
							24	385	1,015	718
							12	770	2,030	1,435
4 kW	6.16	16.2	11.5	12.3	32.5	23.0	48	257	677	478
							24	513	1,353	957
							12	1,027	2,707	1,913
5 kW	7.70	20.3	14.4	15.4	40.6	28.7	48	321	846	598
							24	642	1,692	1,196
							12	1,283	3,383	2,392
6 kW	9.24	24.4	17.2	18.5	48.7	34.4	48	385	1,015	718
							24	770	2,030	1,435
							12	1,540	4,060	2,870
7 kW	10.8	28.4	20.1	21.6	56.8	40.2	48	449	1,184	837
							24	898	2,368	1,674
							12	1,797	4,737	3,348
8 kW	12.3	32.5	23.0	24.6	65.0	45.9	48	513	1,353	957
							24	1,027	2,707	1,913
							12	2,053	5,413	3,827
9 kW	13.9	36.5	25.8	27.7	73.1	51.7	48	578	1,523	1,076
							24	1,155	3,045	2,153
							12	2,310	6,090	4,305
10 kW	15.4	40.6	28.7	30.8	81.2	57.4	48	642	1,692	1,196
							24	1,283	3,383	2,392
							12	2,567	6,767	4,783
¹ Ave. daily PV generation (kWh) assumes a minimum insolation for Binghamton NY of 2.2 hr/day of "full sun" (1000W/m²) in December, a maximum of 5.8 hr/day in June, and an average of 4.1 hr/day annually, a year-to-year uncertainty of +/-11% (NREL data), and an overall PV system efficiency factor of 0.70. ² Recommended battery capacity (kWh or Ah) is set equal to twice the average daily PV generation. For longest life, sealed maintenance-free batteries (aka VRLA, gelled electrolyte, AGM, etc.) should not be discharged to greater than 50% of their total capacity rating (at the 24hr discharge rate). As a rule-of-thumb, recommended battery capacity (in kWh) turns out to be approximately 3x the PV system rating (in kW). ³ Min. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average mid-winter day. Min. recommended battery capacity (kWh or Ah) assumes no other means of battery charging other than the PV system. A battery bank of this size or smaller would be fully charged (on average) by the PV system every day of the year without other charging sources. This is usually the best choice unless expected daily energy usage is more than 1/2 of the min. battery capacity. ⁴ Max. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average mid-summer day. In general, other charging sources (such as an auxiliary generator or utility grid) will be needed at other times of the year to bring the battery bank to a full state of charge at regular intervals (at least once per week is recommended). Without other charging sources, in order for this battery bank to remain fully charged at times of the year other than the summer months, energy usage must be equal to or less than that provided by the PV system on any given day. This can be difficult to achieve, especially during winter months. ⁵ Ave. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average day. In general, other charging sources (such as an auxiliary generator or utility grid) will be needed during the winter months to bring the battery bank to a full state of charge at regular intervals (at least once per week is recommended). Without other charging sources, in order for this battery bank to remain fully charged in winter months, energy usage must be equal to or less than that provided by the PV system on any given day. ⁶ Battery bank voltage is the nominal value, not the fully-charged voltage. 48V values are highlighted due to better economics and efficiency of higher voltage battery charging systems utilizing 48V DC-to-AC inverters. ⁷ Estimated cost of batteries includes batteries, cabling, conduit and enclosure(s) - all of which are required for code compliant systems.